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Old 01-04-2010, 11:46 AM   #1
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Default milling VVT piston domes to reduce compression

Like this poster from another forum says:

Chevelle Tech - View Single Post - 10.25:1 pistons for 71' 402

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Another option is if your TRW`s are in good shape, have a machine shop mill the dome down to give you the copmression ratio you want. Most hi-po machine shops should be able to do this and would be able to figure out how much to take off.
Anyone thought about this? Milling the rectangular pad or "dome" by 0.8 mm will reduce c/r from 10:1 to 9.7, and by 1.8 mm will reduce it from 10:1 to 9.3. I wonder if the 10:1 pistons have more meat here than the 9.4 ('99) pistons.

Y8s, can you measure the thickness of the crown of the 99 pistons you have?
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Old 01-04-2010, 12:43 PM   #2
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I measured the material thickness in the center of the crown of the 01 pistons .... ~ 4mm, yowza!
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Old 01-04-2010, 02:13 PM   #3
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How thick did you expect it to be? Thats the better question.
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Old 01-11-2010, 11:32 PM   #4
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Hoping against hope that Mazda raised the c/r by thickening the crown.
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Old 01-11-2010, 11:59 PM   #5
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I was able to do this with good success on a vw 2.0 16v motor, but there may have been more meat to the piston tops. Droppedt from 10.8:1 to ~9.3:1 cr

Definately a cost effective option if you already have the pistons and there is enough meat in the piston to do this.
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Old 01-12-2010, 12:43 AM   #6
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Quote:
Originally Posted by vw_nut View Post
I was able to do this with good success on a vw 2.0 16v motor, but there may have been more meat to the piston tops. Droppedt from 10.8:1 to ~9.3:1 cr

Definately a cost effective option if you already have the pistons and there is enough meat in the piston to do this.
I agree. If you already have the pistons. It might be cost effective if there is enough meat. I'd check with a machine shop first to see what they charge. Machine shops are generally not that cheap. It might end up being worth it to save for some forged pistons.

Has anyone checked to see if you can by a head shim for the mazda head. I know for my mopar 440 you can buy varying thicknesses of head gasket for this purpose.
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Old 01-12-2010, 12:45 AM   #7
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Originally Posted by djp0623 View Post
Has anyone checked to see if you can by a head shim for the mazda head. I know for my mopar 440 you can buy varying thicknesses of head gasket for this purpose.
you can, but thats the wrong way to lower compression, it can have effects on other things besides compression
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