Crank Threads Keep Stripping....

DeJay1206

DeJay1206

Member
Sep 17, 2013
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Leechburg, PA
So I recently put new seals in my girls blaster. I went to put the flywheel back on and once the nut got tight it wouldn't move the flywheel in and stripped a part of the crank threads. I bought a m12x1.25 die since my die set didn't have that size and used the die. Same thing happened... The nut goes on but feels loose (like it's a size too small or something). I took the same nut off of my blaster and the same thing happened. Not sure what's going on, I just rebuilt this thing a few months ago and the nut went on fine when I put the motor together originally. Can I use my set and take the crank threads down a size and get the appropriate size nut? If so, what's the next size down?
 
Sounds like the crankshaft has had too many rattle gun, or non tension wrench operations performed and has stripped the threads.

A die nut will only clean up damaged threads, and often make the threads that are left sloppy.

It is not recommended that you re thread the shaft as you may not be able to properly tension the nut to get the tapers to grab, but what have you got to loose.

It costs nothing to try and if it fails all the damage you may do is to have a flywheel spin, that seldom causes irrepairable damage.

You will have to choose a die that will cut a full depth thread.

As I am unable to see the thread size we are talking about I cannot recommend a size.
 
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Blaaster said:
Sounds like the crankshaft has had too many rattle gun, or non tension wrench operations performed and has stripped the threads.

A die nut will only clean up damaged threads, and often make the threads that are left sloppy.

It is not recommended that you re thread the shaft as you may not be able to properly tension the nut to get the tapers to grab, but what have you got to loose.

It costs nothing to try and if it fails all the damage you may do is to have a flywheel spin, that seldom causes irrepairable damage.

You will have to choose a die that will cut a full depth thread.

As I am unable to see the thread size we are talking about I cannot recommend a size.
Click to expand...
It's a m12x1.25
I'm not sure what's a step down from that, m10x1.25? I'm kind of out of options and don't wanna have to put a new crank in right this minute, just got it back together and have another motor torn apart right now lol
 
I am by no means a thread size guru but you may be able to modify to American Fine, but whatever you do, do not try to tension it down to 58 ft lbs or stuff will happen.

Find a torque chart to suit the nut you are going to substitute it for.

Flywheels are pretty forgiving if they are not completely tensioned down well, as long as the tapers are in good condition, clean and free from oil.
 
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Did you install the thick wash under the nut? If the threads are stripped, you're going to end up replacing the shaft if you can't get the nut to bite. The next size down is going to be a 10mm but I don't see a die nut being able to cut 2mm off the shaft and produce good threads. If you want to try a standard thread, see how a 1/2 20 feels. I have no idea if it will hold or not. Best bet is to swap out the crank.
 
OConnor Racing said:
Did you install the thick wash under the nut? If the threads are stripped, you're going to end up replacing the shaft if you can't get the nut to bite. The next size down is going to be a 10mm but I don't see a die nut being able to cut 2mm off the shaft and produce good threads. If you want to try a standard thread, see how a 1/2 20 feels. I have no idea if it will hold or not. Best bet is to swap out the crank.
Click to expand...

/\ that :)
and use the thick washer this time, i'm bettin it wasn't when it stripped ?

better lap the flywheel/crank tapers too, so the flywheel has a tight grab on the shaft
 
OConnor Racing said:
Did you install the thick wash under the nut? If the threads are stripped, you're going to end up replacing the shaft if you can't get the nut to bite. The next size down is going to be a 10mm but I don't see a die nut being able to cut 2mm off the shaft and produce good threads. If you want to try a standard thread, see how a 1/2 20 feels. I have no idea if it will hold or not. Best bet is to swap out the crank.
Click to expand...

Awk08 said:
/\ that :)
and use the thick washer this time, i'm bettin it wasn't when it stripped ?

better lap the flywheel/crank tapers too, so the flywheel has a tight grab on the shaft
Click to expand...
I had the washer on when it happened. Are you saying to cut 1/2 20 threads in or just throw a 1/2 20 nut on and go at it? I lapped the flywheel yesterday for about half an hour before I attempted to do all this :(

Idk, what the heck happened to begin with. It was fine a week ago. When I put the nut on I can feel that it's loose, it wiggles on the threads. It'll thread by hand the whole way down to the base, but once about 15lbs of pressure is applied, it stops. It doesn't really "strip" the threads, it just seems like the nut kinda slips over the threads when pressure is applied. All the threads on the nut and crank are perfectly fine. Idk, I hope I can get it resolved without taking it apart. I literally just got it back together!
 
As a last resort remove the thick washer and slightly undertorque... otherwise try cutting a 10mm thread, but it wont be as strong and you cannot torque to same value.
I am not sure about the 'hardening' applied to the crank threads though.
 
Try getting a new 12mm 1.25 nut, it may fix the problem.

It might be the case that the stock nut has stretched and the crank is fine.

The nut should be made of a softer material than the crank.

If the new stock size nut does not work, Try to run a 1/2 20 die up the threads, and use a new nut.
 
you wont beable to use the dies to cut a 10mm thread without cutting the crank to the right size on a lathe and at that point you might as well get another crank
 
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