What options do I have with Pistons ?

[Jean]

Posting it here since the m10 section is dead....
I got a spare m10b18 bottom end at a machine shop for cleaning / inspection. I already have a rebuild head from the same engine waiting for the bottom end. I got the word from the shop that the crank and rods are fine, one of the 4 cylinder walls have a little bit of rust and they recommended to go with 20 over pistons as re-honing the block might not get rid of all of it.

This leads me to my question. What options do I have for pistons here ? I would like to use existing crank, rods and head. So something that would work here w/o having to rebuild the already finished head would be great. I wouldn't mind bumping the compression a little but would rather not go down in compression. If I'll have to use 91 octane , that's ok too.

I know MaxSil sells m10 pistons, but do they only sell them in OEM spec or can I order them like bearings with xx over ?

Thanks!

[PiercedE30]

Get the machine shop to take a look at the existing pistons and have them order them from MaxSil using the numbers they come up with. When you increase bore, you decrease compression unless you compensate with the piston height. Tell them what compression you want to run and they will give you the spec's needed for the pistons.

[Jean]

Thanks! How do I calculate what the compression be after changing the bore from the stock 89.5mm to 90mm for example ?

Trying to learn this :)

[devonjordan]

Compression, IIRC, is the total swept volume of the cylinder plus the combustion chamber volume, divided by the combustion chamber volume... I think? its been a while since I paid attention to this...

[PiercedE30]

Compression = [volume of one cylinder (sans hg/combustion chamber) + (volume of combustion chamber + HG volume)] / (combustion chamber volume + HG volume)

One cylinder volume in cc's = [(Bore/2)^2 x Pi][Stroke]
Head gasket volume in cc's = [(Measured bore of HG/2)^2 x Pi)][HG Thickness]

That's the simple way.