Engine rebore question and clarification

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Engine rebore question and clarification

Postby Jdbfish » Thu Dec 20, 2012 1:15 am

I have my engine at the machine shop now awaiting complete disassembly and measurement, along with a thorough vatte cleaning. Not sure if a rebore is needed or not. The engine was running fine with no smoke but one cylinder was only around 50 psi compression while the others were at 100 or so. I pulled the head and measured the cylinders at the top about an inch below the ring groove. All four measured 2.630 to 2.6315 and were round within .0015
Gss1007 pg28 states that a rebore is needed if taper exceeds .0105 and/or out of round exceeds .0025, which does not appear to be the case on initial inspection. If final dissasembly and inspection confirms this, can I just redone and replace the rings? If so, do I use standard size or 010 over and polish the ring gaps to get proper fit? Please give past experiences and results please. Thanks!,,
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Re: Engine rebore question and clarification

Postby Gary Dotson » Thu Dec 20, 2012 6:52 am

While your taper & out of round may not be excessive, your cylinders have 5 - 6.5 thousands wear, that's a lot of piston skirt clearance. If that were my block and was at the machine going into the vat, it would also be going on the boring machine. Of course, have your machinist confirm your results and make his recommendation.
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Re: Engine rebore question and clarification

Postby Boss Hog » Thu Dec 20, 2012 8:17 am

I think if you use .010 rings you will be too tight at the bottom of the cylinder, if you get the correct end gap at the top, I also think a rebore is needed
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Re: Engine rebore question and clarification

Postby JackF » Thu Dec 20, 2012 8:26 am

It's like Boss & Gary said that's a lot of piston skirt clearance. You’re probably didn’t hear the piston slap because of the steel pistons. Probably the low compression on the one cylinder is from a valve seat problem.

If you’re trying to get by cheap the machinist may be able to expand the piston skirts to compensate for the clearance.

If you reassemble with Aluminum pistons make sure you tell the machinist so he can add more clearance for the expansion. …… Most of all; have your machinist confirm your results and do his/her recommendation(s).
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Re: Engine rebore question and clarification

Postby John *.?-!.* cub owner » Thu Dec 20, 2012 10:07 am

If you decide to rebore, I suggest getting the parts from the machine shop, and letting them reinstall and gap rings, etc.
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Re: Engine rebore question and clarification

Postby mrdibs » Thu Dec 20, 2012 12:25 pm

Mill the head too.
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