rebuilding an '15 RC390 motor


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I think they are a must to properly set the height of the bolts, I personally would take this to a machine shop to do as I dont have the tools necessary.


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I still think that the VAST majority of blown head gaskets are caused not by the head, the cylinder, or warpage... tho I've certainly seen a few warped heads and cylinders, I think a LOT of them COME that way from the factory. No, I think the OEM head bolts are stretching, and it don't take much for the gasket to then let loose. Can the gaskets still fail without the bolts stretching, sure... and I think a fire ring is good insurance as well... but going with GOOD head bolts (like the Spears Racing ones) is FAR more likely to help with preventing a HG failure. $0.02.

Drilling out the first 10mm of threads on the case shouldn't be a big deal. It's just a clearance fit, and the drill should be self guided by the existing bore of the threads so even if you started off axis with a hand drill, the axis should straighten you out as you drill. Setting a stop with some tape for proper depth could work. Even better a stop gauge fixed to the drill to keep you from going more than 10mm deep would be ideal. You are just removing that first 10mm of threads from the case so the wider bolts can fit down inside the case. If there was a mm (or 2) extra clearance, it shouldn't be a big deal. Make sure you deburr the hole after drilling so the cylinder goes down flush against the gasket. Doing this in a garage with a hand drill and some tape is totally possible. Using a Bridgeport mill after spending an hour blocking everything off and indicating the face of the top of the case as would be done in a machine shop isn't something I would think would be necessary just for a clearance fit for the new bolts. Make SURE that the case is taped off well, and blow out all the chips with compressed air before removing tape, and keep building that motor. :)
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