Topic: Broken cam shaft sprocket holders.

[xhevi]

I finished rebuilding this 77 CB750K about 2 months ago.

Replaced camshaft and holders with eBay parts that upon visual inspection and some measurements looked good. Also replaced tensioner and cam chain guide.

Oil pump was rebuilt and I am getting 60psi when cold. I also made certain I was getting oil at the camshaft.

After rebuilt it was spinning fine (with spark plugs off. No load on the cam).

First start after rebuilt was rough. A few minutes of running, I hear a loud clank and rattling so I just shut it off.

Finally after about 2 months I decided to cut the frame and install a frame kit to be able to get to the engine without removing the engine.

Here is what I found. One of the "ears" that holds the sprocket to e cam had sheared off and was resting on the head along with the bolt. The other "ear" had cracked but still attached to the sprocket.

I wonder what happened.

[denward17]

I don't have a lot of experience in diagnosing these issues, but looks like to me the cam shaft grinded to a halt and the crank kept turning.

What would cause the cam shaft to stop rotating?  valve issues, no oil to cam shaft bushings.....etc..

[BenelliSEI]

I don't have a lot of experience in diagnosing these issues, but looks like to me the cam shaft grinded to a halt and the crank kept turning.

What would cause the cam shaft to stop rotating?  valve issues, no oil to cam shaft bushings.....etc..

Good theory. Pop a few cam shaft caps and see if they are badly scored up? Or, valves were tapping pistons (cam timing was off)? Ouch.

[denward17]

BTW, did one of the cam bolts come out?  looks like it in the 2nd pic.

If so, it come have jammed up somewhere to stop the cam.

[xhevi]

The bolt had come loose and threaded out all the way. Would it possible the bolt came loose first and that caused some sorts of stresses on the sprockets holders?

The broken piece and the bolt were both sitting on top of the head, one in each side of the sprocket.

The engine still spins. There is oil on the holes on the cam holder caps. Still have to take one off.

The camshaft still rotates but there is a loud click. Looks like the click is coming from #2 exhaust valve just as it start closing. Kinda like the shaft opens the valve but then just let's go and the valve slaps on its seat.

[denward17]

Well, I think you found the problem.  That bolt backed out and probably got lodged between the cam gear and the head tunnel and the cam sheared.  Do you see any depression marks around the the cam tunnel on the head?

That bolt could have bounced around #2 before it landed where it did after you stopped the engine.

After you take the cam out and investigate you will probably find more damage.

Good luck..

[PeWe]

The cam bolts need to be secured with thread locker Loctite medium (blue).
Strange that bolt came loose that soon. Can not been tightened.

[bryanj]

Ive never used locktite and alo never had one back out, its possible the sprocet was not seated quie right nd stressed the cam when tightened.
That break looks typical of a stres or drop fracture to me and the click may be a valve stem bent by loose bits, as the engine turns i would lift the cam out and inspect the towers, check valve s for seating and fit a different cam, sprocket and bolts

[xhevi]

I would have swore I checked ALL the bolts were torqued to spec, (checked multiple times) ....

But I don't see why it came loose so soon other then not being torqued.

I have a spare cam sprocket and bolts. Need to find some spare time.

[PeWe]

It was correctly tuned so a valve did not get massaged by a piston?
Easy to make a mistake when chain slack is at the front.

Chain tensioner not in place when setting the cam.
I have made those mistakes but with luck, only a minor piston vs valves kissing party. Nothing bent, small marks on pistons at a later check.
That was before frame kit so I had to pull engine...

[ekpent]

That happened to me once also on a good running engine but it had not been apart. I needed a new cam and being it was pre internet went to a swap meet and bought an engine for the cam. Luckily when I took it apart it had a Russ Collins 836 big bore kit hiding in there. Could still get new rings back then so the whole top end got moved over. Still have that engine in a bike after all these years ! Ironically after I bought the engine I went to a guys shop who was a Honda  guru and in his case was a new Andrews street cam for the princely price of $45.00 so that went in also.

[xhevi]

I think I found the culprit!!!!

On the attached picture taken back in July, just before putting the cam cover on and the engine back in the frame, one of the bolts (bottom of the picture) seem to not be seated all the way compared to the other.

I looked into the cylinder bores with boroscope and did not see any any dents on the piston as a sign of kissing the valves. Also the valves seem to go up an down with no binding. So I hope no valves got bent and the guides are ok.

I got the gasket kit from Cycle X,  MLS head gasket. I will call CycleX but just throwing it there:

Is it ok to reuse the MLS head gasket if I have to undo the head to look at the valves?

If you ask me, the frame kit is a must. I would have rather given the bike away then undo all to remove the engine off the frame.

Second pic is my version of oil pressure gauge. Wanted something very small instead of the available kits.

[xhevi]

Yes. The point cover is upside down. Lol.

[PeWe]

I have reused MLS several times.
Last one had done 1000 miles and looked good after use.

I torqued a little more.
Ca: 21 ft lbs first time, 22 next.
No leak after 6300 miles.
I use o-rings at the 2 oil feed holes. Stock size.
This with heavy duty cylinder studs.
The oil pressure will be visible when riding if you have the typical 90* elbow as the oil pressure kits have.

[xhevi]

I have reused MLS several times.
Last one had done 1000 miles and looked good after use.

I torqued a little more.
Ca: 21 ft lbs first time, 22 next.
No leak after 6300 miles.
I use o-rings at the 2 oil feed holes. Stock size.
This with heavy duty cylinder studs.
The oil pressure will be visible when riding if you have the typical 90* elbow as the oil pressure kits have.

Thanks

As for the oil pressure gauge I though more of it as a diagnostic tool rather then continuously monitoring. It turned out much cheaper then the kit.

I don't see myself looking down on it when riding. Might just take it off and put the oem plug when (IF) all sorted out.

[MauiK3]

Time for a new camshaft.
Bummer

[PeWe]

I had the same idea about pressure gauge back in the days. I changed my mind and have had som use of it the last years.
Really hot summer days. I have had thoughts if pressure will be lower when oil has done some miles and need to be changed.

Just a short glance when riding at various revs when my eyes can leave the road for 2 seconds.
Usef K7/K8 stock cams have often been sold cheaper. I have found for $50US.

New cams today cost much more than a few years ago.

[xhevi]

I had the same idea about pressure gauge back in the days. I changed my mind and have had som use of it the last years.
Really hot summer days. I have had thoughts if pressure will be lower when oil has done some miles and need to be changed.

Just a short glance when riding at various revs when my eyes can leave the road for 2 seconds.
Usef K7/K8 stock cams have often been sold cheaper. I have found for $50US.

New cams today cost much more than a few years ago.

i have a used cam that came from a bike with 14K miles on it. it will do for now.

hoping that there were no damages to the head or valves.

[newday777]

I've have been thinking of putting an oil pressure gauge on but up in front of the dash so I can keep my eyes on the road better.  I have never liked the idea of one down below.

[PeWe]

An oil pressure sensor at the usual place connected to an electrical gauge would be safe.
Voltmeter another must have at front.

[BenelliSEI]

I mount the gauge on an 1/8” NPT, 90 degree male/female fitting. Turn the gauge so 50-60 PSI is pointing up. A quick glance is all it takes.

The bike has a spare oil gallery plug in the tool kit, just in case it “falls over”!

[xhevi]

well looks like I got lucky.

Tore up the top end, breather cover to cylinders. looked for damages on aluminum cases and if any other metal from the broken camshaft.

Dropped the oil pan and was in there for a while with a borescope, flashlight and a magnet looking for any steel or aluminum shavings and blew all the oil passages with compressed air. all good.

no damages to the cam chain or pistons, no bent valves.

replaced the camshaft and the sprocket, did the timing and set the tappet clearances.

spins fine, no clicks or clanks

might have dodged this bullet.

[newday777]

Good to hear so far.
Do you know if that came was a stock cam? There was a service bulletin on cam breaks from 10/77, but I couldn't find it to post here. 😢

[denward17]

Good to hear, did you use Loctite on the cam bolts?

[xhevi]

Good to hear so far.
Do you know if that came was a stock cam? There was a service bulletin on cam breaks from 10/77, but I couldn't find it to post here. 😢

The shaft and cam towers that came with the bike were toast due to lack of oil getting to the top end.

The replacement cam that broke was bought on ebay along with the cam towers from the same engine (so i was told by the seller)

I believe the replacement cam broke due to my mistake of not tightening one of the cam sprockets bolts.

The last camshaft, sprocket and cam towers to go in are stock from a spare engine I bought for parts with about 14K miles.