Seized 660cc motor : what happened ?

[samandkimberly]

I realize you asked this to Bill, but since I'm sitting here typing...

a little play on a shaft driven by straight cut gears is no problem at all, too little play can be a *disaster* . I wouldn't mess with it.

My question to you Bill....
What are you doing about the slight side to side slop on the balancer gear ?
Are you going to shim it up some ?
best,
henri

[hb7]

It's the play in the woodruff key slot....which troubles me.
Replacing one cheese like woodruff key with same thing.....
seem wrong. I suggest making a key from better steel.
Here is the gasket #

Clutch side :
GASKET CRKCS CVR 3 for Yamaha
4DW-15462-00-00

[Bill Jurgenson]

hardened keys are available. What is the result? The slot in the shaft ist widened even more, eventually ruined.
What is cheaper? Crank or key?
The purpose of the key is above all to postition and not to retain. The torque on the nut should suffice to to hold when everything is clean.
Do not use Locktite on the nut. Use a torque wrench: 120nm. Be sure all surfaces are clean and use assembly grease on the threads.

There should be be very little slop in the key slot of the gears, The crank slot will naturally have a little play.
The problem ist not in the slot or the key. It is the cushdrive of the gear itself. Whether or not you have any problems at all is dependent on:
- variation of tolerances within the series itself;
- care of engine;
- driving style and use.
But the problem remains real and is today just as much a problem with the new xt660 as it was with the xt 20+ years ago and the xtz because it is one of design.
The only real cure is a different balancer gear.

[hb7]

"The purpose of the key is above all to postition and not to retain. The torque on the nut should suffice to to hold when everything is clean."
120nm = 90 ft lbs.

"The package is then actually only held by the key; both primary gear and balancer gear can turn a bit, slapping against the key in its race. the balancer gear has a recoil spring assembly as hub, so it has quite a bit of backlash and this aggrevates the slapping so the the key itself gets compressed from both sides. "

Cushdrive for the balancer gear....? I don't see any....?

Yes, balancer woodruff key is at 4 o'clock and the slot where it belongs is at 12 o'clock.
Balancer bearing behind it broken through the case.






Did you install the Slipstream gear ? How much is it ?
Any pictures?

best,
Henri
http://moto-science.com/

[hb7]

I would speculate that the wood ruff key will most likely shear off
in 1 or 2 gear. Highly unlikely to happen in 5th gear at crusing speeds.
The one key which I know about, sheared off doing a monster wheelie.
Sheared off the rotor and the balancer keys.

Not me riding, I only bought the wreckage.

[Bill Jurgenson]

here, both gears together;


We are not talking about the gear on the balancershaft, altho I have seen one ruined balancer:

Here, instead the keyway of the balancer is shot beyound repair and just shy of giving out.
This is unusual, tho. After;peening the keyway backdown and removing the weight entirely,I am using this shaft in a balancerless engine.

The gear cost 80£ from Slipstream and yes, I have installed it. I install it in every engine I have to open.

[billr]

For those of us "newer" owners that have read through this thread and are wondering when the bullet will bite...
What mileage did the ones involved have on them?
When is a good time to check this area of the motor?
Has this been limited to bikes ridden aggressively or just running down the road?

My Traveller has 16k miles now.
Looking at that last pic Bill J. posted, I'm about ready to tear into the case and look.
Bill R.

[hb7]

If understand correctly from the pictures, Bill is already using the slipstream cush drive intermediate gear and yet he still
has some impending failure marks on his woodruff key.
I think the failure happens when you suddenly pop the clutch in 1st gear.
This is the most stress on the woodruff key. I can't see this happening in 5th gear.

[Bill Jurgenson]

you did not read correctly!

first the Slipstream gear is the one on the left without a cushdrive, the other is the OEM item, and
Secondly, I just fit this gear to the SZR from which the eaten up key came.
thirdly, I did not write anything else nor write confusingly.
My Skorpion has the gear as do all the engines I have rebuilt in the last two years.
I also wrote that there is no typical mileage. My Skorpion had slightly over 30tkm. i.e.20000mls and I am aware of others sith such low mileage.
And from what I saw in the SZR last week, the key must have sheared of in that one, too. Since I got that one with 32tkm it was about the same.

[hb7]

Thanks for clarifying that. It is the opposite of what I would expect.
Can you explain why the cush gear drive causes this problem ?

best,
henri

[Prosper]

I have all good internal parts to complet a bottom end of these engines including balancer crank complete set of parts inclunding as well the cases if your interested not sure if you are planning on rebuilding if so I have the parts and will sell as needed for the right price had the same poblem bought the parts but then found and engine comple-- the search for these parts will turn up expensive and few and far between
Cb

Ps I will buy the parts bike if you are not going to rebuild

[samandkimberly]

That cush gear seems to me to be an awful lot like the workings of an impact driver. If those springs aren't strong enough then the acceleration/deceleration of the crankshaft is going to act just like a hammer working against the more smoothly rotating counterbalancer.

I'd love to know why Yamaha puts it in there. It's obviously a lot more expensive than using a solid gear like the slipstream, and Yamaha engineers aren't idiots. Not to say that replacing it isn't a good idea.

Sam

Thanks for clarifying that. It is the opposite of what I would expect.
Can you explain why the cush gear drive causes this problem ?

best,
henri

[Bill Jurgenson]

You've got it almost right. Under heavy, fast riding, with lots of shifting you have.
At the extremes of the useful RPM range, idle and just above, the oscillation of the balancer ist very much at odds with the rest of the engine and it slaps back and forth all by itself. That is why it is not possible to set a smooth idle with this engine.

For what it is worth, the similar gear in the Raptor 700 is solid!

may be they did learn something.

[hb7]

2009-2010 700 Raptor US model still shows springs....

[samandkimberly]

Odd; according to the parts list at Bike Bandit it *is* solid:

The gear goes for $63.71, but the mating gear on the balancer has a different #, so no idea whether it could replace the one with the spring.

Sam