Post valve clearance crank failure

[Bluto]

Greetings fellow TRX'ers. My problem started when I decided to tackle the valve clearances on my '96 TRX. It was coming up to the service interval and I was starting to experience a reluctance to pull away cleanly from stationary when out riding. There was also some popping through the exhausts on tick-over. So....
Armed with Haynes' destruction manual I set to work and found most of the clearances where out of spec and would therefore require re-shimming. Cams out, replacement shims in and clearances where now (mostly) in spec. Unfortunately, when I tried to set the timing the inlet sprocket kept jumping back a tooth. The cam chain adjuster was fully extended so I replaced the chain with a genuine Yam one (nearly £60). At this point I realised I hadn't replaced the cam chain top guide (the one that also helps to bolt down the No 2 cam). Yes, I know...Anyway, new chain in, top guide on and all timing marks lined up and the chain wasn't jumping off the sprocket anymore but the horizontal marks on the sprockets would shift out of alignment slightly after rotating the engine as per Haynes' instruction. However, the punch marks on the cams still lined up with the raised lines on the camshaft holders and the "I" mark on the rotor was where it should be. I decided that the sprockets were shifting on their bolts under the pressure from the adjuster and the new chain. As long as the cams and crank were in alignment, surely that is what matters?
Anyway, everything back together and the bike started OK but was reluctant to idle till warmed up. Took it for a spin and it seemed OK though not noticeably better than before.I decided a longer ride could be risked and set off into North Wales. I noticed that high frequency vibes seemed worse than I remembered and the engine felt a bit rough under load. but it still ticked over evenly with no ominous noises.
However, after the bike had rested for an hour or so while it's owner drank tea and shot the shit with an old Welsh boy, and within a minute of leaving the cafe, the engine emitted a loud Brrrr! ing sound and cut out. I pulled the clutch lever in and coasted to the side of the road.
On returning home on the back of a low-loader I stripped it down expecting to find damage to the top end. Top end seemed fine, but when I tried rotating the crank to check the timing, the crank moved but the cams didn't (!). Removing the oil pump cover revealed half of the crank teeth had been stripped. I've yet to dismantle the rest of the engine so don't know what the full extent of the damage is.
If anyone has any thoughts on what may have gone wrong I'd appreciate the input, because the prospect of a major rebuild is daunting enough without the worry of the same thing happening again.
Thanks for reading, Bluto.

[cobbadiggabuddyblooo]

I would suggest removing the cams once again as you may find they have bound from being too tight.
Damage was done on the first long ride as it takes a while to get the heat into the solid cam shafts so it would have munched things up but kept things fluid in a way.
Once the bike cooled then came up to temp on your way home that’s when they locked and the next weakest link is the crank teeth.

[jpe70]

"The cam chain adjuster was fully extended so I replaced the chain"

Did you not release and push it back in before putting it back into engine? If not, there's your problem. It will extend fully and lock when you remove it, has nothing to do with the chain...

Or as cobba writes, cam cap bolt torque is important.

[cobbadiggabuddyblooo]

I’d hedge my bet on overtightening the cam caps.
Replaced a few cam chains on TRX motors and 2 motors had up to 100,000km on the cam chain.
Each and every one the cam chain adjuster was 9 clicks out. With the new chain in place the when back to exactly the same adjustment with 9 clicks out.
Bit of a myth in my eyes to replace the camchain every 36,000km recommended by Yamaha.
I’ve gotten into the habit of priming the oil galley that feed the cam shafts too with a 100mm syringe and pushing oil from the oil bleed point at the exhaust cam end of cylinder 1.

[Tarwetijger]

I’d hedge my bet on overtightening the cam caps.
8<

Me too!
The crank can be fixed but it is a specialist job.
A crank for replacement is not easy to find.

[HolerTogni]

Hi Bluto!

Sorry to hear from your misfortune.

My first (red) TRX engine died the same way after a professional mechanic had done the valve clearances. Only that I could only make about 1.5 km untill the "Brrrr! ing sound" (nice description, by the way).

Therefore, I also put my money on too high torque on the screws for the camshaft slide bearing(s).
10 Nm ain't much, but most of the knowing ones in the German TRX forum recommend only 9 Nm.

It is also recommended to lubricate the slide bearings generously when putting the camshafts back in and to watch the easy turning thereof when tightening the screws.

Another / a further reason might be worn slide bearings (most probably of the inlet camshaft) and/or one or more slide bearings out of alignment.

There are specialists (as Tarwetijger said) that can repair the damage to the crankshaft by welding a new gear wheel on it.

"Quick and successful repair"-greetings from Munich!

[Tarwetijger]

I think mine was not welded. I had to deliver a donor crank with good gear wheel. They literally made one good crank from two broken ones.

Read more here: http://www.trx850.com/phpBB_forum/viewt ... f=5&t=5308
Sadly the pictures don't work anymore.

[Bluto]

Thanks for all the prompt replies.
I did reset the cam chain tensioner before re-installing so don't think that was the issue.
I'm staggered to hear that tightening the camshaft covers to the recommended torque settings could be the problem. It's almost as though the engine was designed to trap the unwary mechanic.
Anyway, I'll check the cams for damage and report the findings.
Thanks again.

[cobbadiggabuddyblooo]

I’ve always tightened to 10Nm but I have a fairly sensitive torque wrench from 0 to 30Nm with point setting between.
It may be the calibration of your torque wrench being the culprit.

[Bluto]

Regarding the crank failure on my TRX. I took the cams out and found some scoring on the inlet journals but no discolouration. However one of the valves on No 2 inlet was fully depressed (bit like the owner). Presumably that valve is bent having got intimate with the piston. A few of the buckets are refusing to come out (using a magnet) and this is something I encountered with two of the inlet valves on No 1 cylinder when I first removed the cams after checking the valve clearances. The journal between the buckets had squished out slightly (can't think of any other way to describe it), creating a very slight lip of metal that obstructed the buckets. I had to pare it off with a sharp blade to release them.
At the very least, it seems, I'm looking at a replacement head, cams, and crank. Haven't got into the guts of it yet, so it might end up being simpler to look for a complete engine ( does the '96 on 900 TDM fit straight in?).
Anyway, any thoughts will be welcome.
Regards, Bluto.