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A15 Compression calculation with GX head
Just can't stay away
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2015/11/10 17:48
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I'll be boring out the A15 after checking and re-checking ring gaps yesterday. When measuring at the top of the cylinder bores, the gaps were well beyond the tolerances. Honing won't work. If I go one size up to 77mm bore size it comes out to 9.12:1. Does this look right?

Felpro HS21199 PT-1. Thickness measurement is on a new gasket so not compressed.
GX Head Combustion chamber 28cc
A15 dished piston -13
Block was decked -.127

I wanted to explore other piston types but they do not show piston dish cc's. Is there a table on this forum that shows this information? Want to be sure I don't get above 10:1 if I go with a lesser piston dish. This will be for street use.

Attach file:



jpg  compression.jpg (168.16 KB)
20677_5ad0ec1b8d24a.jpg 1910X1222 px

Posted on: 4/13 18:48
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Re: A15 Compression calculation with GX head
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Quote:
When measuring at the top of the cylinder bores, the gaps were well beyond the tolerances
Do not measure at top of bore. Measure with the ring -- according to Nissan -- at the bottom of the traveled part of the cylinder. Your machinist should have done this measuring as part of the honing process


Also see http://www.aa1car.com/library/ring_end_gap.htm
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Posted on: 4/13 21:37
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Re: A15 Compression calculation with GX head
Just can't stay away
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Yikes! DD.... was told to measure at top by machinist. You have given me some hope and will check again tonight. Thanks for the tip.

Posted on: 4/13 21:41
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Re: A15 Compression calculation with GX head
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So it's not exactly at the bottom of the cylinder, it is at the bottom of the traveled portion which for A15 is 82mm down from the initial ring position. When you put the ring in the bore, turn a piston upside down and push the ring down with the piston -- this will "square" the ring in the bore. Do this with lightly oiled cylinder so that the ring & piston can slide

Posted on: 4/13 22:26
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Re: A15 Compression calculation with GX head
No life (a.k.a. DattoMaster)
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That looks close but the the 79mm head gaskets have a crush thickness of .8 the felpro gasket should be no more than 77mm with a crush thickness of 1.2mm

You should also be able to find pistons with 10cc dish.

If you are on the stock bore I'd just go up to 76.5mm

As for the rings before you bore the block find someone who can check the actual bore size. Don't automatically assume that because the rings don't meet spec that it's the bore. The Hastings rings for my A12s 73.5mm bore were way to tight and need filling to be within spec.

Posted on: 4/14 6:28
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