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main bearing bore size too large

General Engine Building Discussion

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I just received my block back from the machine shop. One of the things I had done was a line hone of the main bearing bores. I have been measuring the block to double check the critical dimension fall within spec. Unfortunately, when I checked the main bearing bore diameters with my outside micrometer and dial bore gauge, they come out a maximum of 2.1 thou larger than the maximum manufacturer specified size. I measured each bore at 12 o'clock, 2 o'clock and 10 o'clock on both the front and rear of each bore. Now I am quite worried that the bearings wont work. Is there a way to correct the diameter of the bores, or am I screwed?

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I guess before I panic too much, I am going to remeasure the bores one more time for sanity sake.

If the bores are too large, would it be possible for the machine shop to shave some material off the main cap mating surfaces to effectively undersize the bores and then line bore again to the proper size?

Hi Steve, if the tunnel is too large then it may be possible to recover it by taking material off the caps and honing again. If the tunnel is already oversize at the parting face then you'll end up with some ovality in the tunnel but the more critical measurement is at 90 degrees to the parting face. Obviously in the perfect world we want to be on size and perfectly round so it depends how much compromise you're prepared to put up with.

Thank you Andre. I decided to step back and try measuring again. I took the main caps off, cleaned up the stud holes again, torqued them back down to spec and then remeasured zeroing my dial bore gauge to the top size listed in the specs. This time they measured within spec for all except one is a little large near the parting line. All of the bores fall within spec perpendicular to the parting lines, so I am feeling much better about where I’m at now. I’m not sure how I measured wrong the first time, but I’m glad I’m getting better results this time. I’ll measure one more time just to make sure.

Good to hear you're on spec!