Crank problems.

[nickw]

What's this all about. Helping somebody out and after a lot of head scratching found this. Flywheel key in place and intact. Nothing missing from stator plate or flywheel. Engine is a RB22 taken out to approx 230. Cheers for any help.

[Adam_Winstone]

Hi Nick,

Did you or someone previously have to cut a flywheel off (stripped extraction thread)? I don't know how else you would get the mark, as though it has had a grinder taken to it.

Was this a rebuild on an already damaged crank?

Adam

[nickw]

I'm not sure of the history Adam. It has had a recent change of flywheel because the previous one had cracked around the centre bit. I have now found the top of a domed allen bolt broken of the flywheel but that does not appear to have caused the above problem (at least I don't think it has). The crank looked ok before fitting new flywheel (but then again who knows). The gouges are quite big and there is metal left inside the flywheel from the crank. They certainly weren't there when I put the flywheel on. I can't think what could have caused the damage.

[rossclark]

Just to be clear are you saying that you changed the flywheel recently and this damage was not apparent at that time? If that is the case I can't see how you'd have missed damage like that so it must be new.

The only way I can see that having been done was there being something inside the taper on the flywheel when fitted.

[nickw]

Absolutely, no damage visible when flywheel changed. AFAIK there was nothing on the taper when fitted and no evidence of anything. The woodruff key is still in place and keyway looks intact. Looking at damage I would think flywheel has turned on the crank but cannot find what caused the damage. The head of the allen bolt although broken appears intact and does not look as though it has been 'trapped' anywhere.

[holty]

its a very strange one, the key is intact so the flywheel cannot have been loose, almost looks like its been electricity arcing beteen the flywheel taper and the crank and its eroded it away, cant see how though, sadly your going to need another crank and flywheel i think.

[CHRIS in MARGATE]

Clearly you would have seen any damage on the shaft when replacing flywheel because you would have kept the key visible for alignment.
So: Was a foreign body trapped inside the flywheel boss when replacing? It would have been close to the external face allowing the flywheel to locate and slide "almost" all the way home. Something.like a thin 7mm washer ?

[rossclark]

It does appear as if there is some damage to the key on the right hand side in the first photo.

[nickw]

Yes Ross, well spotted. There was some damage to woodruff key. When I took key out it wasn't loose but did have a sort of step on it. I first thought what Chris said, that there had been a small washer in there somewhere, but no evidence of that. Then noticed blue heat marks (see pics) and thought of what Holty said. But where would that amount of heat come from.




I'm guessing the broken metal bits were welded from the flywheel to the crankshaft and were broken when I took flywheel off, which took some doing. Also a bit of a burr on keyway. The engine has not been on the road but only started up in my garage. What could cause that damage on the crank and inside of flywheel? If there was a loose washer then I would expect stator to have more signs of damage and surely there would be no room between flywheel and crank? Apologies for massive pictures, I'm using a new photo hosting site

[Adam_Winstone]

Very odd

[Fat Trucker]

I had a similar issue years ago, the flywheel boss looks like it's a replacement one i.e. If you have a Li flywheel you can replace the center boss for a gp one(Scooterrestorations sell them) I replaced the Li boss for a gp one and used Allen cap bolts and lock nuts provided, drilled old rivets out to the size of the bolts popped them through, nuts on with super loctite. Jobs a good one, lapped with grinding paste, cleaned and pop flywheel on to strobe up flywheel off and on no problem. Had flywheel on for a short while then had to remove flywheel and had nearly identical marks as you've got mmmmmm. After 2 fucked cranks and flywheel bosses I was informed you need to tap the holes for the bolts if not the pressure of the spinning will move the boss very slightly causing friction hence it looks like weld.
Of course I may be wrong as I normally am according to the wife.
Just have a look in the hole the bolt has come fro to see if it's been tapped,
Cheers ove

[dickie]

That's fretting. Tiny movement but large pressure so lots of heat.

I saw similar damage on a pressed steel pin that was supposed to be static but due to massive loads on it, it was moving a tiny amount. A 200mm steel pin totally wrecked.
https://en.m.wikipedia.org/wiki/Fretting

I'd say that either the boss wasn't lapped onto the taper very well or the flywheel was moving relative to the boss (loose screws).

[HxPaul]

I had a similar issue years ago, the flywheel boss looks like it's a replacement one i.e. If you have a Li flywheel you can replace the center boss for a gp one(Scooterrestorations sell them) I replaced the Li boss for a gp one and used Allen cap bolts and lock nuts provided, drilled old rivets out to the size of the bolts popped them through, nuts on with super loctite. Jobs a good one, lapped with grinding paste, cleaned and pop flywheel on to strobe up flywheel off and on no problem. Had flywheel on for a short while then had to remove flywheel and had nearly identical marks as you've got mmmmmm. After 2 fucked cranks and flywheel bosses I was informed you need to tap the holes for the bolts if not the pressure of the spinning will move the boss very slightly causing friction hence it looks like weld.
Of course I may be wrong as I normally am according to the wife.
Just have a look in the hole the bolt has come fro to see if it's been tapped,
Cheers ove

How come the original rivets hold it in place...........the flywheel isn't tapped for them.

[dickie]

How come the original rivets hold it in place...........the flywheel isn't tapped for them.

Rivets will get a much tighter hold than a through-bolt. A tapped hole isn't quite as tight as a rivet but has much better shear strength and is only a little less tight.

[Warkton Tornado No.1]

I'll put my two penny worth in for the craic!

I think the tapers never were a 'fit' because the boss has been moving. & causing fretting, as has been pointed out.

Contributing factors could be the fact that they were never lapped in properly, the key too tall, or, as I have experienced, the boss is machined too big, necessitating a special shim (washer) to be fitted that is larger than the taper nose yet allows the nut to drive the boss on to the flywheel.

(I have bought several bosses & used OEM bosses to re-boss all kinds of flywheels, including Motoplat.

I've been caught out by Scooter Restorations bosses which have all been too big, requiring either the crank nose to be machined or the use of special shims. Of course, it does very little to help with any pick-up alignment as well.....)

Whenever I have re-bossed, I have always tapped out the boss to M6 & used button head high tensile screws just long enough with the washer plates that are from the OEM flywheel. Sod all that messing about with nuts, FFS! Grade 8.8 & above (usually 12.9) have tremendous sheer & holding strength torqued up correctly & I do not advocate it, but on a really light flywheel, I got away with just four for many years.

[Warkton Tornado No.1]

I had meant to say:

Noticably, the crankshaft has become magnetic. How this has occurred might be further indicative of the looseness of the flywheel on the crankshaft due to the boss being mis-machined too big in it's bore (even though the nut may have been torqued up to 50 lb/ft or more)

Some types of steel, will magnetize without a magnet as a result of shock, stress or strain.

[nickw]

Magic. Fat Trucker it was bought from a dealer like that. TBH I hadn't really thought about the boss having been added on like that. The hole is tapped but the bolts are slightly loose so not sure what happened there but thank you for explaining the issue. You can tell the wife you weren't wrong. Fretting. That's a new term. Wish I'd known that before I pulled my non existent hair out. That is really helpful Dickie, thanks. WT no1 another excellent explanation. Again made me realise I wasn't going mad. Thank you everyone for your input. I'm not sure how I will get on with the dealer as the flywheel has damaged the stator and a race crank as well as the flywheel itself. Thankfully not my scooter for a change

[Fast n Furious]

I've seen this happen before.
I put it down to it having an incorrect spec woodruff key, probably one from a Vethspa. Hard to prove once its been sheared of course. A key that is marginally too high will cause the fretting that others have commented on.
I always use gauges to make sure there is a small gap between the top of the key and the bottom of the key-way after the nut has been fully tightened then removed.
If you change the Crank,Flywheel or Key, then its important to do this check to ensure the taper is actually locking concentrically. We can no longer trust that new parts conform to original specs and expectations.
The key doesn't need to be made of steel, which we all know is very destructive when it shears. A softer material would do the same job and do less damage if the taper was to slip.