924Board.org

[Jack_Kirwan]

Hey Guys, can anyone shed some advise on the valve relief cuts on top of the oem pistons. I have the oversized valves that brought of Dan. I put the bottom end together then saw the original sized valve reliefs cut in the top of the pistons. I guess this is done to make it non interference, but do I need to machine the top of the pistons to the new valve size or do I just put the head back on and make sure the clearance is right the valves are at full height.

Do the valves sit flush with the top of the head or do they enter into the bores at all?

I will mill a little of the top of the block to bring up compression ratio.

cheers for any advice.

[!tom]

You'll end up with an interference engine.

If you machine larger reliefs into your pistons, that'll drop your compression ratio, which is probably not what you're after.

Just live with the fact that you've turned it into an interference engine, and you'll be fine.

The valves will pass the block deck when they open.
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78 924 NA
5-lug

[Jack_Kirwan]

Cheers, had that assumption but just wanted to make sure.

Any chance you know how much the valves go below the deck on the stock head?

[!tom]

Given there is no combustion chamber, it's the lift minus the head gasket thickness when compressed.

I don't know the numbers off-hand, but they're easy enough to find.
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78 924 NA
5-lug

[daniel]

They are supposed to be sunk 20thou, not quite flush

either way, dont worry. Put your motor together and just change your Timing belt regularly, including the tensioners
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet