Do bolts fatigue enough to warrant replacement

[psalm3124]

Today, I'm putting on the powervalve housing on my 02 YZ250. It uses five bolts with 8mm hex heads. I'm anal, so I'm using my torque wrench to get it to the factory spec'd 7.5 lbs. As I near 7.5, one bolt breaks off!

I'm wondering, do smaller bolts fatigue to the point of breaking off like that? I'm thinking should I replace the small bolts on my bike the next time I remove them.

 

[splatt]

Was that ft/lbs or inch/lbs? Small bolts like that usually last quite a while as long as they aren't over torqued. I usually just run them down tight with a 1/4" drive nut driver and if your worried about them vibrating loose use a little bit of the mid strength(usually blue) lock tight.

Steve

 

[Kav]

I've never had a Yamaha bolt brake, but that said I'm not a big fan of any of the bolts or screws that Yamaha put on my YZ (I have a '01 YZ250). I've had a few of them strip out on me, and any I've pulled out of the engine, never seem to hold or stay in place when I try to reuse them. I'm a manic for using a tourque wrench, shoot I even have a torque screwdriver for some parts. So what I've done is invest in Loc-tight, and buy new screws from Fastenal. They have stores all over the place. And most of the sizes I've needed were in stock. but I have had to order fastners for my bike. I also like to use them because I don't need to have a minium $25 order or anything like that. And no I don't work for them, I just find them very handy.

 

[nickyd]

I don't use a torque wrench on the small bolts - there are too many variables (and too many OOPS in my years) - if you move the wrench super slow when trying to measure the torque, you're going to get a skewed reading and the bolt will be way tighter than it should be. Dirty threads/bolt can skew the readings....

 

[RM_guy]

If the bolt was over torqued in the past it may have yeilded (streched a bit) and when it gets re-torqued it can snap.

 

[barryadam]

At 7.5 ft lbs, there isn't enough torque to break a healthy 6 mm steel fastener, so you might have had one of poor quality material. If you examine the break, sometimes you can learn more about the failure. Fatigue from a stress crack would show up in a different failure mode than failure by torque. Did it snap off, or slowly yield like a piece of licorice? Were the threads or shaft damaged or nicked? Were the threads clean, and were you able to screw the fastener in by hand before assembly? Did the screw "bottom-out" in the base material perhaps? I've seen that before where a variety of lengths are used in an assembly based on the individual location depth requirments, and the wrong lenght is used in the wrong location. Did the torque increase slowly as you assembled indicating a galling or stripping of the threads? Did you use any anti-sieze lubricant?

When in doubt, all new fasteners are a simple solution. Good luck.

 

[psalm3124]

Thanks all for the input. :cool: I love this forum! Better safe than sorry so I'm going to start switching out all the old smaller bolts as I remove them for various maintenance procedures. :bang:

 

[ochster]

It sounds as if you are using a foot pounds rated torque wrench? In my opinion you will not find much accuracy in dialing in 7ft.lbs on such on tool. I would convert the spec torque to inch pounds and use a quality 1/4 inch torque wrench.

There have been some great threads in the past that have covered this topic indepth, you may want to try a search that can shed some light on the tools themselves.

 

[RM_guy]

Keep in mind too that most torque values are for dry threads...no lube. Lubed threads should be torqued at 75% of the dry value. Friction is a component of the torque and the dry torque value overcomes the friction. Lube reduces friction and you don't need as much torque to properly preload the bolt.

 

[bikepilot]

Also, most tq wrenches have a certain % error, usually 5-10% on cheeper ones. This percentage is typically not of whatever you set the tq to, but of the maximum. That is, if you have a 100ft/lbs tq wrench and you set it to 7ft/lbs you may actually end up with 17ft/lbs of tq. Typically you should try to only use the top third of a tq wrenches range. For small fasteners I just go by feel, I've never had a problem in many years of wrenching (both professionally and on my own stuff).

good luck

 

[psalm3124]

What's the difference between titanium bolts and steel? I was talking to Huffman's mechanic and he uses titanium on his seat bolts, radiator shrouds, and the right side case cover. He feels more secure with titanium than with steel in those places. Is it worth it for a commoner like me?