Refitting Pistons, rings and conrods

Old rings in position

Overall the pistons were in good condition but there was certainly some cleaning required and also the top ring had a gap which was far too large. This seemed to be due entirely to the ring rather than wear in the groove so I went ahead to order and fit new rings.

Piston as removed- 

First step was to remove the old rings which was much easier than those of the smaller pistoned motorcycles I am used to. There is plenty of flexibility and they can be slipped off by hand.

Grooves without rings- muck in top ring

The ring grooves were pretty clean apart from the top one which was dirty with a lot of sticky carbon deposit. Presumably this groove had suffered most because its closest to the combustion chamber and also had the least well fitting ring.

I cleaned the grooves using this Laser tools groove cleaner- great bit of kit and now highly recommended. I then cleaned the pistons using a ro-loc bristle brush in a hand drill.

Cleaned piston ready for new rings

I had ordered a new set of piston rings for the low compression motor (std pistons) from SJsportscars. (Steve's non Lotus alternative). The new top ring was 1.55mm in thickness instead of the previous ring (1.49) and successfully restored the correct ring gap in groove no 1. I am pretty sure that these rings are those described in the handbook as for the 907, but having checked with Steve I was assured that these are also correct for the LC 912. They do however differ from the rings removed; Ring one has no internal chamfer and can be fitted either way up. Ring 2 has an external (note not internal) step which must be oriented downwards and ring 3 is a 4 part system rather than the three part I removed. The lower oil control ring is fitted first. I had never used a 4-part ring before but the instructions I found here:
http://www.deves.com/oilring_inst.html

The set consists of an internal expander (with angular bends), a spacer ring and two rails.

Oil ring set unpacked

Fit the angled expander ring first, aligning the gap with the gudgeon pin (wrist pin)- 12 Oclock

Feed in the convoluted spacer ring, placing its gap at the other end of the gudgeon pin 6 Oclock

Feed in the lower rail, positioning its gap between the others 3 O clock

Feed in the upper rail position its gap at 9 Oclock

This is ring no 2- note the step on the outside of the ring which must be oriented downwards when fitted.

 Add the second ring followed by the top ring orienting their gaps at 1:30 and 7:30 O clock

Ring set in situ.

New  ring set in position, they were all  much more springy than the old rings  and it was particularly hard to keep the oil-ring rails in position in the groove. they tend to spring out easily and have to be checked for correct positioning several times. I checked that non of the ring gaps were in alignment and although there are probably better ways of clocking the rings I was pretty satisfied with this plan.

I removed the old shells from Top and bottom caps by pushing them sideways before clipping in the new ones...



I fitted an oiled ring compressor- checking the ring clock yet again. Its also necessary to check that the compressor does fit closely all the way round so that you can be sure that none of the rings have poked out of their grooves and become trapped on the body of the piston. Note that here the compressor is not aligned at its base (left) - this will allow the rings to spring out when they reach the bottom so make sure that the compressor looks like picture two (right). Remove the liner clamps, oil the bore and add more oil on top of the piston inside the compressor. Insert the con rod into the bore- care not to scratch the liner walls and guide the piston skirt in.  Press the compressor base firmly onto the cylinder liner edge whilst slipping the pistons down to make sure that  the rings have no gap to poke through. However be warned: The compressor does have sharp edges and from experience I would recommend gloves! Tap the piston down gently with the handle of a mallet so that it slips into the cylinder.

As soon as the piston is inside the cylinder refit the liner clamps- you will need to move the piston up and down as you fit the conrod caps and so its essential to make sure that the  liners cannot move.

Add assembly grease to the crank journals and feed the con rod down from the top by pushing on the piston whilst guiding it onto the crank with the other hand. Here the conrod has met the crank...

Before fitting the base cap and tightening the nuts- snug at first. You will need to rotate the crank to get at the nuts cleanly- so make sure the liners are clamped before you do so!

I fitted all pistons and nipped the end caps onto all. Once all were in position I removed the bolts one at a time to oil the threads. Finally I torqued down the bolts to the required 85 ftlb and checked that the crank can still rotate. Its not easy but it did turn.

Here are all 4 pistons inserted and the liners reclamped...

...and all 4 conrod caps tightened down. This motor is an early 912 but despite this it didnt have the oil breather pipe which was left off later models, I left well alone and dint attempt to fit one.











One thing I noticed though was that there was an area of damage on the alloy web of the lower MBP adjacent to the oil pickup pipe.

Some scuffing to alloy edge

This I know wasn't caused by me and checking older photos showed that it was present when I first removed the sump. I think some previous owner has struggled to get at conrod cap bolts by reaching under the oil pickup pipe at some point in the past-. I cant think why as its easier to either remove the pickup pipe or rotate the motor for access but there it is. I need a good seal here as this web will separate an oil from a water filled compartment. I checked that there are no projecting points that would prevent a good fit with the sump/baffle. The sealant should have no problem dealing with these which are not that deep and none of which go right across the web. They were there in the past so presumably had not caused a problem before.

Next job- clean up sump and baffle mating flanges.

Here we go again... refitting the modified main bearing... twice!

Well the amalgamated wisdom from both the Lotus 4seater forum and Steve at Sjsportscars was that I had indeed overdone the size of the notch in the rear lower main bearing! A triumph of enthusiasm over wisdom! When I think about it its now pretty obvious that the groove in the bearing forms a ring around the journal which is filled with oil under pressure. As there is no ready exit for this oil, it is forced out of the bearing between the bearing surface and crank journal thus lubricating the bearing. Obviously any exit of oil through the new notch would decrease the oil pressure in the groove ring and thus reduce the amount of oil spreading out across the raised parts of the bearing. Its not the depth of the groove that matters as oil can only leave where the MBP has been filed and that dimension is fixed, however the width is significant and the wide notch I had produced could allow too much oil through and drive the internal bearing pressure too low. There was nothing for it then but to take the assembly apart again and replace the bearing with one featuring a much smaller notch. Luckily the stripping was relatively simple and the MBP (main bearing panel/bearing ladder) came off again in about 30 mins.

I finally received a new grooved bearing and filed a new notch using a much smaller needle file.

Previous grooved bearing (left) and new one filed with a smaller needle file.

(right) Note that the notch no longer extends right across the groove to help

maintain oil pressure in the "ring" around the journal.

Before reassembly it was necessary to remove any old sealant and this sadly took rather a long time. I had been worried about the thickness of a layer of sealant between the MBP and block but on disassembly it was clear that in fact it had spread out really thinly forming a film across the surface of the metal over the low spots. There was nothing visible at all when the metal contacts were closely fitting. One problem was old sealant around the base of the outer (8 mm) studs). This was tricky to remove and in the end I opted to remove all the studs with a 8 mm stud extractor of the "thread-grabbing" type. This was really successful . Once the studs were out any old sealant was easily removed and I could clean up their shafts easily. I could then use the same thread grabber to re-fit the studs (with oiled threads) and torque them in to the specified setting.

8mm stud in process of unscrewing, note ring of old sealant.

 I was then able to refit the crank by laying it into the inverted block in the engine stand.  BUT... at this stage its essential to refit the spray shield. I actually missed this step the first time through; the shield must be fitted at this stage as it cannot be installed after the MBP is bolted on. Yet another strip down then!!  Fortunately the sealants weren't yet dry and it was a relatively simple matter to separate the block and panel and reseal after installing the spray shield.

This is the stuff to stick the sprayshield
onto the block. Don't use too much as it
 squeezes out when the shield is fitted

Sprayshield installed. It has to be slipped up 

from the MBP direction (step facing the block) and then 

rotated before being pressed into the housing in the block 

behind the flywheel mounting flange and lined up with the 

block on the left hand side (RHS ie out of shot in this 

view as block is inverted) 

Sealant applied to MBP

One other point that came to light since I had assembled this part of the motor was that sealant should not be applied to the main bearing pads but only around the "periphery" of the joint. Sadly this isn't defined or illustrated anywhere, and it is not a completely clear distinction since the front and rear bearing pads actually join to the "joint periphery" so who is to tell where to add sealant and where not? I made a "best guess", and extended the sealant where it seemed to form a natural line, but thinning the film across front and rear bearing pads. Since I now know that the sealant will spread out to a simple thin film I wasn't so worried about it this time around. I refitted the MBP and torqued it down (again). Hopefully this is now the last time! I still need to replace the nylon olive on the oil pick up pipe as I've had to remove it when I separated the panel and block again.

I'd intended to replace the dipstick tube grommet anyway but was surprised to find that the grommet fitted was incorrect. I found three O rings stacked up on the tube to achieve the desired thickness. Luckily I had ordered a new one anyway which I installed. I will need to check that the mating flanges are clean and not distorted so that this will seal.

Three O rings (right) found slipped over the dipstick tube and the 

single correct grommet (left) that I replaced them with.

I left the assembled block and panel to set, having checked that the crank rotated freely and turned my attention to the pistons...

Refitting the crank

I got the crankshaft polished at Southern Rebores near Crawley- great job fellas, really like the satin effect on all the journals.

Polished Crank- clearing all oil ways in the crank... 

I cleared the internal oil ways with a compressed air line.

...and in the block, blasting through from the Gallery along the drilling and also from the bearing holes out

I changed the main bearing shells for the new +10s I got from SJSportscars. One thing I noted was that the bearings as fitted weren't quite right. The lower centre should be a plain bearing and the upper centre a plain-with-hole. All the others are groove-with-hole. This motor had a plain-with-hole fitted in the lower position and a groove-with-hole in the centre upper location. This is probably not a serious problem, at least there was a hole to coincide with the bearing oil inlet, but it wasn't right so I got the right bearings for the 907 motor.

Note incorrect plain-with-hole in centre position of MPB

... and incorrect groove with hole in top centre position
in block

I replaced all the shells with those indicated as correct for this motor in the manual:

Note plain bearing in centre lower position (i.e. in MBP)

Note groove with hole in centre upper position (block)

The rear lower shell required a notch to be filed in its upper edge to allow oil to seep through from the bearing to the newly made groove filed in the MBP. This delivers oil to exiting the bearing to lubricate the rear thrust washer. The manual assumes that it will be possible to obtain a specially modified bearing from Lotus for this purpose. - Perhaps once you could, but these are not available at present, so I had no choice but to file the notch myself. There isn't much to go on when it comes to filing this groove as sadly the illustrations in the manual are of limited use. This shell is a groove-with-hole, yet the manual shows it as a plain; furthermore the location boss is on the left, yet the manual shows it on the right - which none of the various combinations actually have in this position. However the dimension given (0.5cm from the location boss) places the notch within the shell groove which makes sense. However there is no indication of how large the notch should be. The size may actually be critical since too large a hole might allow too much oil to exit the bearing, running it too dry and flooding the rear of the motor leading to rear oil seal troubles. Too small a notch would fail to lubricate the thrust washer. The illustration shows the groove as I filed and fitted it in my first attempt. I formed a shallow notch virtually across the centre groove, but thinking about it later, this may well be too large. I decided to fit the bearing whilst I waited for further information. - OK- foolhardy perhaps but at least it gives me a chance to practice assembly and using the sealant in a dry run. 

Note notch filed in bearing groove below alignment guide allowing oil through to newly
made chamfer. I am not absolutely sure that this is the right size so I may end up redoing this.

I applied red engine assembly lube to all the top shells in the block (block inverted) and including the two thrust washers (copper faces outwards)...

Assembly lube on upper shells (block inverted)

...and then laid the crank in the recess- this was a little awkward as you must ensure that both thrust washers remain firmly back in their grooves or they prevent the crank from dropping in squarely.

Thrust washers pressed back into their recess- journal drops in squarely.

Crank in place in block

The next step was to add Red lube to the shells in the MBP before applying PG307 "Gemlok gasket maker" to the mating flanges of the MBP.  The sealant can be applied as a continuous bead or a continuous film. There are no suggestions as to how this should be done but I chose to do the latter ensuring that the sealant passes inboard of all bolt holes. I also applied it around the bearing shells, I don't think its needed as a sealant here, but assuming that the sealant will have some thickness, I didn't want these to mate at a different distance apart from the periphery of the panel, possibly distorting it or pinching the bearings. 

I don't seem to have a picture of this but it wasn't as easy as it sounds since the PG307 is sticky and tends to form lumps. I do suspect I will be redoing this step of the assembly with a modified rear bearing shell- so if that happens at least I will be better prepared to spread the sealant next time.

The manual states that the nuts securing the MBP are 12mm or 8mm- this is not true they are actually 17mm and 10mm. I sorted out the appropriate sockets plus extensions and pre-set two torque wrenches to the correct torque so that all was ready for quick attachment of the MBP. The MBP then fitted smoothly over the block studs.

A word about studs and nuts; I think a perfectionist would use new studs and bolts throughout the block reassembly. This would be very expensive and I suspect unnecessary because, because the existing ones are in very good condition. I cleaned all studs with a brush and chased their threads with an old tap. All nuts and washers were cleaned ultrasonically before being brushed through with pipe-cleaners or tissue. I oiled the threads and torqued it down. The crank still rotated smoothly once the MBP was fitted so that at least is good news.

Cylinder liners - honing and replacement

Well dear reader, having decided that the liners in this motor were serviceable but not perfect I set about commencing the rebuild. First was to hone the liners prior to fitting new rings. This gives the new rings something to break in against and the (ideal) 45 deg cross-hatching that honing produces, allows oil to cling to the bore.

For honing I used 3-in-one-oil or ATF- whichever I have to hand to lube the bore. I used a Draper engine hone 56246 covering the range 51-177mm. This comes fitted with 180 grit stones and I bought a set of 240 for a second pass.

Honing unit fitted in cordless handheld drill

Honing in progress, avoid the clamps at the top and avoid pulling the
hone out of the cylinder while running!

To get a 45 degree hone the tool needs to be moved up and down in the bore at the same speed as it rotates. I found this was achieved by using the slow speed setting on the drill and depressing the trigger about halfway. Plunging could then keep up with the spin and 45 degrees hatching resulted. although to be truthful this wasn't as good as I had hoped.
A couple of the cylinders came out I thought rather well.

But others wouldn't clean up properly, some marks remained on the walls and no 4 had some evidence of very slight pitting where I assume that some water had rested on to of the piston during storage. I decided it would undoubtedly be best to fit at least two new liners and if I'm doing 2....

... yes, you guessed it and the inevitable mission creep set in- I ordered 4 new iron liners from QED motorsport and a whole load of bits (shells, rings, gaskets etc.) from SJS.

Christmas comes early in Surrey! Two packets from SJS and another from QED- partly unpacked!

I bought a puller to remove the old liners from eBay- £50. It looked rather puny when it came but in fact it worked very well.

Liner puller- threaded studding looked puny to me but it worked well. I'm sure with a little ingenuity this could easily be replaced cheaply with a couple of bars and some studding.

puller in use...

once the seal of the liners was broken they slid out easily by hand.

The sockets into which the liners had been fitted were a bit mucky but looked OK. However once the liners were out it was possible to see that the lower side of the motor had silted around their bases with sediment from the water jacket. Lots of it! This was highly satisfying to remove and clean out.

Note crud surrounding the base of the liner sockets

It was easily scraped out and rinsed away

Clean block sockets

Hopefully removal of this muck will help cooling.

The new Liners arrived but- as you might guess they just wouldn't fit! The old sockets once cleaned and lubed slipped back in easily, but the new ones - no way!... and I was unwilling to force them.

I cleaned the sockets with a drill-mounted wire brush.
This did the trick and liners 4-2 slid in with silky smoothness with
hand pressure alone

Sockets cleaned and ready

I would also say that it helps if you rotate the block to ensure that the cylinders are vertical so that gravity helps them slide in square, they can jam if they don't get dropped in vertically. The only fly in the ointment was the last liner- in cylinder 1. This just wouldn't fit. The other 3 liners would go into any socket and were completely interchangeable, but this liner would go into none of them. It seems to be an oversize- perhaps a quality control issue for QEDmotorsport to think about? I therefore resorted to the hone and after a several burst with frequent checking I did manage to get this liner into the no-1 cylinder position. It was still tighter than the others though.
At this point I checked nip- you will remember that the old liners were on the low side of acceptable at around 1-1.5 thou in nip. Nip is actually only adjustable one-way. You can remove material from the liner step which will lower the cylinder and decrease nip but there seems to be no way of increasing it. Lotus could presumably sell ring shims to go round the liners but they don't. Thus the nip is simply what it turns out to be... and in this case I was lucky in that the new liners sat slightly higher at 2-2.5 thou. All were the same so all was in order to go ahead and use the sealant to fix them in place.

This is the liner sealant I got from SJSsportscars- its Permabond A1044. Seems to be a thread and pipe sealant and  general engineering adhesive I suspect there are satisfactory cheaper alternatives out there- some of which were probably already in my stock!
Anyway it didn't come with any instructions but a search on the web found them. Degrease thoroughly, abrade surfaces with emery paper, clean and degrease again before applying sealant. Obviously the sockets were already roughened from my brushing, so that left only the liners themselves. The use of methylated spirits wasn't recommended as it might leave a residue (presumably the dye?). Anyway I used meths for an initial degrease, scuffed with 240 grit paper and degreased both liner and sockets again with meths followed by cellulose thinners. Then I applied the sealant.

... before fitting the liners. Now degreased and roughened, all liners proved stubborn to fit. No more silky slide in, all of them needed persuasion with a rubber mallet!  I supported the block from underneath whilst I did this as the block is only mounted at one end on the engine stand. I'm not at all happy with this procedure as the silky fit was more satisfying. I suspect there is something I could have done to ensure that this assembly process went more smoothly and if anybody knows what that is please let me know. Maybe sealant around the base of the liner could have acted as a lubricant? I also think that some appropriate 2-4 thou shim washers would have been a great idea. If positioned under the liner clamps then I could have used the clamps to push all liners down to a specified, matched and accurate nip.

All 4 liners in place

Liner clamps refitted- I tightened then to finger tight and then one flat
more on the nut as I didn't want to risk driving the nip lower.

Finally I left all liners to sit undisturbed for 24 hours before I recommenced the rebuild.  I see on the forum that Loctite 567 is recommended for this sealing job and used as a ring around the shoulder...  Ooops! I - hadn't seen that in time. Well I guess my sealant will have pushed up around the shoulder anyway as the liner went in, but to be at least a little more confident  I think I will test the block by filling it with water to ensure that the liners are actually sealed- even at normal atmospheric pressure as I do NOT want my fluids mixing! Actually at this point I decided that I had scratched the crank with the micrometer in my passion to measure it, so I've now decided to get it checked and polished before refitting- I don't want to endanger my lovely new shells! That will be my next task and hopefully I'll get the crank back before I go on holiday.

Overcoming Dysmetria!

Well here I must express my apologies to true engineers- stop reading here, this bit isnt for you. This is where grapple with my inability to measure things accurately and the very tight limits of tolerance given by Lotus in the workshop manual. Since my problem is practically clinical I have coined the term dysmetria!  I am also concerned that the figures in the Lotus tech data section might  not be serviceable wear limits but tolerances in original manufacture so how worn is too worn?
Anyway this blog will be of no interest to the serious and experienced mechanic but it will at least document my struggles with measuring devices and the reasoning behind my decisions (be they right or wrong!)... how hard can it be?

Cylinder nip:
This is the height to which the cylinder liners project above the crankcase surface. You can measure it using a straight edge across the cylinder and a feeler gauge between that and the block. Or you can use a depth

Using a depth micrometer through a hole in the straight edge,
its tilted for photographic purposes- obviously use it vertically.

micrometer measuring through a block also positioned across the liner. I did this both ways, feeler gauges gave a higher value of 1 thou for cylinder 1 and 1.5 for the rest. The Micrometer gave 0.5 thou for no 1 and 1 thou for the rest.
Liner nips (if not the same) should show a smooth gradation from back to front, ups and downs will prevent the head gasket from seating, this appears to be fine. The cylinder liners are magnetic so these are presumably the original iron liners and for these the manual cites values of  1-5 thou for engines after 12478. If the feelers are right then I am in spec, if not then its tight on the low limit. I must check that I haven't depressed the liners with over tightening of the liner clamps. 

Mains and Big ends.
No pictures for these but here is my experience.
Direct micrometer reading on the journals was difficult, it tended to jam, it wasn't easy to see whether you were at right angles and truly across the diameter. This seemed just an easy way to scratch the nice polished surface! Its probably much easier if you have a way of mounting the heavy and cumbersome crank on the bench so it can be easily rotated and examined.
The solution I found was to use engineers' calipers/dividers which were light and easily set to the journal diameter, they were great for testing roundness. The calipers are then measured with a digital caliper gauge. This worked really well. All my journals came out at the expected diameters allowing for the regrinds inferred from the shell markings and there was no detectable out of round at all. Good news here then!

Bores- using the bore gauge.
Setting the gauge: 
Chose an extension anvil of a length close to that you wish to measure- I used 95mm. Insert the gauge at the top and depress it to give one full revolution and clamp it in place. Check that the combined anvil and gauge base is longer than the dimension you wish to measure. 
In order to compare cylinders it doesn't really matter what you set the gauge on, but for actual measurement the gauge must be set up. This is difficult without a setting ring (which is prohibitively expensive) so I have to use a micrometer. In this case set to 95 mm and clamped lightly in a vice for support (I have too few hands for this job). Set the gauge between the micrometer pins using the sprung pip between the wheels and the anvil tip. Even though the surface area of the pins is small, its still essential to rock the gauge to make sure you know where the needle hits its minimum. This is 95 mm so you can set the gauge ring to zero at that point and lock it off with the clamping screw. This is fiddly!

Use
You can then insert the gauge to the required depth in the bore, orient it in the required direction and rock it gently along the plane of the anvil whilst watching the gauge for the minimum reading. 0 represents  95 mm, so reading from zero to the needle maximum deflection gives the difference between the actual bore and 95 mm. Readings counted from zero to the left are oversize, those to the right undersize, provided the needle has passed through the zero (95 mm) point. If it hasn't then they are still oversize so read them from zero anticlockwise. Watch out for the full rev counter although if the gauge is set close to the measurement to be taken  this may not be needed. 

The bore gauge is very accurate and thus tends to generates different readings every time! For reproducibility of  readings its essential to put it in the same position for each measurement. This is tricky. The manual states manufacturer's values measured in the thrust plane (i.e. 90 deg to the crank and in the plane of conrod movement) at 50 mm depth. I drew markers on the gauge 50 and 100 mm up from the measuring tip and aligned these with the cylinder top to take the measurement. My gauge is marked to 0.01 mm per division, I estimated 0.005 by eye when the needle was between divisions.

Bore gauge measuring 90 deg to thrust direction, note depth marks on stem

I would be lying if I said that this was an unqualified success. However I did get a consistent picture. I measured at parallel to the crank, 90 deg to crank and 45 deg to crank in each orientation, i.e. 4 measurements at each point. In all cases the thrust dimension was the smallest at each level and the measurement at 50 mm down was smaller than that at 100 mm in all cases except cylinder 4 when there was virtually no difference at the two levels. Taking the largest recorded value for the bore in each cylinder thus means using the 50 mm, thrust plane measurements which were:
Cylinder 4, 95.355; Cylinder 3,  95.345; Cylinder 2, 95.34 and Cylinder 1, 95.34 mm. The manufacturers spec gives max values of  95.288 and 95.308 for a 912 LC motor depending on whether the liners are type "A" or "B". Pistons and liners come as a matched pair and I have "B" pistons (marked on crown), therefore "B" liners. This means my bores are worn by my "best guesstimate" by 0.047, 0.037, 0.032 and 0.032 mm or 1.9, 1.4, 1.3 and 1.3 thou. Lotus took the view that something was "in spec" or not, they didn't specify wear limits or  give regrind suggestions for the crank; it was right or it wasn't. However most UK motors of the time were more tolerant and a rough guide of 2 thou per inch of bore diameter was tolerable. In fact a typical first bore of say  an Austin A series motor would be 20 thou and by these criteria these cylinders are still serviceable. Remember, the aim here is to produce a serviceable road car not a racer or track car so I'm hoping this will be sufficient. 

Thrust washers.
Rear Thrust washer was clearly worn more than the forward washer.

Rear Thrust (copper side)

Front Thrust washer (Copper side)

Both were measured with a micrometer, front 2.33 mm and rear 2.29 mm. I don't currently know if they both had the same diameter when fitted but it seems likely as the extreme tip of the rear washer was thicker at 2.32 mm. If so then wear would account for an extra 0.04 mm (0.0015") end float. End float I have found  to be right on the max with these washers fitted, so it may be sufficient to simply replace them with new standard sized rather than oversized. Rapid wear of the  thrust washers was a problem with early motors and required a modification (see post: modifying main bearing panel). I will add a pair to my shopping list.

Piston to bore clearance.
This motor has type B pistons corresponding to the design used in 912 Std compression engines, compression ration 9.44:1. Since the pistons have to work in the bore the clearance between them is an important determinant of wear. I am taking this as the difference in bore as measured perpendicular to the crank at 50 mm depth and the piston diameter measured at the widest point which is the skirt. Pistons may vary in diameter at different points to allow for expansion and its not stated at what point the diameter is measured. I measured them both directly using a digital caliper and indirectly with engineers' calipers.  These were very useful to check the diameter around the circumference. I could then measure the calipers using a digital measuring caliper. The measurement was cross-checked with a 100mm micrometer measuring directly on the piston skirt app 10mm up from the lower edge.

Pistons measured at the skirt (mm); 
Piston No: 4, 95.14  ; Piston No 3, 95.14; Piston No 2, 95.18  ; Piston No 1, 95.19
Calculated clearance: Cylinder No 4, 0.215 ; Cylinder No 3, 0.205; Cylinder No 2, 0.16; Cylinder No 1, 0.15
The manual gives the values for piston-to-liner clearance of 0.11-0.15 mm. From these figures Cylinders 1 and 2 are almost within tolerance of 0.11-0.15 mm but 3 and 4 are over. However as with other places in the manual its not clear whether Lotus are providing figures for the tolerance in manufacturing or permitted wear before service is required, and again I'm up against the "as new or replace" doctrine which I just cant afford to do. Taking my cue from Brit cars of the same period, then wear of up to 3x the original spec was permissible and again mine are well within this. Piston measurements are hardly down from stated diameters and I think the existing liner/piston setup is reusable.

Piston Skirt 
Since the piston gets hotter at the top than the bottom, the crown expands more than the cooler bottom of the skirt. For this reason and to ensure that the piston is almost straight when at temperature, the piston is made to be tapered, being wider at the bottom because this will expand less than the top. This skirt flare is detected by an increase in piston diameter from pin level to skirt base. However, if a piston has suffered overheating then the skirt may expand and contact the cylinder wall. When this happens the  piston is forced into  a straighter configuration and does not contract back to its original shape when it cools: the larger diameter at the skirt is lost. Termed "skirt collapse" it can be eliminated by checking that the skirt diameter is greater than the diameter at the pin. In my case this was true in all 4 cylinders. It is possible to check the angle of this flare (grade) but  I was happy to find that skirt collapse wasn't a problem in this motor.

Piston Ring-to-Groove Gap
This is cited for a Std compression 912 as 1.5 - 3.5 thou measured by slipping a feeler gauge in between the ring and the groove land. All cylinders were similar and all gave a higher value of  6-7 thou for ring one and a more likely value of 4 thou for ring two which is within the specified range (just!). A larger than expected gap could be generated by a thinner ring or wear in the piston groove. I was expecting rings one and two to be the same thickness because only one gap is quoted in the manual for both compression rings and piston groves are the same width. However this difference in observed gap size was practically all accounted for by a reduced thickness in ring 1 since when each ring  was removed and measured with a micrometer it was clear that ring 1 (1.52 mm or 0.059 ") is  thinner than ring 2 (1.57 mm or 0.062") by 3 thou. When the actual thickness of each ring is added to its clearance measured by the feeler gauge it generates the groove width, which is practically the same in both cases, supporting the conclusions above that the pistons are probably not significantly worn and still usable if not perfect. Its not clear whether this difference in ring-to-groove gaps is due to similar wear in all 4 top rings, or simply the fitting of a replacement ring set that had thinner top rings to start with! I think these are unserviceable whether through wear or incorrect installation and so it seems likely that new rings would be a good idea.

So Conclusions to this measurement section:
Replace all rings
Fit new thrust washers (Std thickness)
Hone all cylinders.
Fit new big end and main shells- probably not strictly necessary but seems a shame not to do so whilst I am here!
Not perfect but hopefully all serviceable.