Cam Chain Sprocket Bolts Sheared!

Hi all! I'm a newbie here, so please bear with me if this particular problem has been answered somewhere else. I did a search and didn't see anything.

My son's friend asked me to take a look at his XR70R, which had reportedly died while at approx. 1/2 throttle. No compression when he tried kicking to restart.

My first thought was either a broken cam chain or sheared flywheel (woodruff) key. When I popped off the cam chain sprocket cover, I saw that the two bolts holding the sprocket on had completely sheared and and also chewed up a good portion of the inside of the cover. Aluminum shavings everywhere!

Any idea what could have caused this and, other than cleaning up the shavings and replacing the munged parts, anything else I should look at before reassembling?

Thanks in advance!


That happened to me. I forgot to tighten the cam sprocket bolt when rebuilding. They were only finger tight. When I rode it, there was a slight clicking noise. It was cold so I thought it was the rockers clicking because I remembered setting them loose. So then, all of a sudden, the engine makes a terrible sound that no one wants to hear right after rebuilding an engine. So I immediately shut it off. When I get home. I think that a valve broke and start to take the top end off. When the cam cover comes off, I see the problem. the cam bolts fell off. You should pull the entire top end and clutch cover and clean everything out. While you're removing the top end, you will need to inspect everything. My engine bent both valves and warped the cam sprocket. So check the cam sprocket to make sure it's flat. And test the valves for leakage. Pour some liquid onto the combustion chamber and blow compressed air into the ports. If you see bubbles, you need new valves. if you're lucky, your piston will be re-usable along with your head. But when my cam stopped spinning because the bolts cam out, the cam was at overlap. So the valves were both open and not moving, but the piston was moving. So it just killed my valves. Just check everything. And only cleaning stuff out from the clutch cover and top end is a minimum. YOU SHOULD split the cases and clean that out also.

Thanks quoke-q!! Oh man, splitting the case?! Just what I wanted to do on a Sunday! :applause:

As luck would have it, I have another XR70 sitting in the garage for parts, so I stole the cam, cam sprocket, bolts and cover from that one. I may just go crazy and glom the piston and valves from it as well, since the kid's bike smoked like a mother before the bolts sheared and it had been doing that for awhile. Blue smoke, so I'm assuming it needs a ring job at the least.

Also, when I pulled the cylinder head, the top of the cylinder and the valves were completely covered with carbon deposits. Also, oil in the exhaust manifold and pipe. Not good.

Do you agree that this all calls for new piston/rings, etc?


yea, sounds like it's gonna need a complete top end job.. i wouldn't scrimp on it as you'll most likely pull it apart again if you don't do EVERYTHING the first time.... might wanna just swap him for your running motor and ask for a few hundred to rebuild yours, as you're gonna have to split the cases, clean EVERYTHING out, then most likely need a new piston, valves, possible vavle guides and inspect the rest of the cam chain system as the bolt heads could have went down and chewed up some of the other guides and stuff down there

Yeah, I've pretty much resigned myself to the complete overhaul, turboglenn. Thanks for the advice.

I don't happen to have the tool to compress the rings while sliding the piston into the cylinder. Any suggestions on how to do it without that? My local shop is closed today.

Oh, I forgot to mention that I recovered all the bolt parts when I took the cover off. So, the good news is that I don't have to search through the entire motor & tranny for bolt heads! Just hundreds of tiny pieces of metal! LOL!


There's no need for a ring compressor tool, just pinch them with one hand while lightly jiggling the cylinder down into place. A light coating of oil in the cylinder will ease the installation.

There's no need for a ring compressor tool, just pinch them with one hand while lightly jiggling the cylinder down into place. A light coating of oil in the cylinder will ease the installation.

yep... put it on the rod and install from teh bottm... there's a taper like on the oder 2-strokes so you can jsut pinch thena dn wiggle the jug on there easily... i've done several of these motors and never sue a ring compressor

if your gonna rebuilt it.just buy the cheap 88 cylinder and piston from its liek 100 bucks and holds up great. my freind did it instead of rebuilding with stock parts and it worked out great. gives you a descent power gain over stock as well. pick up some valve seals form your dealer as well. dratv also sells the valve springs tool for under 20 bucks. it would be a good idea to do that too.

If it was my (or my son's) bike, I would definitely do the 88 upgrade. But this whole project is pretty much being done as a favor to my son's friend. They're paying for parts but I'm throwing in the labor. The old adage, "No good deed goes unpunished" definitely applies here! :applause: What can I say, the kid's mom is a hottie!! :lol:

Thanks to all of you that replied. Great advice and suggestions from everyone!


a bolt must have worked loose. my want to locktite those next time.

my want to locktite those next time.

Exactly my plan!! I have all the parts I need now and the motor is torn down. I'll be reassembling and testing tomorrow hopefully. I'll let you all know how it works out.

As an aside, this is my first time working on one of these Honda engines. What a nice piece of machinery! It's been a blast so far! :applause:


Yes. These motors are amazing. And there have been only minor changes since the original motors of this design in 1968. Only small things like piston dome and combustion chamber shape. Small gearing changes, CDI, and the oil pump was enlarged in 1982. But generally, these motors are the same. It's amazing.

I finished up the rebuild late yesterday and all went well! Since I had everything pulled apart, I decided to also disassemble and clean up the carb, which badly needed it. The jets were all clogged with gunk. So all told, I put in new piston, rings, valves, cam and cam chain sprocket. BTW, to anyone who may have had trouble in the past getting the cam chain sprocket on, removing the tensioner bolt first was key. It also made positioning the chain for timing purposes a breeze. I set the timing, put everything back together with all new gaskets, and it started right up after two or three kicks. Woo Hoo! Took a few minutes to fiddle with the idle and air adjustment screws and it runs like a champ!

Thanks again to everyone who gave tips and advice. It was extremely helpful and made the job go very smoothly. :applause:


Glad you got another bike on the dirt!

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