My clutch, my clutch!!

[Pep]

History:

Pretty stock 97. About 6 weeks ago, I freshened the clutch (new fibers and plates, typical assembly). Raced the bike 4 motos, it never missed a beat. Clutch action smooth and perfect. Race prep for last weekend was the usual change of fluids air, etc. did not touch the clutch.

Went out for a site lap on race day last Saturday, left the gate and boom, this. No more connection between the trans and crank. Spent the rest of the day on the 98 and came home to uncover this.

The clutch nut was still tight, the clutch screws (threaded ends) all seem tight and no signs that any backed out. All seven screws were sheared from the body of the clutch and ejected all over the clutch side of the engine. Plate teeth warped/bent in spots, no breakage of plates or disks. Cover (where screw heads sit) is damaged from the screws ripping out.

Trans and crank spin fine, no damage to primary, pinion, rotor gear, or the bushing. Clutch hub splines and crank splines look fine.

Never seen one do this - have y’all? Any ideas as to what happened? (Aside from, “your clutch exploded”).

Laurie

[hodakamax]

Ouch, I had this happen on my old short tracker which probably had a modified 92 clutch. Mine also punched a hole in the lower crankcase dumping all of the lubricants on the track and rear tire. This was my very first use of epoxy which had just been invented. (that's how long ago it was!) It was the only case I had ever seen until yours. I haven't a clue what started that chain of events then and now, or at least I can't remember. Maybe some close inspection of parts will reveal some cause.

Maxie

PS--My first thoughts are the screws were a bad batch that didn't meet specs.

[Pep]

Well I guess I was lucky compared to you Max!
Closer inspection has revealed no other clues.
Even though the screw remnants still look tight, all I can imagine is that one must have loosened enough to destabilize the cage and domino into a failure.
Smarter folks than me might have alternative theories.
I'd like to never see another one of these happen again
Laurie

[Zyx]

This was my very first use of epoxy which had just been invented.(that's how long ago it was!). Maxie

Wow. You were racing a model 92 in 1938??? That was a long time ago.

[hodakamax]

OK AZ, It was new to me!

Maxie

Be good to old people

[RichardMott]

Took me a few minutes of re-reading the post before I got the joke.

"when first invented"

[Bullfrog]

Here is my SWAG.

The clutch screws were not fully tightened. OR something caused one or the other of the clutch covers (where the screw heads are or where the clutch screw threads are) to not properly seat "squarely" with the cage as the screws were tightened. In this case, the screws were tight . . . and subsequently the offending cover found its properly seated position against the end of the clutch cage . . . and presto - all 7 screws were loose. Clutch life with loose screws is very short on the track.

Note that all of the screws visible in the photo are broken at the base of the threads . . . that indicates "racking" to me . . . with the break happening at the "change" of cross section of the screw. And "racking" indicates screws which are not tight enough to keep the cover where the threads are from sliding/rotating on the end of the clutch cage. In the "factory" assembly sequence ONLY the static friction between the outer cover (where the threads are) and the cage keeps the outer cover from being able to "move" relative to the cage. ONLY fully tight clutch screws can create the large clamping force to produce the necessary static friction to keep those two parts from moving relative to each other with the "factory" assembly sequence. The factory screws are high tensile material with rolled threads. I still like 'em, though others don't. Regardless of which screws are used, looseness of the clutch screws just isn't survivable.

I highly recommend looking up my relatively recent post on the "Phil Chapman" clutch pack. Phil's brain storm greatly improves reliability for clutches on pumped up engines. But true craftsman-like care is required upon clutch assembly.

Ed

[thirdstone]

This exact thing happened to me about 2 months ago. I had no other damage apart from the clutch arm (on the outer case) locating pin was sheered. I have been told that the stock screws are too soft so now I have some high tensile screws fitted and lock nuts to stop the screws coming loose, I know if you do proper lockwiring they can`t come loose but 30 years in the aircraft industry tells me locknuts are better than lockwire.

[Pep]

Thanks Ed, I believe you nailed it (of course).
Further inspection revealed "one of these things is not like the other..."

The lower right hole is undamaged, and that was the likely culprit - backed out enough to destabilize the cover. 100% mechanic error. That's a new failure for me - live and learn
I use a press to assemble, and sometimes secure with locknuts, sometimes wire, rarely locktite - this was a wired clutch.
Split the cases yesterday, no other damage found, so that's lucky.
Thanks for the input!
Laurie
P.S. Yes, I found the latest clutch thread on the forum very enlightening.