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I got hold of a m30b35. After breaking it down, cylinder number 6 is borderline bad. I can feel the ridge, and it was been homed to about the max. All others are within specs. Is it a good idea or bad idea just to bore one cylinder? The cost of doing all six would blow the budget out of the water. The plan was/is to "refresh" the engine. Plan B keep looking for another block.

Your description sounds like you have mic'd the cylinder bores in all cylinders and found that #6 is worn more than the others. You really want all cylinders to be the same size, so either step up to a complete rebuild with oversize pistons or go shopping for a short block that's not worn.

Sorry for the bad news, but maybe Santa will be good to you. You have been a good boy, right?

How much wear is there? I think spec is a max of .004" wear which is quite a bit. I've even run more than that before without any problems at all. Spec is much tighter on taper, but since all the wear is at the top, it's always in taper. If it's .004" or less wear, I'd run with it and not worry. Be sure to measure your pistons, I've seen blocks with minimal wear and pistons that were down .020" Those I don't reuse.