Old M30 issue

Falkenberg · Post by **Falkenberg** » Tue Aug 28, 2012 1:39 pm

Any thoughts on what could cause this?

Mike W. · Post by **Mike W.** » Tue Aug 28, 2012 8:55 pm

Only guessing without checking everything out in person, but it could be as simple as too thin a head. Looking at it I'm guessing 2.5 or 2.8, probably 2.5 given the contact and the fact that it has a higher crown to give it the same compression. Stock thickness is 5.079" or 129MM. I've cut heads somewhere in the .025/.030" range, but someone may have gone even further to try and get some performance.

It probably wore in, but that must have sounded terrible at first start up.

Falkenberg · Post by **Falkenberg** » Wed Aug 29, 2012 2:58 am

I am suspecting the head as well. It's a 2.8, zenith carburetor version, they had those high crowns.
I was not there at first start up last year, but the mechanic did not report anything out-of-the-oridinary.

Engine did have a bad knocking sound around 3.000 rpm, which is the reason that that I pulled it out in the first place (besides oil consumption).

I will measure the head. I think I can compensate the loss with a thicker gasket?
In never figured that the tolerances are that small. When I saw the damage, my first thought was that maybe somebody put in the pistons the wrong way.

Mike W. · Post by **Mike W.** » Thu Aug 30, 2012 9:30 pm

If you feel the head is good, even if it's a little thin, I'd just "clearance" it and maybe the pistons a little bit. Some minor work with a die grinder on the head and you'll have plenty of clearance. You know where to use it. Increasing the combustion volume by a few MM2 won't affect compression. Pre assemble with thinner washers/shims than the head gasket and test for clearance. A thicker gasket will only be thicker by .3MM, which isn't much.