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thrust rod broken

ThompsonJP
Posts: 52
Joined: Thu May 22, 2014 11:45 pm

thrust rod broken

Post by ThompsonJP »

thrust rod and piston.jpg
Hey, I have a 1974 R90/6 with about 75,000 miles and recently the clutch stopped working. After disassembling it I discovered the thrust rod had broken (photo attached). A little history on the bike: in Dec. 2020, I replaced with transmission with a donor from a 1975 R90S. In June 2018 at 61,000 miles I had the clutch reconditioned by Southland in California.
Everything worked great until about two weeks ago, I got excess play in the clutch lever. I thought it had gotten out of adjustment, but I couldn't improve it and finally the clutch went totally slack.
Before I order a new thrust rod, I want to get help on what may have caused this so I'm not back in the same boat later. I'm including a bunch of photos in case someone can see something I can't.
Thanks so much in advance.
Attachments
pressure plate side 1.jpg
pressure plate side 2.jpg
compression ring 1.jpg
compression ring 2.jpg
clutch disc 1.jpg
clutch disc 2.jpg
diaphragm.jpg
JPT

User avatar
schrader7032
Posts: 8223
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX

Re: thrust rod broken

Post by schrader7032 »

Maybe it was just a defect in the rod? Maybe the alignment of the transmission on the studs wasn't the best. Also, be sure that the forward end of the pushrod has a good tacky grease on it where it rests on the pressure plate. That must be able to spin when the clutch is actuated and the rod is pushed forward.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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srankin
Posts: 537
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

Re: thrust rod broken

Post by srankin »

I can't tell from your pictures, did you have the felt clutch rod ring in place? Other than that, I can't see anything blatant. St.
Owner of a 84, R80RT and 78, R100RS

ThompsonJP
Posts: 52
Joined: Thu May 22, 2014 11:45 pm

Re: thrust rod broken

Post by ThompsonJP »

I did have felt, but it came out a torn mess. I think some things may not have been greased properly when installed last time. I'll make sure to do that this time.
Thanks
JPT

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srankin
Posts: 537
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

Re: thrust rod broken

Post by srankin »

Some of the parts are oiled internally by gear oil in the trans. Did you perhaps not have enough oil in the transmission? Otherwise it is a who knows what happened, it just did. Sorry, it is the best I can think of. St.
Owner of a 84, R80RT and 78, R100RS

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vechorik1373
Posts: 118
Joined: Fri Oct 27, 2006 3:00 am

Re: thrust rod broken

Post by vechorik1373 »

LOOK carefully at the throw out bearing parts. The thrust washer surface, the caged needle bearing and the rear thrust piece with the seal on it. If you see pitting, or rough surfaces on these pieces, REPLACE all 3. The bearing assembly must turn effortlessly, if it seizes, it will break the clutch push rod. Don't simply replace the clutch push rod and felt..
And, an assembly procedure, in case you don't know.
1. Take a bore brush for a gun, and thoroughly CLEAN the inside of the input shaft.
2. With a new (oiled) felt, on a new push rod, it must be installed in the front of the input shaft, and it will be tight, and require some wiggling to get the felt into the input shaft. That's why you want the hole through the input shaft CLEAN so it does not bind up when installing the clutch push rod.
Vech
Technical Adviser, Former owner, Bench Mark Works
662 312 2838 cell 9 am to 4pm CST PLEASE!

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srankin
Posts: 537
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

Re: thrust rod broken

Post by srankin »

So Vech, is it possible when he assembled the unit last time, the felt was out of place and blocked oil passage?

My teflon memory is acting up so I will ask; "is the unit oiled from inside the transmission by gear oil"? St.
Owner of a 84, R80RT and 78, R100RS

User avatar
schrader7032
Posts: 8223
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX

Re: thrust rod broken

Post by schrader7032 »

Yes, splash oil from inside the transmission is supposed to oil everything aft of the felt. The felt seal is there to keep the oil from running forward and getting on the clutch.

I don't think this is the model bike, but I recall a situation where the pieces didn't fit inside the tunnel well and/or swelled a bit when heated. Vech talks about honing out the tunnel, but I was under the impression that the piston, the larger piece at the end of the rod had to be machined in some way to reduce the diameter to fit the tunnel.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

User avatar
srankin
Posts: 537
Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

Re: thrust rod broken

Post by srankin »

Kurt, there have been postings about newer aftermarket or OM pistons swelling causing sticking problems and such. I could be wrong but I think they may have been made of plastic? Sorry I only skimmed the posts because I didn't have that year bike and have never had the problem written about.

Vech, as always makes sense as did you in your first reply. Having put one of those felts in myself a couple of times, I could very well see how they could be installed improperly.

Hopefully this doesn't happen again, but what a surprise it must have been! St.
Owner of a 84, R80RT and 78, R100RS

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vechorik1373
Posts: 118
Joined: Fri Oct 27, 2006 3:00 am

Re: thrust rod broken

Post by vechorik1373 »

"Vech talks about honing out the tunnel, but I was under the impression that the piston, the larger piece at the end of the rod had to be machined in some way to reduce the diameter to fit the tunnel."

No, not the tunnel of the transmission. You misunderstood me. I was talking about the hole through the input shaft.
You need to get it CLEAN before installing a new felt and push rod.

You must think about what caused the push rod to snap in two, in the first place. And, what I forgot to mention previously, was that you need to examine the female tapered hole in the spring plate, that the push rod seats into. If it is galled up, it will chew up the tapered end of the new clutch push rod rapidly. If it is in that condition, replace the spring plate.

But the clutch push rod can only snap in two, if there is excessive drag inside the input shaft, and/or if the clutch was incorrectly adjusted which caused the throw out bearing to be under pressure and spinning all the time. If I recall the owners manual correctly, they said to never pull and hold the clutch lever at a stop light, while the transmission is in gear. Always put the gearbox in neutral, and release the lever. This keeps prolonged pressure off of the tip of the push rod, and the throw out bearing at the rear of the push rod. Repeatedly sitting at stop lights in first gear will accelerate the wear of these components.
Vech
Technical Adviser, Former owner, Bench Mark Works
662 312 2838 cell 9 am to 4pm CST PLEASE!

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