It's always what you least expect

[Moto]

Ok I bought a 1972 240Z with a L28 engine. I assumed (yeah yeah I know) that the engine was just transplanted from a newer car, since thats what the guy told me. N42 castings on both the block and head. Anyway, I had really low compression on number 6 cylinder. To save myself the time and trouble, I buy a complete head with low milege, from a guy, buy all the gaskets, and new head bolts. I take off the old head, and low and behold some one rebuilt the engine and put flat top pistons in it, meaning that the exhaust valves where all having intimate realationships with the pistons.

My question is, for a quick fix until I have the money to rebuild the engine. Can I use a dremel to notch out the piston?

[SteveJ]

How much contact? Take pictures and show us. You might be able to get away with a thicker head gasket.

[Moto]

I don't know if this helps but all the pistons where the valve hit have the same depth.

Also the new headgasket I bought is 2mm I believe.

[Moto]

another thing I just noticed. At TDC the pistons extend about a half mm past the block.

[Moto]

.013 past the block

[Moto]

After further examination of the head. I think I need to change the pistons out for the correct dished ones. I would have to put a really thick head gasket to keep the exhaust valve from hitting the piston. The new gasket I have is 1.5mm thick and the valve would still hit. Does anyone make pistons that are notched for the exhaust valve. Or do I have to order the dished pistons? I like the idea of the higher compression is why I ask. 2ndly I need to find pistons that are .75 over stock.

Thanks Guys

[Moto]

I think there is a mistake on the pistons. It's stamped .75 on the piston. Should it not be .075. I think .075 is about 2mm over size. But if the stamp is correct at .75 it would be close to 20mm over stock. Is this possable?

[Arne]

Probably .75mm, or .030 over.

[easyguy71]

Have you tried adjusting the timing (mechanical) to see if it is simply out?

F54 block engines had the flat tops that rise above the level of the block as well. He may just be out a link on timing.

[ajmcforester]

I don't know if we want to blame the piston people run flattops in Zs all the time. The other thing is we need figure out why before we make a fix (even a quick one) otherwise you will throw money into this that won't fix the problem.

First off are we working with a factory cam? Someone might have put a bad combo together. Also did the timing match correctly for the cam?

[Diseazd]

I agree with Arnie....1mm is .040 inch overbore...... .75 would be .030 over. The idiot that built this engine missed a couple teeth on the timing chain. .013 out of the block shouldn't be a problem unless you've got a very high lift cam. Use your new head....... make sure your bright links are where they should be and you should have no problem. All IMO. Guy

[Jeff G 78]

With quality calipers, measure the thickness of the head. It should be 108mm thick from the valve cover surface to the bottom. It might have been machined too much, causing the valve-to-piston contact.

That happened to me 25 years ago before I was smart enough to double check the machine shop's work. They told me that they were going to take a 10 thousandths cut - the max allowable per their spec book. What they never did was to check the original thickness. It had already been cut at some point. It was my first rebuild and I didn't clay it or check their work. After a few minutes of running, my pistons looked just like yours. :stupid: