I have a iron 6.0L block that was advertised as being a 4.030 bore. One would think I would have to order pistons 0.30 over After measuring cylinders (which are finished honed) I get a bore of 4.0261 to 4.0274. Is this normal or are they way off on their machining? On one hand…. the pistons I want to use GMPP 89017479 - LS-Series Piston are 0.020 oversized and if within tolerance will give me a pretty good piston to wall clearance. On the other hand… if the bore was 4.030, wouldn’t 0.030 oversized pistons not work.
I know most people finish hone to the pistons, but what do these manufactures that finish hone their blocks expect the customer to do? Any encounters welcome.

 

I always get the pistons and then have the bore matched. I would take that block to the machine shop and let them measure it and tell you which oversize to get.

 

confident on my measurements and piston selection…would use the pistons mentioned instead of taking a block to a shop…kinda makes more cents$.

 

Pistons sizes are for the bore. For example, 4.030" pistons will actually measure approximately 4.026" so the clearance is built in already.

I would get the cylinder rehoned. Over .001" tolerance (4.0261" - 4.0274") is pretty sloppy.

 

thanks KCS...thats exactly the feedback i was looking for. so .020 pistons from GM are going to be around 4.016?