Valve to piston clearance

[DanThe]

How much or little should I be aiming for with my build?

Ive got an M50 2.5 iron block that im rebuilding with the M52 2.8 crank, rods and pistons.

With the 2.5 crank gear in I noticed the pistons protruded above the sealing face of the block, with the 2.8 crank gear in (which ive just built up) the pistons dont reach the block face, they sit about 3 or 4 mm below.

I am planning on having the block decked to raise the comp a bit, but how much do I have taken off to keep it within limits so to speak. The comp will already be raised slightley due to the difference in combustion chambers.

[m-dtech]

hehe, leave it as it is and fit a massive turbo

[Andy335Touring]

from what i've read you need to aim for about 1mm squish height and piston/valve clearance

[Andy335Touring]

Also consider the size/diameter of the valves/valve cut outs if you are pick'n'mixing componants

[DanThe]

Thanks Andy

The valves are the same diameter, maybe slightly taller than the 2.8 but not massively different, pistons have valve pockets also.

So 1 mm is the target, that aint a lot at all

Just out of interest what do the 2.7 M20 builders go with?

[Andy335Touring]

So 1 mm is the target, that aint a lot at all

Yeah, it's a tight clearance, i ended up with about 1.2mm squish clearance with a thick H/G, but i might have it apart and put a normal H/G in which will give me a 0.9mm clearance

I had to reshape part of the squish area in my heads combustion chambers when i used an Alpina bottom end because the pistons protruded more than normal ones.

Standard M30



Alpina M30



New squish areas

[DanThe]

That looks a big combustion chamber Andy, have you measured the volume?

Maybe its the size of the valves making it look bigger

I measured mine at 26 ml

[Andy335Touring]

Yeah it is a big chamber, i CC'd the chambers to make sure they were still equal after the grinding, i think they were some where around 60-65cc ?

[DanThe]

I dont think I want to be squishing too much if those are the differences

Any idea on your CR?

Standard M52 is 10.3 or thereabouts, seems it would be easy to get it up to 11

[Andy335Touring]

Standard E34/2 M30 C/R is 9:1, now it's about 10.6:1

I dont think I want to be squishing too much if those are the differences

each chamber was with in 1cc(or 1 ML) of each other, i said 60-65cc because i couldn't remember the exact number.

[DanThe]

Youve done well there then,

I meant differences between the M50 and M30 combustion chambers