Author Topic: CB750 tear down.  (Read 1692 times)

Online K2-K6

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Re: CB750 tear down.
« Reply #15 on: February 09, 2021, 11:02:22 AM »
Yes the restrictor would potentially change which parts are showing wear.

It's there to hold the crank and big ends at full pressure,  while not letting the camshaft bleed away to much of the oil supply.

In practice,  and if the camshaft main bearings are good (they'd be recieving more oil than planned) their sealing isn't bad,  and so this may have helped the crankshaft pressure stay up and so avoid significant wear. If it's got away with a little more wear to shell beatings, then that's not a bad outcome.

Obviously,  putting back to new shells and correct restriction control should cover it off.

Online Bryanj

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Re: CB750 tear down.
« Reply #16 on: February 09, 2021, 11:03:10 AM »
Restrictor missing, the way i understand it, lowers the pressure at the crank causing shell/journal wear and to my mind would reduce flow on the side that still had the restrictor fitted as the oil will take the path of least resistance i.e. the "open" hole. Happy to be proved wrong though

Offline Trigger

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Re: CB750 tear down.
« Reply #17 on: February 09, 2021, 12:07:10 PM »
I have no evidence to suggest this engine had been dismantled by previous owner other than the fact that there was one oil restrictor missing! There is minimal wear other than the camshaft lobes. Crankshaft is good main bearings big ends and small ends good, pistons and rings good. Scars on one or two of the mains so will replace them as well as primary and camshaft chains, tensioner etc. Maybe wounds from when the drive chain decided to break into the crankcase. The pistons are standard size Honda stamped, but why would somebody decide they were in the wrong way round? Bores very good no scores and no lip. So the stupid question is did the restrictor missing impact on the delivery of oil to the camshaft or did it deprive delivery elsewhere with the reduction in pressure?

That engine has defiantly been apart before as, the PO would never of got all the broken bits out from the case damage  ;)
With only one jet in, the engine would of destroyed the crank in no time. I have known owners to drill out the oil jets out to a bigger size thinking, that they would get better oil delivery to the cam. Yes, you do get loads more oil to the cam but, blow the crank out  :o

Offline Johnny4428

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Re: CB750 tear down.
« Reply #18 on: February 09, 2021, 01:23:53 PM »
Thanks for comments, I only doubted the complete stripping of engine because of the nature of the repair! Over patched rather than repaired properly. All makes sense Bryan regarding oil restricted on one side that’s where there was slightly more wear when measurements were taken. Especially at the end of camshaft (No 4). Think I am reasonably lucky to come away with a good crank by the sound of it, and I am so glad I didn’t put this on the road this year without inspecting internals. I get Triggers logic in stripping even if it has been restored. No hiding place for “skeletons”.
1952 Cymoto on Triumph bicycle.
1961 Matchless G3
1974 Honda CB550K1. Running resto,
1978 Honda CB550K3.
1999 ST1100 Pan European 50th Anniversary.
1975,1980,1984,1986 Honda C90’s
1973 Honda CB750K3

 

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