YZ450F STM Slipper Clutch question

So, the YZ450F that trashed the main shaft input bearing is nearly back together. Except that the engine builder doing the work phoned me to say that the STM Slipper clutch is missing the secondary spring stopper washer. I took off the clutch in the process of diagnosing what was wrong with the bike (transmission locked up), but I'm sure I didn't lose the washer. Not being familiar with the STM clutch I wouldn't have known about it, but I had a tray strategically placed while working and that caught any stray bits including the 6 balls. I had to pull the components out separately as the primary given gear/basket would not pull off until I did. It was jammed to the kickstart gear by a piece of broken bearing race.

 

I've emailed STM about a replacement, but I'm not too hopeful. It's part 003YAM007 on this photo of the installation leaflet (thanks a million to the person who posted this up on the internet somewhere!).

 

He also reports that a couple of the steel clutch plates are warped and need replacing. I'm wondering if someone may have had the clutch apart previously and left out this washer by accident or design. Might it reduce the secondary 'spider' spring pressure to create even less engine braking? Or perhaps leaving it out ended up warping the steel plates.

 

So I'm faced with buying a complete new (or used if very lucky) slipper clutch for the sake of a washer, or OEM hub, pressure plate, thrust plate, nut, lock washer, springs and screws to convert back to a standard clutch. My lad won't be happy about that as he complains about the 350SXF not having a slipper clutch. He brakes much later and harder then me and is used to just banging down through the gears without the clutch (this is on tarmac, running a road race full wet or a supermoto cut slick on the back).

 

I'll get the engine back rebuilt except for the clutch tomorrow, so I can experiment to see if it would work at all without the missing washer, but it looks as if it's unlikely to be back in action in time for the next speed hill climb as the OEM parts will take at least 10 days to arrive. If I'd known before I could have ordered them along with the new clutch basket/gear and main shaft :angry:

clutch.jpg

I've been through many of these clutches over the last many years and have never seen that washer in any of them. It's quite possible you didn't take one out. You won't find it easy to get anything from STM so going with out the washer is the way to go if you want to ride!

I've been through many of these clutches over the last many years and have never seen that washer in any of them. It's quite possible you didn't take one out. You won't find it easy to get anything from STM so going with out the washer is the way to go if you want to ride!

A friend in Italy tried phoning STM for me today but predictably got no answer as they are probably closed for the summer holidays.

 

I've been beating myself up convinced that I must have lost it somehow, but I really can't understand how. The builder says he has got it back together - except for needing new steel plates. Unfortunately he is unfamiliar with the STM clutch and is relying on that instruction leaflet. I'll have a good look tomorrow. Seeing if it works at all without that washer may be worth a try. I'm struggling to figure out what effect that might have on the action of the clutch but I wonder if was left out rather than fitting a lighter secondary spring. It certainly seems to have no engine braking whatsoever when I have ridden it.

Edited by 72degrees

Looks like the washer's purpose is to act as a bearing of sorts between the spider spring and the stopper hub.  Probably not a hugely important part, but it would reduce wear between those two parts.

Looks like the washer's purpose is to act as a bearing of sorts between the spider spring and the stopper hub.  Probably not a hugely important part, but it would reduce wear between those two parts.

 

That makes sense. It's not always easy to tell dimensions just looking at a drawing like that but I assume that the spring stopper hub comes up hard against the toothed washer inside the stroke stopper hub when everything is torqued up. As you say, that washer prevents direct contact between the inner part of the secondary spring and the spring stopper hub.

 

I'll pick up the engine anyway tomorrow, as refitting a standard clutch is definitely within my capabilities if necessary and I ought to learn how to re-assemble this STM unit myself for when it inevitably needs new plates. Speed hill climb starts are hard on clutches. It probably gets much more abuse, trying to achieve a sub 2 seconds time to the 64ft lights frequently on a grippy surface with a sticky tyre, than a MX bike gets in the same number of hours use.

There have been a couple different revisions on that clutch.  It seems that part numbers and instructions are vague at least.  That washer is just a buffer for the spring.  It doesn't spin on there so it as not a big deal.  I actually ran one of these clutches with a primary spring that was broken in half for over a year.  Sold the bike with the broken spring and the new owner has continued to ride it.  Once everything is put together it all stays in place.  The clutch is nice but parts are not to be had.  If you do replace go with the Hinson.  Better design with pretty much the same feel and performance.

Well I just got it back from Revs. Nick showed me how the slight wear pattern  on the stopper and spring suggest it had been running without the washer anyway. They had already done the trickiest bit for me by assembling the drum, hub and balls on to the shaft. I think I'll swap in the new plates this time with the engine out and on its side though.

A dab of grease keeps those ball bearing in place on the ramps during assembly.  Just a bit though.  Don't want any on the plates.

Well it lives again. My lad had started again second kick. The clutch seems to work in the normal way OK but no way to check the back torque function yet. Noticeably mechanically quieter than before but not amazingly so. The main shaft input bearing must have been grumbling for a while.  Even so, given that all the other transmission bearings were still OK (though now changed as a precaution) it's a mystery why it failed.

The bad news - STM finally replied to my email after a phone call from a friend in Italy. The shim is no longer available.

 

The good news - my lad reports that it seemed to be working as well without it as ever at Prescott Speed Hill Climb today and he set a personal best for that hill.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now