KX 125 Woodruff Key

Joined
Mar 22, 2010
Messages
22
Likes
0
#1
I rebuilt my engine a couple of days ago and it started once. It wouldn't start again so i took the flywheel off and saw the timing was out by alot, and it has sheared the woodruff key apart.

I was wondering if the engine would be ok for a few days without the woodruff key if I make sure the flywheel is in the correct position. I figured the woodruff key doesn't do anything anyway in terms of holding anything in place because it is so soft, so the flywheel is held in place by the taper. Would the timing slip out without the woodruff key?
 

dirt bike dave

Sponsoring Member
Joined
May 3, 2000
Messages
5,349
Likes
3
#2
The timing will absolutely slip out without a woodruff key.

This could be dangerous out on the track or trail. You could get stranded or hurt.

FWIW, I had a woodruff key shear just starting a trail ride (the bike was a '84 KDX200). We had driven over 2 hours to get to the riding area and did not want to pack up and go home.

We found a washer in my buddy's tool kit and snipped pieces off with some tin snips. Used the washer peices as the woodruff key and got in a long day of trail riding.
 
Joined
Mar 22, 2010
Messages
22
Likes
0
#3
dirt bike dave said:
The timing will absolutely slip out without a woodruff key.

This could be dangerous out on the track or trail. You could get stranded or hurt.

FWIW, I had a woodruff key shear just starting a trail ride (the bike was a '84 KDX200). We had driven over 2 hours to get to the riding area and did not want to pack up and go home.

We found a washer in my buddy's tool kit and snipped pieces off with some tin snips. Used the washer peices as the woodruff key and got in a long day of trail riding.
Oh thanks good job I waited for a reply then. What I don't get though is how come it makes soo much of a difference, because the woodruff key is soft and can't possibly hold the flywheel in place. I'm going to try the washer method, thanks.
 
Last edited:
Joined
May 19, 2006
Messages
1,500
Likes
1
#4
Woodruff key really does hold the flywheel in place. And yes, they are soft enough to sometimes shear, but it doesn't happen very often if torqued correctly. If you look in the hardware drawers for oddball hardware parts at lowes you will probably find one that will work until you get an OEM. Make sure washer is beneath flywheel nut. You are partially correct in that a perfectly machined taper and matching flywheel with interference fit will hold parts together like you are thinking, but these parts aren't machined with enough interference to do the task without the woodruff key.
 

RM_guy

Moderator /
Damn Yankees
Joined
Nov 21, 2000
Messages
6,777
Likes
292
#5
Guys, I hate to break it to you but the key is NOT there to prevent rotation. It is only there to insure proper alignment. The taper in the shaft along with the proper torque is what keeps the flywheel from spinning.

It is very important that the taper (on both the flywheel and the shaft) is perfectly clean before assembly. Any oil or contamination will mess up the joint. You shouldn’t even touch it after it’s been cleaned. Then the nut has to be torqued to the correct spec.

If the key does shear and the flywheel has been spinning for very long it can damage the tapers and you may need to replace both the flywheel and the crankshaft. They can be repaired if it isn’t too bad but if not done properly it will just spin again.

So, yes you can run it without the key but it has to be aligned properly and, more importantly, if it has already spun the tapers must be inspected to insure they are not damaged.
 

IndyMX

Crash Test Dummy
Joined
Jul 18, 2006
Messages
5,546
Likes
1
Location
Amo, IN
#6
What size do you need? I had to buy some from a fastener company due to lack of availability of parts for a 1982 KTM engine.

So, I have 24 keys in my box that I can't use.

They are 4mm thick by 13 mm long by 5 mm tall.

If you or anyone else needs this size, I'd be happy to drop one in the mail to you.

Just email (not PM) me with your particulars.
 
Joined
Jun 20, 2007
Messages
1,020
Likes
0
#7
great stuff indy. :cool:


one trick we have used before and me in particular when fitting a heavier flywheel than stock in one engine was to lap both flywheel and crankshaft with grinding paste. some have glued with thread glue both parts.

if you need to ride with out a key, please make sure the timing is dead on or slightly retarded. do this only if the flywheel on your engine is light. or fabricate a key the way dave has pointed. that should work for a few days...
 
Joined
Jul 8, 2006
Messages
827
Likes
1
#8
I never used a key on kart race engines. I used valve lapping compound set the desired timing with the flywheel and torqued the flywheel nut. Never ever had one to move.
 
Joined
Mar 22, 2010
Messages
22
Likes
0
#9
IndyMX said:
What size do you need? I had to buy some from a fastener company due to lack of availability of parts for a 1982 KTM engine.

So, I have 24 keys in my box that I can't use.

They are 4mm thick by 13 mm long by 5 mm tall.

If you or anyone else needs this size, I'd be happy to drop one in the mail to you.

Just email (not PM) me with your particulars.
Thanks Indy. I ended up getting one from a mechanic down the road. I had to grind it down because it was too bid but problem sorted now. Thanks anyway :)
 
Joined
Sep 13, 2010
Messages
296
Likes
0
#10
+1

RM_guy said:
Guys, I hate to break it to you but the key is NOT there to prevent rotation. It is only there to insure proper alignment. The taper in the shaft along with the proper torque is what keeps the flywheel from spinning.

It is very important that the taper (on both the flywheel and the shaft) is perfectly clean before assembly. Any oil or contamination will mess up the joint. You shouldn’t even touch it after it’s been cleaned. Then the nut has to be torqued to the correct spec.

If the key does shear and the flywheel has been spinning for very long it can damage the tapers and you may need to replace both the flywheel and the crankshaft. They can be repaired if it isn’t too bad but if not done properly it will just spin again.

So, yes you can run it without the key but it has to be aligned properly and, more importantly, if it has already spun the tapers must be inspected to insure they are not damaged.
Exactly what they told us in trade school, and as the tutor was into old and very old bikes he also told us about the valve paste and even brasso (for polishing brass candlesticks etc) as a finer finisher.
 
Joined
Jan 5, 2015
Messages
2
Likes
0
#11
Boy i sure am late to this party. But I will add my two cents in anyways. Not having a woodruff key , for karting is fine. But the danger with not having one on your mx bike is a different story. It may not slip, and it could run great. But with Motocross I don't think it's safe for a novice - pro level. Let's say your hammering on your bike and now your in your last moto, that big double at the finish line is waiting for you, let's just say it does slip before your ready to hit your throttle before the double. Bogg bogg cough cough goes your motor, and your front helmet on top of your handlebars smacks as your body is slammed into your bike when you just cased out . What's scary is I've seen a good intermediate case a double because he forgot to turn his gas on. I mean the odds of havin your gas off and gettin ready to open on a big double then have your float run dry .... It can happen. When it happend to my friend he walked away with my help along with broken collar bone. Be careful , Ride Smart, and if you are not sure about your bike , triple check it.