M30b35 piston swap

[anthony]

After collecting parts I have started to rebuild the bottom half. I have 6 noral pistons (manufactured in 1991)and a crankshaft from a 91 e32 m30b35. They are going into a m30b34 block with m30b35 head. (the b34 block mounts straight in to the e12)

The crankshaft went in no problem. There are two issues with the pistons. 1) the size stamped on the donor pistons is 91,985. vs 91,97 pistons from the b34 bloc.. I figure there is wear on the block so this should not be a problem. My digital caliper goes only two places past the dot.

2). The donor pistons tops are .014 in. above the deck of the block. I measured with a feeler gauge. For what it's worth it is the pistons pin location that raises whole piston so the rings are raised as well. Compared to the original pistons which are flush.

I can't figure if I'm better or worse off with this set.

[anthony]

After doing more research. Not good. First, I did not mention that I was referring to the edge of the piston. The b35 piston is raised and does extend above the deck of the head but not the edge of the piston.

All solutions are more of a problem than finding the correct piston which is in a earlier m30b35.

[Mike W.]

A bit above deck height shouldn't be a problem, Motronic 3.3s were something like .016 high of the block, and a head gasket is something like .067". The diameter of the piston does give me concern though, it's possible it has enough wear, but half the clearance of stock would worry me. I'd try fitting the piston in the block, with a very light lube like WD40, and sticking a feeler gauge in there. .0015 is the smallest common one, it's awfully thin and bendable, so if you can get it in you will be fine, spec is .0018, but I wouldn't worry about 3, ten-thousandths of an inch. But measure all the pistons, not just one. Well in, top and bottom, not just at the edge. Pistons wear in a barrel shape, the ring land area is small to start with for thermal expansion, but below that and down the skirt it's tighter. I'm usually concerned with too much play, not too little, and the B35 is a slightly different beast, but I doubt that much different. And you want to measure east and west, or perpendicular to the piston pin, again, pistons aren't perfectly round.

Now, after all that if they seem too tight, machine shops have hones that are almost a cross between a bore and a hone, they could probably do the half a thou you may need. You will not be able to with a hone and a drill, you would be there for days before you made any difference. The machine shop tool is a whole different animal than a hone you or I buy. Or maybe take the pistons to a machine shop and have them measure them, they have better tools and can get closer to a ten-thousandth measurement rather than a thousandth like we can with more modestly priced tools.

[anthony]

Given cylinder #1 is east, to my right and #6 west, to my left. I am able to slide the 0.0015 feeler through. There is more room on the east west side of the piston than at the north and south. I was able to get .002 on the east west side but not north and south.

Now I did have to use WD40 equivalent, oil was to thick for the thin feeler.

Will check all pistons in the respective cylinders before proceeding, but this enough good news to feel optimistic.

Thank you Mike.