250 Engine mount bolt failure

[Darrell]

I give my bikes an occasional pre-flight inspection. This time I noticed the hex head of one of the two rear engine mounting bolts on my 250SL had snapped off flush with the shaft.

Has anybody else experienced a failure with these bolts, or is my experience likely a one-off?

My options are to replace it with a NOS bolt (M10x250mm), or source a 3/8" diameter bolt from an industrial supplier. The latter will be Grade 8 rated so I will know it has at least adequate tensile strength. However, the diameter at 9.525mm will be slightly less than stock.

What does the collective experience and wisdom of the Hodaka world think? Thanks in advance.

[thrownchain]

It may have failed for a couple of reasons. First would be a defect in the bolt, second is it was loose and fractured the bolt head off, third is it could have been massively over tightened and the bolt was stretched and failed. We're any of the other bolts loose? Are the bolt holes on the frame round? Or egg shaped? I'd go back with the stock set up.

[Darrell]

It may have failed for a couple of reasons. First would be a defect in the bolt, second is it was loose and fractured the bolt head off, third is it could have been massively over tightened and the bolt was stretched and failed. We're any of the other bolts loose? Are the bolt holes on the frame round? Or egg shaped? I'd go back with the stock set up.

Nope, the other three are still tight and there's no evidence of ovalling or vibration damage to the bolt holes, which might indicate a sudden failure of the bolt.

[thrownchain]

Replace with a stock one and hope for the best.

[viclioce]

May just have been a rogue bolt. I’d replace it with whatever you trust! Riding without trust in a product always leaves a little uneasiness while riding. Victor

[Darrell]

Replace with a stock one and hope for the best.

Hope springs eternal! I sent off and order for that bolt and some other stuff.

And then I just noticed that a tank gas tank mounting bolt also sheared off flush with the boss on the steering head

I think I could use an extended warranty...

[TheBevman]

Darrell, I have two bikes that each have one of the front tank mounting bolts sheared off. One the DPO's really did a good job of NOT getting it the one in my M94 and butchered everything around it and the other is just sitting their flush on my Ace 100. Let me know if you come up with a good way to get it out. Normally, I'd center drill it and use and easy out but M6 is so small and I think I'd mess up while drilling the hole. Good luck!

[matt glascock]

You can use a small broken screw extractor. They are available for screw/bolts of that size. I have managed to work out small screws/bolts using an electroengraver held to the broken screw end tangentially. Use a bit of penetrating oil (Kano-kroil is my fave - spendy but very effective) a day prior to the attempt. You have to fiddle with the angle and approach a bit to get the screw moving in the right direction but all you need is to extract it enough to get purchase on it with a locking pliers. Vibration more so than torque is the force applied to the job. Either way, mask off the area carefully to reduce the chance of marring the frame finish.

[Darrell]

It was a nail-biter but I got the broken tank bolt out using a double-ended damaged screw remover bit. One end drills (counterclockwise) a divot for the other end that does the grabbing and extracting. These bits are actually designed for damaged, but intact, Phillips and/or square drive screw heads.

The fracture point was actually slightly recessed and a loose thread was causing some binding that made the first turn the hardest.

The worst case, I think, is if the broken bolt bottomed out in the frame mount/boss and was binding under compression. The only way out of that "bind" I think would be to drill it out straight through the steering head then re-tap a size up. In that case a thread locker might prevent any grease migration or sweating around the threads.

[Hosk1956]

Interesting, I am just rebuilding my 250ED and not that same bolt has been replaced with a standard M10 bolt as well, I was assuming the previous owner just lost the original (but how?), now I am thinking it may have sheared of as well

Wayne from Oz

[dirty_rat]

I have had good luck removing broken off tank mounting bolts using bolt extractors. The main thing is to insure that you are drilling in the center of the bolt and take your time. Then insert the bolt extractor and work it out (using penetrating oil and heat). Don't force it. If the extractor breaks it is impossible to drill out as it is hardened. On any that won't budge, I drill them out completely and put in a 6mm heli-coil, to keep the bolts consistent.