A bit more than a year on now....
I've just acquired what I believe is a DG to strip, hoping I've got really lucky and it's a DJ, I doubt it. Anyway I hadn't measured the pistons before I returned home so started looking at the specs and realised that the bores are the same for DG/DJ, the difference is in stroke. Now I've had a bit of prior experience messing with customising pistons to correct an "engineers" erroneous block shaving, so just as a refresher and out of intellectual curiosity I decided to do some quick calcs for the T25.
I back calculated the compressed volume from the compression ratio and then did a google search to see if I could find the piston volumes, which is why I am here.
Taking the DJ, by deduction compressed volume must be 55.5cc, and if you take off piston volume of 35cc and head volume of 23.3cc as stated above, you are left with a deck volume of -2.8cc. This equates to a deck height of just -0.40mm so the top of the piston is just proud on the barrel...... assuming all the measurements and calculations are accurate. BTW I normally separate deck height proper from head gasket height, but that's not so appropriate for our engine design, I understand why you left the compression ring in place.
Duplicating this for the MV the deck height should be -0.49mm and for the DG, with a CR of 8.6:1 (I think you mentioned 9:1 above ?) the deck height is +.66mm, but I just assumed the piston volume was the same as the DJ, probably wrong, 42.5cc would give the same deck height.
That's the theory, I think you have done the calcs on the assumption that the top of the piston is at the top of the barrel, maybe it isn't and I have stripped my engine without measuring so have no further insight at this point. I can see where your calcs came from however
What is clear is that small changes in compressed volume do change CR quite a bit. Did you happen to measure head face to compression ring so that piston clearance can be evaluated for each engine or gudgeon pin to top of piston, that could possibly provide further light ?
BTW, I've just gone through another exercise with changing head gasket thickness on an engine because a water leak (i.e steam) eroded part of the block. It's a turbo engine so we wanted to get back to the same CR. The engineering shop had different measurements for piston volume compared to the piston manufacturer. Make of that what you will.
