Piston Protrusion??

rockjeep73

rockjeep73

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Jul 8, 2007
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Well I am in the process of assembling my 24 valve which I am rebuilding. The block had to go .020" over to clean up and I went with genuine Cummins pistons. I only have .010" piston protrusion and am wondering if this is fine or will I need to get a different "grade" piston. The service manual specs are anywhere from .016"-.036".

Also, .010" was taken off the deck of the block.

Any insight on this would be appreciated.
 
you should be fine. If the protrusion is less than it was when you tore it down, then the compression will be slightly lower, which is no big deal. Just more room for fuel and air. :D
 
Well I never checked it before I tore it apart, but I guess I could put one of the old pistons in and check it. Honestly I was trying to keep the compression up on this one as its a daily driver and I am only looking for about 600hp.
 
What about using a thicker HG to compensate for the .010 you decked off the block.
 
rockjeep73 said:
Well I am in the process of assembling my 24 valve which I am rebuilding. The block had to go .020" over to clean up and I went with genuine Cummins pistons. I only have .010" piston protrusion and am wondering if this is fine or will I need to get a different "grade" piston. The service manual specs are anywhere from .016"-.036".

Also, .010" was taken off the deck of the block.

Any insight on this would be appreciated.
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Run it. Cummins oversized pistons are not graded anyway. Just the std sizes. Your good.
 
So what your saying is that the oversize pistons only come in one protrusion and thats all you get?

If thats the case then I will continue on assembling it.

I figure that it will put my compression down to about 15.5:1 if I use a standard thickness gasket.
 
rockjeep73 said:
So what your saying is that the oversize pistons only come in one protrusion and thats all you get?

If thats the case then I will continue on assembling it.

I figure that it will put my compression down to about 15.5:1 if I use a standard thickness gasket.
Click to expand...

that decompression is quiet a bid for a DD. 15:1 is about as far as you'd want to go but, even there.... you'll have a time on cold starts. You may end up with compression knock too. I don't think you've taken enough away to decompress that much anyway. I have bored cylinders, decked block and head with Cummins oversized pistons with a 24v gasket and it's not decompressed that much.
 
If you're worried take another .010 off the block, that will give you at .020 pertrusion. Thanks Steve B.
 
triton said:
I have bored cylinders, decked block and head with Cummins oversized pistons with a 24v gasket and it's not decompressed that much.
Click to expand...

Same here except for MLS gasket, and flycut pistons... the flycuts being the difference. I was told only about 1/2 to 2/3's a point loss of compression from the flycutting. Done from calculation and not measurement.

I don't know the accuracy of my situation, but I am pretty confident he didn't go from 17+ down to 15 from what he describes.
 
a stock 24 valve engine has a compression ratio of 16.25:1, with a .020" thicker headgasket it will drop it too about 15.3:1, which is how my truck was set up for the past year or two. Cold starts are a little harder, but it doesnt really get cold here in California. I was hoping that the protrusion ended up about the same as stock and I was planning on using a standard thickness gasket to get the compression ratio back up to around stock. I feel that with what I plan on doing with the truck, (daily driver, tow rig, bracket racer, around 600hp) there is no reason for me to lower the compression ratio.

The 15.5:1 number is just an estimate. I figure that if the stock protrusion was around .025", which I am going to check, and I am at .010" now that will give me .015" more clearance if I use a standard gasket than what I previously had with the .020" gasket.
 
ok, I didn't realize the 24v's had a lower compression than the newer CR's. My 325hp 5.9L stock is 17.2:1
 
Ya, the high output 24 valves were 17.2:1, the standard outputs were like 16.25:1
 
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