I should have kept at least one old 912 piston and measured the volume of the bowl in its top, I need this to begin calculating how much compression difference 0.1mm of shimming wil cause (in theory).
You sure? you can find the bore and stroke easy enough, and knowing the theoretical compression ratio will give you the theoretical volume of the final combustion chamber, then 0.1mm change times the area of the bore would be the increase, right? I could measure a piston for you, but I bet the theoretical calculation is more accurate.
the engine compression is often changed on engines by the position of the top piston ring not the piston bowl, on the 912 motor the piston is measured using soft lead or solder to give clearance between the head and piston so they do not clash under load conditions, its not to alter the compression, every barrel could be a different length, plus a piston might be fractional different, its nothing to do with compression just a set standard of running clearance.
The reason I thought of the piston-bowl was to calculate the effect of shims in terms of compression psi BECAUSE my 1st compression tests fall too low. The rings haven't seated but I'm not sure if that's so much of a factor. When I did the shims & fire ring (gasket) ritual I aimed to end up 0.15mm above the 1.00-1.20mm spec (I don't like too little crown clearance, fearing pistons slapping the head at exhaust TDC if a muffler is removed for example). I got this pretty well where I wanted it with the soldering lead pieces compressed to between 1.33-1.38 of which very narrow range I was quite proud. So this time I was really looking to get an idea of just how many psi difference my 0.15mm of extra shimming could account for. I thought someone might have seen just this kind of tabulation somewhere. With bore/stroke of 100mm/120mm and a 'presumed' chamber (bowl) volume of 59 cu-cm I got to about 16:97 ratio and 239 psi which is close enough for the canonical 17:1 & 236. After posting my question I fingered out that 0.1mm over-shimming would translate to 16:75 & 236 psi meaning a drop of only 3 psi. At a full 1mm (which I never got close to) this would be 209psi and 14.98:1 or a psi drop of 30. My compressions range from 130-145 psi which are easily 40% low and I have no explanation for this. I made my own adapter from an old injector which doesn't leak as far as I know, my gauge has no check-valve so I watch the bouncing needle, and beyond these I'm open to wisdom. NONE of the sleeve kit rings had to be filed, all were at spec gap or, greater, or much greater (another looooooong topic discussed here over previous months).
Has the engine ran? or are you testing it still? I think you need a check valve, but 145 is still way low, but I've never compression tested new rings so that might be normal? I'd think a check valve would be needed for an accurate reading as the pressure will build over many cycles.
Hasn't run yet, was liberally oiled with assembly oil, rollout & first start is is planned for May. I vaguely remember having read somewhere that spec-compression should never be 'expected' and certainly not at this stage but have no idea where I read that. Off-hand I'd like to see around 175psi. Tomorrow I'll try to borrow a gauge with a check-valve to see if there's any difference. BTW I lied about the shimming data I've dug up the record and the close-range figures I was so proud of were mathematical solutions, the finals wre a bit wider is scope after almost-procedural torquedown.
You’re overthinking this. Set the piston crown clearance to the spec and don’t worry about it. And yes, the compression gauge needs to have a check valve or you’re just wasting your time
The engine will need to be run and given some work before you even think of compression testing the cylinders although why bother, if the bores and the rings are good with the bump clearance as near to spec' as you can get things your flying, with the engine hot and working hard the exhaust type is not going to effect piston clash with the head, the time things can start to tap is when the engine comes off load, a straight pipe will let the heat away faster obviously but the din will help nobody, the air cooled engines only need 1/3rd of the blown air of a water cooled engine, to do a proper compression test you need the engine cranking over fast, a check valve is sweet but a gel filled gauge will do fine, I do tend to think you might be making a meal out of I don't know what, the compression target per cylinder per that engine set correctly is printed in a book, read it after the motor is working if you want.
Thanks for the reassurance, I don't do an engine a day so it feels good that what I thought I had read before is substantiated. I've had a few issues with these sleeve-kits but nothing so major as to have a significant impact. I tried to borrow a gauge with a check-valve today but couldn't find one (I probably wouldn't wanna lend mine to anyone either, lost many good tools that way already)
Got me a new gauge and all 6 jugs are looking good 1 240psi 2 200psi 3 210psi 4 250psi 5 240psi 6 220psi Since my adapter is an old injector, I stuck a copper washer to its tip with glazier tape. Even this was not good enough on second tries with the same gum by then bathed in fuel. So I re-did the whole prep for each cylinder, beginning with brake cleaner and new gum. The engine has run about 10 minutes total so far. No issues.