E30 Zone

maybe you guys can be of some help

The engine that i started out with was from an early 325i '87 (i think). however when the block was with the machine shop getting decked one of the pistons got mashed up on the machine. dont ask how lol

i got another couple of pistons from a boy up the country same make KS but from a later 325i (post facelift i think)
thought they were the same until i was cleaning them tonight and i've noticed a difference in the valve cut outs my original pistons have a noticably deeper cut out for the valves



dimensionally they seem fairly similiar and weight ways they would probably all balance up

would there be any issue with me using one or two different pistons is their a difference in compression ratio? what effect would this have?

any thoughts would be greatly appreciated

Willy

there are two M20B25, a low and high compression.

i think one is 9 something and the other is 8 something....

Not to be mixed.

Do not mix. You ideally want a set of the ones on the left. The higher comp ones. And in re to cleaning, it's the sides and oil grooves etc that ought to be mint. Avoid cleaning the tops. The carbon build up helps oomph, as it raises compression some more.

Even things like Pistons are in the Wiki, if you look at the Engines:

http://www.e30zone.net/e30zonewiki/inde ... 20#Pistons

thanks guys

Three different (stock) M20 b25 compression ratio's exist, all with different pistons, although I understand there are more then three variations of piston.

engine code 256 E1 - 9.75:1 (prefacelift motronic 1.1)
engine code 256 E2* - 9.4:1 (early facelift motronic 1.3)
engine code 256 K1** - 8.8:1 (facelift motronic 1.3)

*Produced for around six months on the first of the facelifts.
** K for katalyser (catalytic converter), I believe the compression was dropped to avoid pinking on unleaded fuel.

It's well accpeted the higher comp engines feel better and tend to lose less horses over time, officially they all made the same power.