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Snapped ONE cam shaft holder bolt

  • Thread starter Thread starter nabrams
  • Start date Start date
N

nabrams

Guest
Hi All,

So I got the engine out of the bike (1981 Honda CB750K), installed the new head gasket (the old one was missing a 2-inch section) and got the cam shafts and cam chains all back in place nicely. At this point, I'm feeling pretty good about this, even though I've invested at least 10 hours into the project thus far.
THEN the nightmare - I overtightened just one of the cam shaft holder bolts and it snapped :evil: :evil: . It's the "inside" bolt on the "H" holder. The "H" holder is the one on the intake shaft closest to the engine center-line, without the oil pipe.
So out of 12 bolts on the intake shaft (6 holders, 2 bolts per holder), I snapped just one.
Can I leave it like this?
The bolt will not come out of the bolt hole easily and there's less than a millimeter sticking out. Drilling, tapping and helicoil is my only option - but with such a thin bolt I am very likely to get the drilled hole off center enough to make things worse.

Your advice is appreciated.
 
Take it back apart and take it to a machinist to be fixed. Either its right or its not, There isnt anything inbetween. You messed up, accept the consequences and remember for the next time.

Earl
 
earlfor said:
Take it back apart and take it to a machinist to be fixed. Either its right or its not, There isnt anything inbetween. You messed up, accept the consequences and remember for the next time.

Earl
Click to expand...

The best bet is to do what Earl says, but if you're feeling lucky though, file the top of the broken bolt so it is flat, bolt the cap back on with one bolt and use the remaining hole as a guide and drill an indent in the top of the bolt with a drill the same size as the hole in the cap, then remove the cap and drill down enough so you can use an easyout to remove the remaining stud. Chances are very good the stud is not frozen in the hole. I hate using easyouts because if you break one in the hole they are virtually impossible to get out. SO DO NOT BREAK THE EASYOUT!!! If the stud seems frozen and does not come out with little more than finger pressure stop and go to the machinist. Sometimes the local tool store will have drill bits with a reverse curl and are meant to drill in reverse. If you can get one of these, use it to drill the first indent, if you drill slow enough it might catch on the stub and spin it out. Good luck but remember "DO NOT BREAK THE EASYOUT!!!"
 
stud

stud

Ok so you got a small amount sticking out, pull the cams back out, take the bike to a tig or mig welder, mask the whole topend but leave a hole with the stud sticking thru, have the welder weld one of his old tips to it and then grab with some small vise grips and it will come right out, now if your real smart you will replace all of the cam cap bolts, and use a torque wrench to replace them with. Now please tell me that you at least used a straight edge to check the head for flatness :roll: :roll: , a little hint you can use some foil and contact cement, tack the foil around the stud to protect the suface around the broken stud in case your welder sucks
 
Isn't that Honda DOHC setup a doozy! (what were they thinking? 2 chains and 2 adjusters?). Anyways, keep in mind that a lot of these cam bolt breakages occur because of oil in a dead-end bolt hole. The oil will not compress and it can not get past the threads very fast. If you are just looking at the bolt going down to where it should be seated, you may try and force it because it is stopping early (could also be messed up threads). Back it out and check why it's stopping (it may even be the wrong bolt, lol).
 
Got it out!

Got it out!

G-d had mercy on me.
After an hour of VERY careful drilling, I had a shallow, centered hole in the bolt that I stuck a small flat-head screwdriver into - then the bolt backed right out!
Using a center punch and going up in drill bit sizes very carefully was the trick to it.
I then spent another 3 hours wrestling the engine into the frame.
I've got to get the carbs, airbox, tank, seat and exhaust back on and then I'll be ready to test. That will be another 3 hours at least.
All told, this head gasket replacement job looks like it will tally to 20 hours - I hope I don't have any other problems for a while.
By the way, I did use a torque wrench - on this bolt I "spaced-out" - it was really late at night and I'd been working on the bike for hours.
Thanks to all for the advice.
 
First the good news:
I got everything back together and there's no whistling/chirping out of the head and not a drop of oil leaking anywhere.
I thank all of you for your technical advice and moral support when things looked grim.

Now the problem:
The bike runs strong and smooth above 3000 RPM, but will not hold an idle below that even after fully warmed up.
Before I tear into the carbs again (I cleaned them several times when I got the bike 4000 miles ago), I'd like some input from the experts.

My observations with the bike idling at 3000 RPM:
When I pull the plug wire for 2, 3 or 4 the RPM immediately drops and the bike stalls a couple of seconds later.
But, when I pull the #1 plug wire I get no RPM change at all. I visually verified spark on #1, and I'm guessing the bike would not accelerate so well above 3000 RPM if #1 carb was not getting fuel at all.

If I do let the idle down to 2000 RPM I hear a knocking noise and the bike stalls a couple of seconds later - I'm guessing that knocking noise is #1 not firing at all at that low RPM.
I did have a bit of a choppy idle before the head gasket went south, but the bike would idle fine at 1500 RPM.

Maybe the butterfly valve for #1 is stuck open?

Thanks again.
 
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