Broken Honda Cb750k '79

Michigandude

Michigandude

New Member
Hey Folks,

I just joined and have been enjoying reading all your threads. I am trying to learn how to work on my own bike, but I ran into a problem. Oil was leaking from the valve cover, so I checked one of the bolts and it was free spinning. So I took it out and found it was broken. I got the bolt out but then after trying to retap the steam inside broke. I feel pretty overwhelmed any advice on how to resolve this issue would be most appreciated.

Blake
 

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Did you try to re tap without removing the cover? Looks like you have tried to re tap the thread down the shaft of the broken bolt. Anyhow if you did not remove the cover you will have to now as you will all the bits of thread in the head and you need to clean them all out before you attempt to start the bike again or you could have bigger problems if they get washed around with the oil.
 
I did start it but then shut it right off after thinking about what could happen. But now that piece is broken what would be the next step?
 
Take a pic from farther away, it's a pain to try and imagine where this bolt is that you're dealing with. Is the bolt broken in the cover itself, or are the threads down in the head? The valve cover comes off easy, super easy. Take it off. Then grab a magnet and a shop vac and scoop every and any scrap of metal you can possibly find out of there. Once that is done, find out what size the bolt is that's stuck, and get a drill bit that is the thread size, i.e. the size of the hole the bolt is going into, and drill it SLOWLY while the shop vac is running and sucking any shards out of the hole. When you reach the bottom of the hole, you'll know it. Clean it out, tap it (USE PLENTY OF OIL WHILE TAPPING THE THREADS) When tapping, cut 1/4 turn, then back off, then another 1/4 turn, then back off, repeat until you reach the bottom of the hole.
 
Lob! Dude you helped a lot I opened her up and found the problem I am trying my best to get all the stuff out. I got most everything I see a few tiny tiny ones do i have to take the cam out or if the majority is out am I good. This is the most I've ever worked on an engine feels good!
 

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That is your cam cap... You can't just replace one of those, they are installed on the cylinder head during manufacturing and then line honed to size. the only proper way to repair this is replace the cylinder head. If you take one from another used head and install it your oil clearance will be out of spec, and the hole will not be centered any more, causing a broken or seized camshaft.
 
Hard to tell from the pic how badly damaged the cap is, but you might be able to take it to a machining shop and get a new shoulder welded back up.

But like already said - you can't just throw any other cap on there or you'll have excessive cam float / seizure (and by the way - all the caps are numbered and need to go back on in the same order).
 
Yes - but the problem is the arc under the caps are all bored to different sizes between different caps / heads. Get the wrong size cap and you can ruin the cam.
 
Actually they are lettered not numbered so if I find the same letter cap it should work also I have the right torque for each
Bolt should that help?
 
No - the letters are there to show the position of the caps, but the caps are specific to the head they were bored on. Getting another cap from another head will (more than likely) be the wrong size because it wasn't bored on your head.

Also, the cap bottoms out on the head so changing the torque on the bolts doesn't change the bore gap.
 
Then why would the manual say that they can be taken off and put back on? That doesnt make sense to me, why even make it so you can unscrew it if its not possible to put back on? Do you have any advice where I should go with this next. Sucks I was riding it today and felt good. :/
 
You can take them off and put them back on - but they have to go in the SAME PLACE.

They are bolted to the head at the factory and the cam tunnel is line bored with the caps in their respective spots.

So, the cam cap is matched to that spot on the head. THAT head - not another head.
 
Okay, so the problem is not the caps going on (with the right torque and placement), but it is getting a replacement that will be hard?
 
Yes - I'm not even sure you can buy them as individual pieces.

Best bet is to find a machine shop and see if they can re-build the top of the broken cap.

Or, you can find a breakers / boneyard / wreckers and either get a whole new top end, or get another cam cap and just hope it works out (it might).
 
Man, this bike ran like a dream and thats what i get for trying to fix the little oil leak do to broken bolt. I have contacted a shop to see what they will do. Thanks guys. Im trying to learn about bikes.
 
So for all the nay Sayers just want to let you know I ordered another mount and guess what fit perfectly! Same tokens mount same lettering. ;)
 
??? ::)

Its not about whether it will bolt on, of course it will bolt on.
The problem is the very small variances that you can't see but your cam spinning in there at 10,000 rpms really doesnt like.
 
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