Thread: Crank key failed, flywheel slipped. Rod Knock?

Alright so I was out cruising today, the bike was running great. After about 20 minutes of solid cruising (30mph) the bike suddenly sputtered and came to a halt. I kicked it and it didnt start, only backfired. So I walked it home and pulled the flywheel and saw that the key had seemed to of been pushed into the crank slot more and it also showed signs of wear. I swapped it with a new key, retimed it and the bike fired up. It ran good for a second, I gave it a little rev, it came back down to idle then it started a horrific rod knock sound. Has anyone ever had this happen to them? Does anyone know what it could be for sure?

Also, it seems to have a misfire at times now. Its timed correctly though. Is it possible that the piston is broken? I will be removing the head to see what it looks like soon.

Take the flywheel back off and see if it sheared this one too...that could cause a knock. I doubt that the sheering of a flywheel key would cause the bike to have a knock. I've sheared my fair share of keys...on everything from from a stock 70 to a 143cc engineering marvel.

When putting the nut on, I always put my impact gun to 80lbs and use blue locktite on it. Although the torque spec is somewhere around 40, I never had had a problem since taking this approach.

Hope this help.s

Originally Posted by fatcaaat

Take the flywheel back off and see if it sheared this one too...that could cause a knock. I doubt that the sheering of a flywheel key would cause the bike to have a knock. I've sheared my fair share of keys...on everything from from a stock 70 to a 143cc engineering marvel.

When putting the nut on, I always put my impact gun to 80lbs and use blue locktite on it. Although the torque spec is somewhere around 40, I never had had a problem since taking this approach.

Hope this help.s

Yea, I jumped the gun without checking more....the flywheel is slipping still. Cant seem to get it to hold well. I will try an impact on it too.

Alright, I'm not sure whats going on with this key. It appears to be alright, one of the keys is a little beat up, but it wasn't the key that was in it when it slipped. The flywheel slot is in excellent shape, nice sharp edges. The crank looks alright.

Do you guys think that the flywheel just loosened up? (I didn't use lock-tight like I should of). Take a look at the pictures and tell me what you would do..

Thanks too...

You didn't say if you examined the inside of the flywheel. If a flywheel key spins, there is usually a lot of damage to either the crank or the flywheel. Validate that they are ok.

How are you putting the flywheel on? If you are putting it on by hand, its going to keep coming off. Unless you have a strap wrench to hold the flywheel still while you are tightening it, and you are getting at least 40lbs on it, it will keep looseing up.

If it comes loose too many times...you WILL destroy the crank and or flywheel. Take my recommendation...get some blue locktite and an impact wrench and you'll never have the problem again...just don't overtighten it to where you strip off the threads of the crank...sure its hardened, and you are more likely to ruin the nut...but still.

And...while you are at it...pony up the $3 for a new key.

Originally Posted by fatcaaat

You didn't say if you examined the inside of the flywheel. If a flywheel key spins, there is usually a lot of damage to either the crank or the flywheel. Validate that they are ok.

How are you putting the flywheel on? If you are putting it on by hand, its going to keep coming off. Unless you have a strap wrench to hold the flywheel still while you are tightening it, and you are getting at least 40lbs on it, it will keep looseing up.

If it comes loose too many times...you WILL destroy the crank and or flywheel. Take my recommendation...get some blue locktite and an impact wrench and you'll never have the problem again...just don't overtighten it to where you strip off the threads of the crank...sure its hardened, and you are more likely to ruin the nut...but still.

And...while you are at it...pony up the $3 for a new key.

The slots on the crank and flywheel both look good. The key looks good. The issue was not having the correct amount of torque on the flywheel, you were right. I torqued the nut down to 50lbs (should be alright if you go up to 80 with it). I kicked the bike over and it started first kick.

I guess this thread can be a lesson learned and also insight for other members in the future....torque down that flywheel correctly. Sorry if this seemed like a useless thread too, sometimes its easier for me to figure things out if I'm sitting here writing out the issue. Thanks for the help...back to riding..

Better yet guy's, lap the flywheel to the crank taper. If a flywheel fits the crank right you dont even need the key,it's there to align the flywheel not hold it. Use valve grinding compound to do it, keep lapping until the high spots are gone.This is done on big bikes too for the same problem.

Originally Posted by Gary

Better yet guy's, lap the flywheel to the crank taper. If a flywheel fits the crank right you dont even need the key,it's there to align the flywheel not hold it. Use valve grinding compound to do it, keep lapping until the high spots are gone.This is done on big bikes too for the same problem.

This works great! Had to do it a couple times.

Hopefully you didnt smoke a valve when it jumped time...

Common mistake, i did it too, you really have to torque them up, most people think that the woodruff key stops the flywheel from shifting but its the taper once torqued that holds it in place, the key just holds it in place for you to torque it up