Inliners International Forums General Discussion Engines Max overbore for 250?

I've been reading this forum for a few years but this is my first post here. I need to know the maximum overbore for a chevy 250. I've read on other forums that .060 is the max but the Leo book says anything over .100 should be magnafluxed, implying that anything under .100 is okay. I know that the chevy 250 is a popular engine here so I'll value any advice that you are willing to share. Thanks.

Jeff

Jeff,
These 230/ 250 and 292 blocks are a thin casting. I will only bore them to .060 . For street use.
If doing oval track or Drag then a bigger overbore can be used with sonic testing and by adding filler to the block to add rigidity to the bores.

I bored a mid-'70s 250 block .125 once and ran a stroker assembly in it twice. First time without block fill, second time with block half fill and a rigid hone to true it up. I did install head bolts on spacers around each bore while honing as a poor imitation of torque plate. Unfortunately the block is gone now so I can't sonic check it. Got a couple extra bucks at the scrapyard for that one.

Saw some irregular wear in the bores after running in both cases. Whether it was inaccurate machining, deflection from head bolt tension, or running loads I can't say.

I interpreted it as head bolt tension deflecting the bore. But nothing broke and it ran great. Based on the irregular wear I say .125 was too much. Running stock 250 type rotating assembly and a proper torque plate hone would likely have improved my results.

One example. Your risk tolerance may be different than mine.

The reason for my question is that I have a remanufactured shortblock that was bored +.080". I'm going to build a street engine; no racing. The re-man company assured me it's good but I value the opinions of real world engine guys over a salesman.

The best tried and trued method of determining how much a block can be safely bored, is to have it sonic checked. As a rule, the Mexican blocks have always been considered a better, thicker casting, and more desireable for HP usage. But I have seen some of those that wouldn't be thick enough to safely bore to .060 over either. Most non-Mexican blocks will not be thick enough to bore .100, and would definately need to be filled if they are thick enough, as they are just barely thick enough at best.

http://cgi.ebay.com/DIGITAL-ULTRASONIC-T...=item45f3671185

This looks just like the sonic checker I have. Works great. I radiused the probe to use on cylinder walls. Purchased after scrapping the above mentioned block so I don't know what that block wall thickness was.

The blocks I've checked (230,250) have thick walls on the thrust side, .250 inch or so. Thinnest spots are between cylinders, typically around .120 inch. The smallest reading I've seen is one block has a very small spot that shows about .080 inch with .120 or better all around. That spot is about 2/3 the way down the bore and between cylinders. I think that particular block was already at .060 over.

CNC, I'm interested in your experience regarding what sonic readings are cause for concern relative to location in the cylinder and planned usage.

Stroker, as you mentioned, the major thrust side is of the most concern. I always tried to select a block that would be at least .125 or thicker in that area for race or HP use, after it was finish bored. The 250 blocks do seem to be a little thicker than the 292's for some reason. Also, as you mentioned, the area between the cylinders seem to be thinner as well, but I still would try to maintain at least a .125 thick as a margin for use. As long as I didn't find a spot that was bigger than a quarter that was thinner than .125 directly in line with the major thrust, I would consider it for use if it was perfect in all other areas as in magnaflux. Now thats not to say you couldn't/shouldn't use a block that is thinner than that for street use or mild recreation. I also would never use a block that had or required a sleeve for a race engine. And again, not to say that one needing a sleeve couldn't/shouldn't be used for street or mild performance use.