Caliper Mounting Holes on Fork Legs: Can They Be Tapped?

[ViperRon]

My take on the issue is that there is no sheer force worry in mounting the caliper. The issue is really with having a solid mount that can resist the vibration without loosening up. Brakes and their parts experience vibrations and various harmonics which have different effects on different material. I am sure that the size bolt that is used is to handle the torque to resist vibration rather than sheer force. Actually a softer bolt may be better, I do not know. I do know the main issue is to get the same torque and solid mount as what was originally there and check it to make sure it does not loosen. A back nut insures it can not loosen a lock washer can help.

I wish you many worry free miles.

[crazy steve]

Everybody is over-thinking this..... Remember, probably 90% of the bolts on these bikes thread into cast aluminum (including all the caliper bolts), not steel, so there's your limiting factor on thread strength. With that said, as long as thread engagement is roughly the same you'll have the same strength bolted connection (all else being equal). Having the proper torque is more important, as that determines bolt stretch, and that's the 'spring tension' used to keep the bolt tight in use. Lock washers are used where possible movement may exceed bolt stretch. Less precise non-machined mounting surfaces, things that vibrate/flex, or where you have gaskets that allow more movement is where you'll find lockwashers. Note that you'll find very few lockwashers inside a motor; they allow too much movement here.

As to the difference between bolt grades, I'm neither a metallurgist or an engineer. But I have picked up some info over the years... As bolt grade goes up, so does tensile strength, but bolt stretch/elasticity goes down. In other words, a grade five bolt will allow a wider range of stretch before failing compared to a grade eight. In terms of shear strength, much the same; the serious off-roaders have found that grade eight bolts will break under shock loads where a grade five won't, and even the hot rodders are experiencing failures in some applications . One important thing to note; if you cross-reference bolt sizes/factory torque specs to a 'standard' torque chart for metric bolts, you'll find that the OEM specs are almost all at/below the specs for a grade '8.8' (the lower of the two main ratings for metric bolts), roughly the same as a grade 5 SAE. Very few (if any) of the 'standard' factory bolts are the higher grade; there's only a few 'specialty' fasteners where that may not true.

One last thing about fastener strength; thread length engagement. And I consider this more important than bolt grade. The 'rule of thumb' is you want the bolt turned in about 1.5 times the diameter of the bolt to get full strength. So a 10mm bolt should have 15mm of thread into the bolt/hole. The caliper mounting tab on the 'standard' forks is 14.5mm thick, so you're only missing it by .5mm, or a bit over 3%. If 96% of the factory 'strength' isn't enough, then the whole OEM mount is suspect IMO...

With all this said, on a street-driven motorcycle it's very unlikely to make any difference one way of another. The weight/stresses involved are pretty low compared to automotive. As long as you make a reasonable effort to semi-duplicate the factory design in terms of bolt size/attachment point, you'll be fine. If a higher-grade bolt or extra nut makes you feel better, do it, but it's almost positive that it will be overkill.