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Question about Engine Mounting Bolts on YZ-250?


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Rebuilding a 1994 YZ-250. Putting things back together. The only bolt in the pile the right length (4" , or so) for the Forward-most engine mount to the frame is thick on both ends (8mm?) and tapered (thin) in the center of the bolt. Is this the correct bolt? If so, why is it tapered like that?

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That is the correct one. I don't know why they are tapered like that but is is factory not a result of over torquing.

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Could that thin center be an attempt to let it flex, while being tight? Still, at some level it would be nice to give the engineers a nut kick or junk punch just because of things like this.

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My guess would be that it's just for weight reduction, takes a good amount of material out of the area it's not needed.

In some cases fasteners like this are for torque-to-yield applications, where the bolt is torqued down until the material reaches the point of deforming. Common in automotive head bolts, rod bolts, etc.

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My guess would be that it's just for weight reduction, takes a good amount of material out of the area it's not needed...
That was my first thought, kinda like what they do with spokes.
In some cases fasteners like this are for torque-to-yield applications, where the bolt is torqued down until the material reaches the point of deforming. Common in automotive head bolts, rod bolts, etc.
And dirt bike triple clamps.

When upside-down forks first came on the scene, it was discovered that the upper fork legs could be crushed by over-tightening the tripleclamp bolts.

Torque-to-yield bolts helped keep that from happening -- as it appears RCannon noticed. :banana:

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And dirt bike triple clamps.

When upside-down forks first came on the scene, it was discovered that the upper fork legs could be crushed by over-tightening the tripleclamp bolts.

Torque-to-yield bolts helped keep that from happening -- as it appears RCannon noticed. :banana:

If triple clamp bolts were torque-to-yield it means that properly torqued you'd begin to deform the bolt, and it would only be usable once. Remove your forks and reinstall? You'd need new triple clamp bolts.

Now, being designed to yield if you exceed the proper torque value so you don't crush the fork legs into an egg shape, that's something else all together.

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It happened years back when I did the triple clamp thing.

I had a very expensive torque wrench at the time and was attempting to tighten the bolts to the recommended 15 lb/ft.

I watched the numbers go up to 15, then they just stayed there. I made almost 1 full turn, but the readign did not change. I then decided to remove the bolt and it had this hourglass shape to the center. It must have gotten oil or grease on it.

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If triple clamp bolts were torque-to-yield it means that properly torqued you'd begin to deform the bolt, and it would only be usable once. Remove your forks and reinstall? You'd need new triple clamp bolts.

Now, being designed to yield if you exceed the proper torque value so you don't crush the fork legs into an egg shape, that's something else all together.

I stand corrected (I mis-understood your terms).

I meant a torque limiting (or sacrificial) bolt. Though similar to a torque-to-yield bolt (they both become "plastic" and stretch at a pre-determined torque value), limiting bolts can be reused as long as they have not been "over"-torqued. If that happens you get the situation RCannon experienced.

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