I'm too Bored?????

[Von Writter]

Hi All I'm wondering if my 250 short block has been over-bored too much? It is at .060 over and I'm worried that may be too much for 9.5/1 compression. It won't be seeing any high RPMs (4000 maximum) but have the
Cylinder walls been compromised? The re-builder thinks they will be fine but I'm not sure so I thought I'd ask.

 

[LaGrasta]

Although you're at max, you'll be fine.

 

[rbohm]

8) i agree, the maximum safe bore is .060, but if you have concerns, have the block sonic checked and see what the wall thickness truly is. i dont know how much the cost of this procedure is, but racers use it all the time to check blocks for core shift, especially if they are going to go for the maximum bore possible in a block.

 

[Von Writter]

OK Thanks guys, I was getting worried and confused as the performance hand book was saying it was OK to boar to all standard sizes (I believe .060 is a standard size) and it also said in the back of the book that .040 was the Max for these engines. It may have meant a full on race engine but didn't say so.

 

[CZLN6]

Howdy Back Troy:

Yes, .060" is a standard overbore size. That is because you can fairly easily find over size pistons for 200/250s in .010", .020", .030", .040" and .060". Our recommend is to overbore no more than is necessary to achieve true bores. The reason is that the more you bore away the less material there is to maintain structural rigidity. The thinner the cylinder walls get the more susceptible they are to distortion and flex. The first distortion occurs when the head is bolted to the block. The next occurs as cylinder pressures rise and fall during the compression/combustion cycle. The higher the pressure the greater the likelyhood of cylinder wall flex.

As cylinder wall distort and flex, piston ring seal to cylinder wall is lost. In worst cast scenerios, cylinder distortion and piston heat expansion can lead to contact of the two. Not good.

So the advice is to overbore no more then is necessary to achieve true cylinder walls. THe small amount of extra torque and power from a few more cubic inches of displacement is not enough to risk cylinder wall distortion. The higher the performance levels- HP, torque, and RPM the greater the risk. Also high loads due to lugging with the high cylinder pressures of a stock cam can lead to problems.

SO Troy, Yes, .060" is considered a standard overbore size and, yes, you can safely use them is a rebuild, but, our reccommend is to try for true bores at .030" and .040" first. I'd use .060" only as a last resort and then only in a street type engine build. I'd also add an after market, mild cam, to help to reduce cylinder pressures. I'd also probably spring for the few extra bucks to have my machinist do some sonic testing on the cylinder walls, just to be safe.

That's my two cents, for what it's worth.

Let us know your progress.

Adios, David

 

[Von Writter]

Thanks for the clarification David! I really appreciate it. The Author himself!! ...what a forum I do have a RV (264) cam in it as you suggested. Any idea what the minimum thickness would be on a 250 if I have it sonic tested?