Piston ID help needed

[crm103]

So i'm starting to rebuild a m20b25 with 180,000 miles for boost. Planning a full rebuild, o-ringed block, MLS, ARP's, the works basically. Just want a nice solid motor.. So I start tearing it apart, and I notice that the number one piston is different. It has different marking than the rest and the valve reliefs are larger. So, my question is: Can someone ID the different piston for me, because i'm have a hell of a time finding info. Thanks in advance...

Piston top in number one cylinder.


Pistons 2-6 all look like this.

[jlevie]

I don't know what piston that is, but it doesn't look like an OE part. I think you need a new set of pistons. But before going there the cylinders need to be checked for wear with a bore gauge. If the engine had over 150k on it the odds are good that it needs to be bored and over size pistons used.

[reelizmpro]

If i were to guess...aftermarket high compression pistons. 9.7:1 was a common high compression ratio for the M20. Don't remember if 83.98mm is first or 2nd over or not. Look into that. Wait a second...is that a "KS" logo below the "Made" ?? Looks like it but hard to tell from the pic. Could be Kolben Schmidt pistons. Only two companies come to mind out of germany...Mahle and KS.

[crm103]

If i were to guess...aftermarket high compression pistons. 9.7:1 was a common high compression ratio for the M20. Don't remember if 83.98mm is first or 2nd over or not. Look into that. Wait a second...is that a "KS" logo below the "Made" ?? Looks like it but hard to tell from the pic. Could be Kolben Schmidt pistons. Only two companies come to mind out of germany...Mahle and KS.

I believe 83.98 is the factory piston diameter. I may be wrong. And it is not KS. It says 530 with 3 vertical lines then 1000 under that. No idea what that means.

So from what I am gathering after tearing into the motor a bit more that maybe at some point the motor dropped a valve. There are some knicks in the combustion chamber for the number 1 cyl. ( the same Cyl. as the 9.7 piston) which would explain the mismatched piston. I guess for a $150 motor, there are gonna be some things to work out.

Does the other piston I had a picture of look to be the factory piston? How would i be able to tell? Maybe the best thing for me to do would be to buy a matching set of pistons? I'll measure the block this week to see if it has been bored before.

[e30leigh]

its two different pistons. the valve notches look bigger on the one with the markings.

if your just doing a hg then id leave it alone if it was running well. if you are doing a rebuild then get a good set of matching pistons. or take them to an engine building. have them measured, and checked. there might be nothing wrong with them.