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.