# piston question.

Discussion in 'The Garage' started by colbystephens, Jun 15, 2005.

1. ### colbystephens1 ton statusGMOTM Winner

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so i have a 6.2l diesel motor in my '83 k5. i'm doing a complete overhaul and build on the truck, and when i pulled the heads yesterday, i found divits worn into the tops of the pistons - sort of looks like two overlapping circles. when i was asking a buddy of mine about it, he said that this was a good thing and that if i were to have that milled down flat, it would screw up my compression. while the wear on the piston makes sense, his reasoning seems kind of funny to me tho. anyway, i'm just learning and need some input. my guess is that i'm going to end up having to bore out the cylinders, so i'll have to replace the pistons anyway. i suppose this could end up being a moot point, but i'm still curious. thanks for the input!

Colby Stephens.

For the story of my build, check out www.web.pdx.edu/~colbys

2. ### Iron_WeaselRegistered Member

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Your friend is right. What you most likely have is a dished piston (and those overlapping circles shouldn't be from wear - rather they were machined into the top of the piston), which is fairly common. You can get "flat top" (or dome) pistons though.
And yes, if you go from one type of piston to another, then you will change your compression ratio and compression pressure.

Just as an example....say you do change to domed pistions from dished pistons. You will be increasing your compression ratio and compression pressure because you are removing volume from the combustion chamber.
In this scenario, we'll use some arbitrary numbers.
Let's say you have a compression ratio of 8.5:1 and a compression pressure of 180psi. Now by using dome pistons, you're increasing both the ratio and pressure (remember, you removed available volume from the chamber, so you're cramming more air/fuel into a smaller space). So with your new dome pistons, you're at a compression ratio of 10:1 and have a compression pressure of 210psi.

Also, boring out the cylinders and leaving the heads the same size, you would be increasing the compression ratio for the same reason as going from a dished to dome pistons. You now have a cylinder that's say (for example purposes) 5.250" in diameter that was originally 5.220" in diameter. The heads were designed with a (again, for example purposes) 72cc combustion chamber. By adding that extra .030" of space, you're trying to force more air/fuel into a smaller space than it was designed to. Granted, the increase is small, but still there.

Last edited: Jun 15, 2005

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