Crank shaft steel type

SIMBAD · 06-12-2012, 03:13 PM

Hi just wondering what kind of steel would be used in a crank for a 450 ktm dirt bike.
A mate snapped the tread off on the flywheel side of the crank using a rattle gun to do it up..

So he wants me to to tig weld the snapped bit of thread back onto the end of the crank. I just want to use the right type of tig wire to weld it back together.. Cheers.

Ps. Haven't looked at it yet, but if it looks like too much I won't touch it as I'd hate to it fail in any way if it is holding a flywheel back

BigBlue · 06-12-2012, 03:54 PM

SIMBAD,
Your call but I would not even think about welding that.

Seriously , the legal liability consequences if it fails (and given the mechanical ability of the owner after reading your post ; it most likely will) your reputation , bank account , legal standing will all be around zero.

I would tell this bloke it is a no go and buy a new bike or OEM part.

In my opinion not worth the risk and exposure.

Peter.

aquahead2001 · 06-12-2012, 04:06 PM

Run, run for your life!!!! The crack shaft is a finely balanced piece of material, it is also precisely ground, heat treated and toleranced piece of steel that has work under incredible stress. Welding it will create a heat effected zone, losing the heat treatment and most likely bending it.

Tell your mate that the only solution is to replace the crank... And never use a rattle gun to do anything up!

Sprint · 06-12-2012, 04:20 PM

on a postie scooter ...maybe

on a performance offroad bike......no way

SIMBAD · 06-12-2012, 07:53 PM

Looked at it, doesn't seem to hold anything really, just looks like it puts pressure on the flywheel and holds it locked onto a taper lock.
Anyways, still left it and went fishing.. Trolling, looking for that big cod instead..
It's a bit quiet here though lol so thats why I'm having a squiz at the forums right now

XRtowcar · 07-12-2012, 06:46 PM

Depends if the crank is heat treated. Would make it impossible to repair.
Trying to get the thread square too would be difficult.

SIMBAD · 08-12-2012, 07:44 AM

Mate worked it out himself, I suggested tapping a thread on the inside as it is hollow with a 8mm grub screw and then screw the snapped bit back on, was about 10-15mm broken off. Loctite and he is happy.
I did no work to it.

FGII-XR6 · 08-12-2012, 08:16 AM

not even on a postie scooter , here's why

any crankshaft is balanced, welding will unbalance it
mass production crankshafts are cast iron, when you weld cast iron you need to heat treat it or it will soon crack just up from the original break
even crankshafts on smaller engines have significant stress on them due to rotating mass
when someone gets injured you will loose everything you own in the court case, there are no mates when it comes down to public liability

don't even think about it, run as fast as you can away from it

Quote:

Originally Posted by Sprint

on a postie scooter ...maybe

on a performance offroad bike......no way

SIMBAD · 08-12-2012, 10:05 AM

Quote:

Originally Posted by au3xr6

not even on a postie scooter , here's why

any crankshaft is balanced, welding will unbalance it
mass production crankshafts are cast iron, when you weld cast iron you need to heat treat it or it will soon crack just up from the original break
even crankshafts on smaller engines have significant stress on them due to rotating mass
when someone gets injured you will loose everything you own in the court case, there are no mates when it comes down to public liability

don't even think about it, run as fast as you can away from it

I did not work on it, or touch it. I made a suggestion and then went fishing. He did everything himself.

Maka · 08-12-2012, 11:12 AM

http://crankshaftrebuildingservices.com.au/crs1_014.htm

Just in case!

cheers, Maka

2011G6E · 08-12-2012, 02:10 PM

No. Just...no. Do. Not. Do. It. Ever. Especially not on something making a lot of power like a big KTM.

The crankshaft...unless it's from something extremely old or exotic or simply unobtainable...should be seen as a sacrificial item. They can be ground down or built up for new bearings, they can be lightened, they can be balanced better than standard...but when they physically break, bin them.

The "flywheel" side is, I am assuming, the alternator side? That usually sits on a tapered end with a key to hold it in place. That little key is also a sacrificial part...do not replace it with a piece of ordinary steel or a lump of tool steel if you have recurring problems with it or can't be bothered to buy a proper key...it's meant to break first. If it keeps breaking, then that's a sign that something is out of balance somewhere. Trust me on this...you will smash the keyway out of the end of the crank and wreck your rotor. Don't ask me how I know...

Rattle gun...? I have heard of that being done before to put pulleys on cranks for some reason by someone who doesn't know better, but jeez...