removing lower frame rails under engine

[jscgdunn]

Could it be a purposely designed weak link? These soft M8's shear off before the frame cracks, by design?


[/quote]

Pat,
On my 92 two bolts were sheared off (one on each side) and the frame was cracked on the left side.

Jeff

[simi_ed]

Could it be a purposely designed weak link? These soft M8's shear off before the frame cracks, by design?

Maybe some of that "designed in flex" to avoid chatter, like the MotoGP bikes? 

[markmartin]

I just did this yesterday. I found 3 of the 4 rear cradle bolts sheared off. 
After I "easy out" the broken bits ( why do they call it 'easy out'? Ain't nothing easy about it) I'm gonna drill and tap to replace the soft M8x1.25 button head bolts with some stainless M10x1.5 bolts.

The cradle is already sized for the M10's I wonder why Yamaha used the smaller M8's?

Holy wow Pat!  3 out of 4 bolts sheared off!!??  I'm paranoid now.  Did you have a clue something was wrong? What happens when 4 of 4 bolts shear off?

I get the feeling that I should be checking these out on my bike for myself. Great directions on how to approach the job and what will be encountered. Any further advice is welcomed!

Mark M

[Mark Olson]

Wow this is a great annual  maintenance tip to add to the spring list.

Check engine mount bolts.

[movenon]

Wow this is a great annual  maintenance tip to add to the spring list.

Check engine mount bolts.

+1 
George

[The General]

The properties of stainless steel already make the same size bolt more resistant to shear failure. Going up a bolt size makes it that much stronger.

Exactly my thinking Mark. The 10.2 grade stainless is a step up from those oem Yamaha M8's

Yes, a direct drill and tap into the frame, no inserts. Paul Lawson made some drill bushings which fit into the cradle holes that hold the drill bit dead nuts center for drilling into the frame.

Doug, I'm thinking about your comment on not using stainless bolts.

Now here's the deal... The Yamaha engineers are smart folks.
Why would they design a lower cradle sized for M10 bolts and only put in M8's? ...and not a high grade steel M8's at that. It is a sloppy fit.
This is in a newly designed frame for the rubber mounted late model FJ's. New stuff here. Fresh start.

Could it be a purposely designed weak link? These soft M8's shear off before the frame cracks, by design?

mmmm....I hear ya Pat. I replaced existing Stainless bolts on my sidecar front end with hardened steel (can`t remember the grade) cause my understanding and received advice, is they have a little flex and less likely to sheer....which sounds ridiculous, but have been told such a number of times. But no matter, unless we can work out why that adjusting nut /screw is there on the motor rear, and what is it`s correct adjustment, I don`t think our process of elimination can reach a logical conclusion on why those bolts break. imho   

[X-Ray]

On my bike that rear adjustable engine bolt just spins, the threads are gone. One day I will drop the engine out and redo/check everything. (Might help get me to 100% vibe elimination)

[ribbert]

But no matter, unless we can work out why that adjusting nut /screw is there on the motor rear, and what is it`s correct adjustment, I don`t think our process of elimination can reach a logical conclusion on why those bolts break. imho   

Doug, on my bikes the screw does not and cannot be made to contact the engine in the resting position.

My theory is it's a buffer to limit the torsional movement of the motor in the rubber mounts under extreme load.

With the sprocket well to the left of the centreline, the engine will want to twist when you dump the clutch. I believe it is no coincidence that stop is in line with the sprocket.

The clearance is somewhere around what you might expect from movement under extreme conditions (big and fast clutch dump in first)

I would imagine the adjustment to be no more than a mm or two and no contact under normal riding conditions.

If these are only on the rubber mounted models, it would add some weight to this theory.

It is also reasonable that if this is not set correctly, the load is taken on the frame, it's got to be taken somewhere, there is a huge amount of torque to harness, hence the sheared bolts and cracked frames.

Well, that's my theory.   

Noel

[Pat Conlon]

Mark M, Mark O, George and others, if you find any broken rear cradle bolts and you wish to drill/tap to M10 X1.5 bolts, keep me in mind, I have a drill bushing you can use, sized for the correct 8.50mm drill bit.
This little bugger keeps the drill bit centered in the cradle holes as you drill into the frame.

Noel, if torque twist is the culprit (I also think so) wouldn't the bolts just shear on one side of the cradle?

I just can't get over that sloppy fit with the M8's in M10 cradle holes.

Good discussion, thanks!

[ribbert]

Noel, if torque twist is the culprit (I also think so) wouldn't the bolts just shear on one side of the cradle?

I just can't get over that sloppy fit with the M8's in M10 cradle holes.

Good discussion, thanks!

Pat, this was not a well thought out theory, just off the cuff.

The bolt would just shear on one side and then the load would transfer (and be increased) to the remaining bolts and then the next one shears because it is now taking the load and then the next one............
I believe there would be a point (3 bolts?) where the frame now has sufficient flex to relieve the load on the remaining bolt.
All three bolts are not going to break at once, only as the load is transferred to them, a chain reaction over time.

The motor wants to naturally rotate around the sprocket because the wheel on the ground is providing resistance, it also wants to twist because the output shaft is off centre. What forces are exerted in what direction and in what proportion is anyone's guess.
The adjuster, I imagine, limits that movement so as to not over stress the rubber mounts.

As for the "sloppy" hole that seems to trouble you. Bolt shafts are never a machine fit through the non threaded piece and always have tolerance. Ever noticed how much you can move a car head around with bolts in but not tightened?
Bolts in this application clamp the pieces together, they don't align them, the clamping secures the location, not the bolt. 
It wouldn't surprise me if the holes were a little on the large side by design to provide some tolerance for experienced or anticipated manufacturing discrepancies.
Maybe achieving a fine tolerance fit at that join couldn't happen, it's quite a long piece, joined to another piece that is welded to another piece.....
A lot of stuff on the FJ's has the appearance of being hand done rather than machine done, including some of the welds.
All food for thought........ hmmmmm, snag in bread.......it's dinner time here, I'm off, it will be interesting to see what comes of this discussion but I'm not plagued by the need to know. If there is no outcome, as there was last time the General raised this, I won't lose any sleep.

This also provides a rare (me not knowing it all) opportunity for some punk to put a notch in his belt and prove me wrong. 

Noel

[The General]

But no matter, unless we can work out why that adjusting nut /screw is there on the motor rear, and what is it`s correct adjustment, I don`t think our process of elimination can reach a logical conclusion on why those bolts break. imho   

Doug, on my bikes the screw does not and cannot be made to contact the engine in the resting position.

My theory is it's a buffer to limit the torsional movement of the motor in the rubber mounts under extreme load.

With the sprocket well to the left of the centreline, the engine will want to twist when you dump the clutch. I believe it is no coincidence that stop is in line with the sprocket. (Good point...never thought of that!)

The clearance is somewhere around what you might expect from movement under extreme conditions (big and fast clutch dump in first)

I would imagine the adjustment to be no more than a mm or two and no contact under normal riding conditions. (my thoughts too!)

If these are only on the rubber mounted models, it would add some weight to this theory. ( )

It is also reasonable that if this is not set correctly, the load is taken on the frame, it's got to be taken somewhere, there is a huge amount of torque to harness, hence the sheared bolts and cracked frames.

Well, that's my theory.   

Noel

Thanks Noel, my focus is now on info re correct setting of that screw. 

[markmartin]

With reference to the schematic below, could someone please clarify which bolts are the rear cradle bolts ?  15 & 16  ??

Many thanks,

Mark M

[Pat Conlon]

15 and 16

[markmartin]

Thanks Pat.

Did the head of the bolts turn out with little effort revealing an obviously sheered off bolt?

If I put a medium amount of counterclockwise torque on the bolt and it doesn't turn out, would I be relatively safe to assume the bolts are OK? 

I'm trying to avoid my next post starting with, 'well, they weren't sheered off when I got there, but they sure as hell are now' . 


thanks,

Mark M

[Pat Conlon]

Of the 3 that were broken, 2 were missing the button heads and the 3rd button head fell out when I removed the aluminum side plates. The breaks were flush to the frame.

These button heads are soft, kinda like your oem rotor bolts. I would use butane heat on the frame and a impact driver to get them broken loose. They are easy to deform with a Allen wrench...
Still not to worry, if you do deform them...you can slot the button head with your Dremel and try again with your impact driver, if that fails at least the button head's Allen holes provide a neat centering function for your drill bit when you have to 'easy out' them.

Working with a easy out on the broken bolts (without the button head) is a different matter.  It's hard to get the drill bit centered on the broken piece...That's where the drill bushing comes into play.