Loose drive sprocket nut

[LAB3]

Earlier this year I replaced the sprockets on my 450t and was rather alarmed that my drive sprocket nut was loose, as in not even finger tight! The side of the lock washer was properly bent to keep it from spinning and I was hoping there wasn't an internal problem inside the transmission causing it to happen. After replacing the sprocket and tightening the nut to spec everything worked fine so I forgot about it.

About a month back a pretty substantial leak showed up which turned out to be the seal where the push rod that disengages the clutch goes into the case. Ordered up a new seal and after a two week wait it finally showed up. Yesterday I went to replace it which requires removing the drive sprocket.

Sure enough, the nut holding the sprocket on was again very loose and again I torqued it to spec. Everything seems be OK but I'm still puzzled on why this is happening. The lock washer is definitely keeping it from spinning so how the heck is this happening? It's been 8k miles since I first noticed this, seems to me that if something was wrong inside the transmission it would have shown up by now.

 

[Brendan W]

Could the stack not be clamping properly?
If you have wear behind the sprocket or it's a bit on the thin side the nut is bottoming out before clamping the sprocket?

 

[LAB3]

Could the stack not be clamping properly?
If you have wear behind the sprocket or it's a bit on the thin side the nut is bottoming out before clamping the sprocket?

There's no wear on the case and no side to side wear on the sprocket as near as I can tell. The first time I took it off I believe it was the original factory sprocket, I bought the bike with 7500 miles on the clock and had 19k when I replaced it. Never did measure the thickness of the old one vs new.

 

[fastbysuzuki]

I have to go with Brendon on this, if the nut is torqued up to the spec and the locking washer is correctly fitted both on the shaft and the nut then the only way the sprocket can come lose is that somehow it's not fitting correctly and when the bike runs it moves position in effect it's not the nut that's coming lose it's the sprocket.
strange as you say this is the second sprocket that has come lose, wondering if both were after market ones?
perhaps fitting a shim of around 005" - 010" behind the sprocket would be worth a try should not effect the chain alignment that much.

 

[LAB3]

I have to go with Brendon on this, if the nut is torqued up to the spec and the locking washer is correctly fitted both on the shaft and the nut then the only way the sprocket can come lose is that somehow it's not fitting correctly and when the bike runs it moves position in effect it's not the nut that's coming lose it's the sprocket.
strange as you say this is the second sprocket that has come lose, wondering if both were after market ones?
perhaps fitting a shim of around 005" - 010" behind the sprocket would be worth a try should not effect the chain alignment that much.

As mentioned, the bike had 7500 miles on it when I bought it and a continuous chain leading me to believe it was the original. As to shimming that's probably worth a try, the lock washer seems to be mighty thin for the job. As near as I can tell the condition isn't causing any trouble but then again seems to be only a matter of time before it might.

 

[Brendan W]

On the back of the nut is there more wear towards the inner diameter?
This could be a sign that the nut is bottoming out on the shoulder of the splines rather than clamping down hard on the lockwasher.

 

[LAB3]

The nut is dished with just a thin ridge around the edge that contacts the lock washer. Since I'm still on the road for the summer I've needed to rent a torque wrench when needed. I'll be visiting a friend this weekend or early next week that has one and report back then.

 

[Guest]

I have had the same issue with mine, that nut kept coming loose.

The only way I've prevented it (at least last time I checked) was to go to almost the maximum torque, and ensure the washer is flat and hard up against the edge of the nut.

I'm confident I know the cause of the issue though... when I bought my 450 way back in 1998 I found out from a dealer when they swapped the sprocket out (yeah I know, I didn't know better then but they were pretty good too) that the splines on the counter shaft were worn from the sprocket being loose. They recommended just replacing the sprocket and ensuring it was kept tight. I suspect you may find the same...

 

[LAB3]

Tore into it Tuesday and sure enough, it was again only finger tight. There didn't seem to be any obvious wear on the nut and the sprocket sat firmly within the splines. Found a thicker washer which a local ATV shop had in stock and put that on and tightened it to spec. As of this morning it seems to be holding firm, I didn't break into to check the torque since you could move it a tad even with the old lock washer in place. I'll keep an eye on it, about all I CAN do at this point.

 

[bwringer]

Both my chain drive bikes specify Loctite on the sprocket nut.