cylinder bore /

[bobterisch]

Hi all,
I need help for an new automotive student please. Can someone please tell me why when boring a cylinder for new pistons it is >020", .030", ,040" or .060"; but never .050"?
Thank you,
bob

[maxwedge]

The most logical reason is to cut down the issue of so many different size pistons, if you need .050 just go .060 if the block can handle that over size. Oe mfg used to make .005 o/s pistons in the old days, on average with todays thin wall casting, about .010 is about it. Not every engine design has all those o/s pistons available, usually .030 or .060. Some engines can only go .040 or .020, this is more of a reason for the various sizes.

[curtis73]

I think its probably because whoever came up with the logical oversizes couldn't count past 4. Then someone later added the .060". That's my story and I'm sticking to it.

Actually, there just isn't a real market need for it. Many of the overbore sizes are in the increments they are because any less won't be adequate for cleaning up the bore. Most engines that need to be machined will require at least .020 or .030 to true up the bores. Then if it needs done again, it probably needs another .030 for a total of .060.

If you do the probable math, that means the first overbore needs to be 20, 30, or 40 over, and the second boring needs to be 20, 30, or 40 over. The chances that .050 come up are slim, so to keep it simple most of the market can be served with those sizes.

Its also pretty typical to only rebuild classic american V8s twice. Once they've reached their maximum bore (often times around .060") they aren't worth risking metal fatigue to invest money on sleeving it and doing it again. If you have an engine that is 60-over, chances are it has 300k miles on it unless someone was looking to get max cubes on their first try. Engine cores are pretty common, so most builders don't risk it and just move on to another block.

[534BC]

ASK AN OLD guy mechanic why. I think it has something to do with wear.

Many sizes are available including .05, some might be metric eqivalant nowadays. Some times you can get whichever size you spec in any increment you need provided a blank is available.

Lots of times you can go to a different engines piston and get an odd-ball oversize if needed for a particular application.

How about bearings?

.001 .002 .01 .02 .03 .04 .06 .1

King bearings used to have many more undersizes than the big manufacturers and they were cheaper too.

[bobterisch]

Thanks Guys.