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Discussion Starter #1
I have a 4bt that Im rebuilding. The head gasket blew and the must have been driven for quite a while because it has a divot between the cylinders. My question is how much can be taken off the block and still use the stock head gasket?

Thanks
 

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One thing you need to check is whether your block has been decked previously. If Cummins decked it, they stamp an X or XX on the top of a pad on the exhaust side. X means it's been cut .25mm (.010") and XX is 50mm (.020"). What you have to keep an eye on is piston protrusion. Max limit for a stock gasket is in the .023-.028" range. Anything higher would require pistons to be cut or you can get a thicker gasket from Cummins. You can measure that as it currently stands to see where you're at. Any significant cut to the deck will require a thicker gasket or cut pistons or both. Also remember that this has an effect on compression ratio.
 

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Thanks for the reply. The machine shop ended up taking .004 off. I didn't see any markings before he decked it. But should probably check anyways. Just out of curiosity, is there a formula for calculating how much the compression ratio increases per thousandth? Thanks
 

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You must measure piston protrusion. That is the only way. You assembly all the pistons and rods in the block and measure each one, in 2 places at TDC.

Don't mistake the "we removed .004"" as meaning that the deck is only .004" lower than it should be. You have to measure.
 
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