How much can you shave a 94-97 1.8 head before CRASH! - Miata Turbo Forum - Boost cars, acquire cats.

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Old 05-18-2011, 02:43 PM   #1
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Default How much can you shave a 94-97 1.8 head before CRASH!

I have a friend trying to increase compression and needs to know the limit to shaving head before issues like piston meeting valve and before tensioner no linger works. Like to see 10-10.5 : 1

He will be using adjustable cam gears to correct that.

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Old 05-18-2011, 02:50 PM   #2
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Or use 01+ pistons to get to 10:1
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Old 05-18-2011, 03:21 PM   #3
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i shaved mine .042........just saying
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Old 05-18-2011, 11:38 PM   #4
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There's a spec in most manuals, I believe measuring from the valve cover mating surface to the bottom of the head. Not sure what that is though.
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Old 05-19-2011, 12:10 AM   #5
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Make sure to get adjustable gears and check piston to valve clearance after. Shaving it too much may impact how much retard/advance you can run on the cams.
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Old 05-23-2011, 09:53 PM   #6
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I have done .050 and [email protected] compression pistons. It wasnt a watDa mod, but the engine felt more peppy, had a raspy sound all across the powerband. The cam gears is a plus like falcon states. Although not recommended by many, I also removed the top and lower layers of the 4-ply metal head gasket. I ran it like that for years without any issues.

Later ran the engine with untouched head, [email protected], 2-ply in the head gasket and 15 psi of boost, no problems, no leaks. I love those BP engines, they for sure are indestructible.

It is not if piston hitting valves, but these engines are designed that if the timing belt snaps it will not mess up the CH. Once you shave the head that margin of error is gone, non-interference engine.

I still got a cylinder head with .070, but will hit with [email protected] (lower compression is fine), but I needed to run 94 or more octane and a good tune. With .070 the center notch of the top of the piston touched the CH. It was to much trouble reverting to stock pistons and in a way defeted the purpose so its kept on the garage for some all motor project someday, or as spare.

So to answer your question, with .070, 2-ply head gasket, and [email protected] pistons only the center dome hit the cylinder head, but did NOT bend any valves. If the belt snaps, thats another story... will let you know when it happens.

Last edited by mazpr; 05-24-2011 at 09:27 AM. Reason: JDMPOWELOL
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