Setting the valve clearances-shims and tappets!

I got delayed by Christmas and the cold weather but eventually I got around to re shimming the tappet buckets for the correct valve clearances. On the 912 these are 5-7 thou inlet and 10-12 exhaust. I made a simple Excel spreadsheet to sort out the arithmetic and guard against mental aberration whilst calculating the shim size needed. The spreadsheet is here if anyone wants to have a look- as usual comments appreciated. Please download a copy for yourself  rather than trying to use it on the web. Make sure you have the latest copy of Google Sheets,  click the three dots in the top left and select "sharing".  Click on "make a copy" to download to Google Drive.

It should work for any engine just change the max/min clearances specified for your motor in the top boxes, and then enter your measured values for shim thickness and valve clearances. Work only in mm or thousandths of an inch - don't mix measurements (-obviously). There is a conversion calculator at the bottom just in case. You should enter values only into the yellow cells, the others are formulae and should be locked.

Having made sure that the tappet buckets were a good sliding fit in the housing (see post on cam housings), I then measured all the shims (which were still paired with their valves). It seemed that many had been filed or ground, and this meant that they were now tapered. This was an unexpected complication as I needed accurate measurements in order to calculate the new shims needed. In the end I measured their thickness in the centre where they would touch the valve stem tip. I measured the full set twice using a digital micrometer and tabulated the readings for comparison. Most were identical but readings were highly dependent on where you measured them which is obviously not good. I will change all of these so that all shims have parallel sides. I then reassembled the head and cam housing including shims making sure that they sat squarely down in the valve retainer head. I oiled the tappet buckets before refitting the housing- but without sealant (The housing will be on and off several times rather than clean up and replace the gunk each time I have chosen to ignore sealant thickness in terms of affecting the valve clearances). I set the head on two wooden blocks (taking care not to obstruct the valves) and then nipped the nuts up tight, but not to the full torque (This was a mistake... see on!). I could then rotate the camshaft to  measure the clearances under the heel of the cams.

Cam housings reassembled onto head

Head raised on wooden blocks because...

The valves will protrude below the head as the camshaft operates them. This shows maximum opening, ignore the screwdriver lower right and below valve head, its on the bench behind.

NB as the cam houses are angled, I measured the clearance when the cam tips were in line with the centre moulded dots on the housing arches.

Cams lined up with central moulding marks on cam house "arches"

I used a bent set of feeler gauges for this as they also coped well with the angle - this set is marked in both mm and thou and are very useful. Sadly the selection of sizes isn't as thorough as you might like so some ingenuity is required to get the feeler size you actually want! Again I went through the whole series twice to make sure I trusted the values. It was usually a case of finding the first size to slip under and that made it quite clear. There seemed to be a lot of  latitude in the camshaft position... Easier than some bikes!

Angled feeler gauges. I used the Draper 12 blade set. These are great because the angle allows easy access to the cam base but the extra width of the gauge allows you to check the whole of the contact between cam and tappet at once. I know this shouldn't make a difference, but some of my shims turned out to have been tapered by poor grinding in the past so the clearance varied across the contact areas. The gauges only go down to 8 thou though, so great for exhaust  or determining "too high" clearances but you will need something smaller for checking a correct inlet setting- or anything that's closed up smaller than that.

I found the inlet valves were at least in the right area. All had closed up though due to the seat regrind and a couple had no detectable clearances at all ... which means less than 1.5 thou and I wasn't sure how to deal with these, but in the end decided to assume a value of "0" and deduct the full clearance value from the shim. I may be lucky but more likely will need another round of measuring for these. The shims themselves are quite cheap but the minimum postage costs at SJS are a bugger so I hope to avoid an extra order if I can! The exhaust clearances were all too high- this was expected as I've used new, non-Lotus valves from KPH engineering and these are a little shorter in the stem than the originals. However, as I had the seats recut this will have compensated to some extent by moving the whole valve upwards into the head. Even so I will need thicker shims for all of these .

I calculated the shim thicknesses required and ordered them all from SJS. They arrived promptly so after the Xmas break I set about installing them. The 95 thou seemed to have been supplied as an empty bag but apart from that all the other 12 were fine so I just fitted a spare of approximate size for reassembly. I had expected to be able to reuse another 3 by swapping them around to positions where their size was appropriate.
Having assembled the houses again I measured the clearances once more - now all were very close but most were actually  smaller than expected - and it is then Dear Reader, that the truth dawned. As I hadn't torqued the cam carriers down to the specified setting, I couldn't reproduce the tightness accurately. Variations in the tightness actually affected the clearances; I hadn't expected any significant effect at all! Anyway I was then obliged to torque both housings to the specified 15 ftlb and re-determine all clearances for yet another set of shims! My calculations had aimed for a midpoint value in the permissible range, but the effect of this torquing error was to close down the clearances by 1-2 thou and  8 valves were now in spec (although on the lower limit), leaving problems with 6.
Of the two valves that had previously given zero clearance, one was now measurable although still too small, and the other had still got no clearance and so will need to be tried again with a thinner shim. One further valve gave a very strange high value (20 thou) well over the expected, calculated  clearance*. (* I had attributed this to my bad measurement but in fact it later turned out to be a supply error, the shim in question being 95 thou rather than the 105 ordered and specified on its label). Of the remaining 3, one still needs a 95 thou shim (which was missing from my order - presumably having been supplied as the 105) and the final 2 had suffered from the torquing problem and closed up too far giving clearances 2 thou lower than expected and sadly now one thou below acceptable! In any event as expected I will need a few more shims (in fact 6) in order to try again to obtain the correct clearances all over. I will also order a spread of sizes for the valve with no clearance to ensure that I can get it in the right range.

Well friends, it took another couple of iterations to get it right but got there in the end.  As I later discovered the labeling error it meant that I hadn't actually got a 105 thou shim at all so I had to machine down a 170 shim to 105 in the end rather than wait for yet another order. This turned out to be surprisingly straight forward ... but I had to fit the shim into the lathe using a magnetic pick up tool from the front of the lathe jaws to make sure I could hold it squarely as I tightened the jaws. It worked pretty well and ran true after a couple of tries. Even so I don't think it would be possible to work on shims less than 100 thou as there simply isn't enough to grip in the lathe. Maybe there is some mandrel that could hold them whilst their faces are skimmed?

I have to say that finding a whole set of clearances spot on is a very satisfying thing! Anyway job done. I will refit everything with sealant and new O rings under the oil feed spigots before fitting the head back on the block. My new workshop crane has come now (2 tonne long reach) so its soon going to be time to start swapping the motors about!

Anyway before leaving the shimming exercise here is a summary of what I have learnt from this experience:
1. If shims are tapered chuck them away, you can use any old shim to measure an initial clearance but NOT if its tapered.
2. If you get new shims don't assume they have been put in the right bags, check them as they arrive. One of mine was supposedly 105 but in fact was a 95 and this caused no end of problems.
3. Check and double check that the clearances and shims you measure have been recorded against the right valve. I found it very easy to forget whether I had counted from  valve no 1 or no 8!
4. Always torque the cam housing down fully before checking clearances.

Well that's it for this job; hopefully I won't need to do it again in the immediate future!