Crank Shaft Runout

During a recent tear down of my 06 450F engine, I inspected the runout on my crankshaft. Holding the crank on centers as the manual calls for, I get an indicated total runout of 0.005". The manual says to replace if over 0.002" runout. According to a machinists handbook, the "runout" is taken as 1/2 the indicated runout as obtained by taking the maximum minus minimum reading on a dial indicator runing on the bearing journals. Is this interpretation of "runout" correct? If that is the case, my runout is 0.0025"

I have already purchased a new crank, in part due to the runout issue, and in part due to heavy wear on the cam chain drive teeth on the crank. The bike had the heaviest flywheel weight you can put on (651 grams total flywheel weight - about double stock flywheel weight). I noticed that there is no material removal on the flywheel weight for balance, which leads me to believe that the flywheel could be out of balance. What is the liklihood that an out of balance heavy flywheel could have been the cause of my excessive crank shaft runout?

Even if the flywheel weight ring is balanced itself, a slightly loose fit onto the flywheel, or misplaced welds (see attached pict) could cause an imbalance. With that much weight that far from the center of rotation, it wouldn't take much of an offset to cause a problem. Has anyone run into issues with this before? I just can't think of anything else that could have thrown my crank out of true.

As a reference, my new crankshaft has a runout of 0.0007" (0.0014" indicated). I have both the stock and heavy flywheel and am wondering if I should go back to the stock flywheel if the heavy one will throw my new crank out of true. Any experienced opinions?

Thanks,

Scott

Flywheels.png

What seems to happen on the 450 cranks is that they spread opposite the crank pin I would guess that if you looked at the run out gauge you would see that it was "low" at the bearing surface on the crank pin side of the wheel. If you had a good cam chain sprocket ( stupid that is not replaceable) you can just press the crank together in a vice 180 from the pin and get it trued up. the only way to prevent this is to weld the pin after truing. which makes rebuilding a pain. These cranks are not pressed together with very much force. I guess the heavy flywheel cant help but they do it even with out one.

You are exactly correct, my crank was spread opposite the crank pin, and I did see a low runout as you describe. Having nothing to loose since I was planning on buying a new crank, I did try to true it up in a vice, but managed to overdo it and made the runout worse in the opposite direction. Do you know of shops that balance flywheels? My local shop says it is done, but they can't do it.

Thanks

You understand there's a difference between truing and balancing, right?

Ask for the crank to be trued and you'll have better luck.

Even if the flywheel weight ring is balanced itself, a slightly loose fit onto the flywheel, or misplaced welds (see attached pict) could cause an imbalance. With that much weight that far from the center of rotation, it wouldn't take much of an offset to cause a problem. Has anyone run into issues with this before? I just can't think of anything else that could have thrown my crank out of true.

If you can track down Jeff Fox at Fox performance in Colorado Springs he has a specialty balance shop and could do the weighted flywheel.

You understand there's a difference between truing and balancing, right?

Ask for the crank to be trued and you'll have better luck.

I do understand the difference, but just thought that an unbalanced flywheel could cause the crank to become untrue. However Geoffit indicates that cranks going out of true is not an uncommon occurance, which implies it might not be related to my modified flywheel. I just wanted to try to get it balanced just in case since I am putting a brand new crank on it (which is already running true).

Geoffit, thanks for the lead for Fox performance.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now