Safe over-bore size?

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mok4200

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Feb 16, 2015
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Ram Year
1998
Engine
5.9
Rebuilding my 5.2, summit offers a 3.940in piston bore size. Is a +.030 OK to overbore? The spacing between bores looks adequate but i was hoping to get input from someone who knows more than i do on this subject.
 

Ricks Ram

Senior Member
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Ram Year
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Hemi 5.7
The spacing between the cylinders doesn't make any difference in how much you can bore a cylinder. The limiting factor is cylinder wall thickness. Some engine blocks have thicker cylinder walls than others due to the way they are casted so they can be bored to a larger size with less chance of having an issue. Even if a certain type of engine is known to have thicker cylinder walls it's a good rule of thumb to have the cylinder walls checked for thickness before you bore it out. Any good machine shop can do this. The best advice when it comes to boring an engine is don't do it unless you have to or you are using it for a special application and need every ounce of displacement you can get. If you bore the engine .30 over you are not going to feel any appreciable difference in power. You will gain approximately 5 cubic inches of displacement. Oversize Pistons are basically made for engines that have cylinders that are worn past the recommended clearance between the piston and the cylinder. The block is bored and the next size piston is installed to give the correct clearances. In most cases it's not a good idea to bore an engine block unless it needs it. The thicker the cylinder walls the more Integrity the engine has plus it has a tendency to run cooler. If you are rebuilding your engine I recommend having the Machine Shop measure your cylinder walls and not bored unless necessary. If it's necessary only bore it to the size needed to get the correct cleclearance
 
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