Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: MikeSB6 on May 02, 2018, 11:54:32 AM
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Hello, I'm currently checking the main bearing sizes for the K6 engine which I'm restoring. The main bearing crankshaft pin second from the alternator measures 35.985 in the mic. and the crankcase sizes are all B. With plastigage, the clearance on this bearing measures approx 2 x .045mm using a black bearing shell, i.e. the plastigage is only flattened to half the width of the .045 measurement i.e. about .09mm.
The mainshaft pin third from the alternator measures 35.980 in the mic. but .045 clearance with the plastigage. Given that this pin is smaller, why does the plastigage read half the clearance when the main bearing crankcase opening is the same size and the pin is smaller? Is this due to the B crankcase measurement having a tolerance? I've checked the mic. against its standard 25mm test piece and it seems o.k. Also, The Haynes manual says that the limit on the crankshaft is 35.94mm but bearings aren't available for this size. Are thicker bearings shells available? I've checked with black, which are the thickest but the clearance seems excessive. The crank looks in good condition with no bluing or pitting. I'm tightening the crankcase bolts to the torque spec.
Any help most appreciated,
Cheers, MikeJ
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why does the plastigage read half the clearance when the main bearing crankcase opening is the same size and the pin is smaller?
Because the total clearance is the same gap twice. If you put another piece of plastiguage dead opposite your first one, the clearance is the sum of the two.
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Hello Mr Davo,
thanks for the reply, but I'm comparing the clearance on the second bearing from the left (2x.045mm=.09mm) with that on the third bearing (1x.045mm), and I'm using the same parameters for both bearings, so the same problem applies i.e. one bearing has twice the clearance of the other, even though the crank pin on the one with greater clearance is larger,
Cheers, MikeJ
P.S. Has anyone tried reclaiming a CB750 crank using say spraying or welding and was it expensive?
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Check noone has painted a bearing carrying face, it happened to me, ruining the crank and main bearings. It would be cheaper to get another crank, as low mileage as you can find than reclaim one. It could be done, but getting a ground surface within Honda's tolerances would be the hard bit.
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O.K. Mr Davo,
thanks very much for the tip,
Cheers, MikeJ