Topic: CB750 K Camshaft Failure Analysis

[Hinomaru]

Anyone want to surmise why this CB750 cam failed?

I don’t have much information other than:

1970 KO engine: CB750E-1038498 with 25,000 miles and the cam is marked R8

As you can see, just about every lobe and journal surfaces has damage and/or scoring and I suspect it’s probably bent too.

(click on pics for a larger view) 

[speed demon]

Lubrication or lack of

[Curtis750]

Lack of oil.

No doubt about it

[wrenchmuch]

The oil jets in the head are blocked  , oil clearance at the crank is too big , worn oil pump , oil contaminated with fuel . My cam look like that on 1 end from a blocked oil jet in the head . Lots of silicone had been used on the barrels when bored for a 900 cc Henry Abe kit . A little chunk of it came out when I back flushed the jet ( non removable k1 type ) . If its early sandcast the jet will be just a drilled hole in the head instead of the metal disk with a raised center that was used until the removable jet came along . How did it run  ? The lobes look awful . Most of the damage on mine was at the bearing journals . I think it happened suddenly . Yours looks like more of a long term thing . Curious to see how this goes .
Mike

[GammaFlat]

My bet is that someone used silicon sealant on the engine.  The oil jets in the head get plugged by the little bits of silicon and you get what's in the pictures. 

[754]

 If that is a factory KO cam, they do indeed have  VISIBLY fat lobes.
 check the end for other markings.. might be regrind ?

[Hinomaru]

754, you may be on to something here. After re-inspecting the cam, it’s most likely a regrind. The difference in the lobe profiles looks a bit suspicious when visibly compared to my known ‘69 sandcast and diecast examples. Also, the black finish appears to be non-standard as compared to stock Honda. Since there are no other markings found other than the factory R8 stamped next to the cam sprocket, it will remain of questionable origin. If that cam could only talk ...

[Hinomaru]

That combination of lobe & journal damage throughout the cam was a new one on me. The true cause will forever remain a mystery, however the evidence suggests the failure is primarily due to a (long term) lack of proper oil pressure and sufficient oil volume. This seems to be the most logical explanation. I can only imagine what the associated valve train parts must have looked liked. Sure would have liked the opportunity to personally tear that engine down. Since there is not any more history available and engine parts to inspect, who really knows?

Thanks to all for the replies! 

[KRONUS0100]

picture 6.what is up with those sharp defined ridges on edgges of lobes?   that is almost like the rocker wasnt lining up with the lobe correctly.  but it certainly looks like top end oil starvation.

[MCRider]

The short answer is lack of oil. Then you have to dig deeper.

The R numbers are mold numbers. In my expereince they didn't get to mold numbers as high as R8 until K2 or so. K0 were R-1 to R-3.

So without further evidence i would speculate someone has been in the engine and replaced the cam. Possibly with an R8 that was welded and reground, which was done back in the day.

And as others say, the oil jets were at least partially plugged, usually with sillycone. The most common cam failure is a complete plug of one side or the other causing the cam to seize in the cam holder and break in 2. The engine continues to run on 2 cylinders. THough since they are not the 1-4 or 2-3 pair, it is not an ignition problem. Then the owners take the valve caps off and see one side is motionless, causing a sinking feeling.

Kronuso: The appearance of the sharp ridge is common. The pad of the rockerarm doesn't cover the whole lobe. And where it doesn't, the lobe doesn't wear and you get that ridge. Doesn't mean there was any mis-alignmnet though.

[Hinomaru]

picture 6.what is up with those sharp defined ridges on edgges of lobes?   that is almost like the rocker wasnt lining up with the lobe correctly.  but it certainly looks like top end oil starvation.

Along with MC Rider .... That lobe (yellow circle) is actually deeply worn and the difference is approximately .042” from low to high. That ridge is part of the lobe that didn’t have contact with the rocker arm pad. Several other lobes have the same wear characteristic, but not as bad. 

[754]

I took 2 lobes further down than that, think it was sticking guides..

[crazypj]

At least two lobes show tight valves (should have clearance on base circle - wear/polishing shows zero clearance)
Most likely valve is sunk into head through lack of maintainance or dirt ingestion? (valve face wears and takes up clearance)
 I would say lack of maintainance as it looks like burned on oil (the 'black death' Castrol advertise about)
It was well known in Europe about 30 yrs ago (the Germans found out first as they had higher speed limits  )