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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 14 Mar 2008
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Is it worth repairing (Again) ?

Hi all,

I have a 1991 XT600E that had its engine professionally rebuilt about 6 months ago, at about 85 000 miles (140 000Km). The work included replacing all the crankshaft bearings, new con rod, re-bore etc. This cured the oil consumption and loss of power but not the excessive amount of vibration that had developed during the last few thousand miles before the rebuild. Unfortunately, that mechanic has now emigrated.

Initially I thought that this would improve when the engine had done some miles but 4000 miles later, no improvement. Then I thought tight spots in the final drive chain were the cause, but replacing this didn’t help either

A friend and local mechanic has had a look and discovered that there is some play at the end of the crankshaft, visible when the circular cover is removed from the left side engine case and a screwdriver levered against the nut on the end of the crank. He is reluctant to investigate further as he believes that at 90 odd thousand miles the crank cases will have suffered a lot of strain where the main bearings locate.

Is he right?

I don’t know yet if the (new) bearing is at fault or if where it locates in the crankcase is damaged. If the case is damaged, is it practical to repair?

Is it worth spending even more money on?

Thanks in advance for your thoughts

Mark
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  #2  
Old 14 Mar 2008
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A few mates who have had jap motors fail have gone for the second hand engine approach this is a bit of a risk but most of the dealers will give you a warranty and it means you have lots of spare bits for your engine and it sometimes works out cheaper than an engine rebuild
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  #3  
Old 14 Mar 2008
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Depends on if your the kind of guy that would put a old loyal family dog down instead of caring for it in it's old age.

If your attached to the ride then fix it, if not take it off roading set it alight and claim on the insurance, oh I had an accident on the bike it went down and caught alight, pay me now!

Legal disclaimer I in no way condone this kind of activity as it would not be legal.
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  #4  
Old 15 Mar 2008
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Play

The crankshaft bearings on the XT/TT models have seemingly a lot of play when cold. For info, the designation is C3. This has been designed like that because they get hot ... hot, the play when cold serves a purpose. Therefore, checking this with a cold engine does not tell you much.
If there really is play between the bearing outer race and its seat, I'd guess you would already have had a big bang ....

I would rather check tightness of all the bolts holding the engine to the frame, it is not for nothing that Yamaha prescribes high torque values for these.
Check this first:
- The 3 bolts on top of the cilinder head: 58 Nm / 43 ft - lb
- The 4 bolts connection the lower end with the front frame downpipe: 58 Nm / 43 ft - lb
- The swingarm shaft / bolt: 100 Nm / 73 ft lb
- The 'hidden' bolt below the swingarm shaft, something like 80 Nm

Tighten up and see how that helps. If not, post again.

And, remember, it is a large single .... not the best design to imitate turbine like running behaviour. Ever compared with another one?

Auke
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  #5  
Old 20 Mar 2008
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Thanks for your replies

All

Thanks for your replies.

I have checked that all the engine mounting bolts are good and tight, though not with a torque wrench.

I am familiar with how much these bikes vibrate, I have a 3AJ Tenere to compair it to, and I can remember that when we first got the 600E I was supprised by how little it vibrated (compared to my old XS650 and Z650). It is very different now!

I will compare movement on the rotor end of the crank between the two XT600s, with engines cold and hot, by removing the circular plastic cover andusing a screwdriver underneith the nut to lever it up (without using much force).

However, I was hopeing to get opinions on whether the crankcases are likley to be so stressed by 95 000 miles of use as to make it foolish to rebuild the engine with the intention of using it for another big trip.

Also, if the crank case location for the main bearing is worn, causing the observed free play, is it practicle to repair it?

Thanks

Mark
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  #6  
Old 20 Mar 2008
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are you sure that when they rebuilt the crank they put it back together within 1.5 thousandths? That's typically what I shoot for when rebuilding a crank. It may be that the shop assembled the crank improperly. I'd put my bets on the crank being out of round myself rather than the cases.
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