SOHC.co.uk Forums > CB750
Help! What's caused this?
Bristolbadger:
--- Quote from: haynes66 on March 25, 2022, 04:36:29 PM ---i was told by an old honda guy to turn the engine over with the rocker cover caps off to see if oil arrives at the head. i assume that's a good idea?
--- End quote ---
F#@#####CK!! I can't think why I didn't do this with the engine in the bike not just on the bench, I assembly lubed everything as i went and was fixed on the pressure being good and having seen the oil getting up there on the bench I assumed all was well..... so moved on to the next thing. TW@T!@!!
Here's that vid:
https://youtu.be/TnXZu-yCPGA
Like you all say rockers 3&4 got starved of oil and just running dry and melting the cam and holders. I don't think, & hope - until I can investigate further - that the head itself is damaged, pray be.
Thanks muchly for the call and support Roo. These are sour times man
Bristolbadger:
--- Quote from: Nurse Julie on March 25, 2022, 05:22:32 PM ---I suppose in theory, even though oil is observed getting up to the cam on a newly rebuilt engine at initial start up, it could take a few miles of riding to dislodge any debris that is in the bottom end, such as the dreaded silicone sealer, blobs of glass bead etc, etc.
--- End quote ---
Julie I reckon you're spot on and this is what happened. I'd been pretty surgically clean and really tried to make sure all oil routes were clear but I guess something somewhere blocked a jet and gave me the two fingers!!
So painful to fall on the last hurdle like this after a couple of years but I guess I'd be looking at ANOTHER tear down of the head etc anyway even if I'd discovered the oil blockage before meltdown. Just this is now even more of a costly one for parts and it now not going anywhere (will cancel my insurance on it now as it's just started) and almost beyond me putting anymore into it, moan moan f@#k s@*t
Oddjob:
Any blasting media used on the engine cases etc?
I ALWAYS spin the motor over with the camshaft out and holding the camchain up on a screwdriver, plugs out of course. You can then compare oil feeds to see if one looks weaker than the others.
K2-K6:
"Like you all say rockers 3&4 got starved of oil and just running dry and melting the cam and holders. I don't think, & hope - until I can investigate further - that the head itself is damaged, pray be."
Not quite as bad as it seems.
The rockers do get hot on the cam from no oil (you can see colour gradient in the rocker web indicating the range, through blue to staw colours) but it's not that that takes the cam bearings.
The bearings without oil quickly pick up through their own friction causing the aluminium to stick to the cam, as this continues it's more like a snowball and wrapping more aluminium onto it.
The "good" bit is that the above is usually totally contained within the cam, carriers and rockers, which obviously go on replacement list. The head usually escapes anything serious if anything at all.
For historic (other's looking in here later) this is where Graphogen assembly paste outperforms virtually anything else, in that if you do have a feed failure during initial assessment, it just lasts longer and is remarkably good at preventing metal pickup from one component to the other, even with zero oil.
Oddjob:
Honda are now recommending using Molybdenum Oil on the cams and shimbuckets etc when assembling. Evidently you can make it by mixing MolyB grease and engine oil 50/50. I've tried it and it seems to work very well.
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