RC46 front right brake caliper - bolts snapping!

Hi all

The stretch bolts on my RC46 1999 fi x right side front brake caliper keep snapping, even though they are new bolts. They will not tighten to the specified torque of 31nm. Instead the torque reduces and the bolt snaps. I've inspected the threads on the caliper and they look ok, also there is no metal in the bolt threads.

The weirdest thing I've ever experienced doing mechanics!! I've ordered another caliper (used) but has anyone got any ideas why this happens?

yours flummoxed,

Nidgy

 

Yeah I doubted my torque wrench so borrowed a really expensive one and it still snapped bolts at 26nm

 

That makes no sense. I have had the front end apart on all my VFRs multiple times and never an issue with bolts snapping. Are you using genuine bolts?

 

1. Is it #29 that you're snapping?

2. sure you're looking at NM scale on torque-wrench and not lb*ft?

3. are these brand-new never used bolts from plastic bag bought from Honda dealer ?

4. Post photos of broken bolt pieces. Zoom in on 2 sides of break. How deformed is break? Can you re-assemble cleanly or is there lots of distortion?

5. Test fit any other clean bolt by hand (no threadlocker), are you able to spin it all way in by hand?

 

usually, finger-tight + 1/4 -1/2 turn works right----we hope no cheap chinese bolts

 

Yes genuine new honda bolts p/n 90107mt3003

 

1. Yes i think so (21 on my pic)
2. Yes nm on sealey digital torque wrench
3. yes p/n90107mt3003
4. wildo
5.will try this too
6. thx!

 

You're welcome!

I'm thinking there may be something inside bracket threads which may be damaging bolts?

5. Since these bolts come with dry-threadlocker compound, it may be possible there's some build-up or some contaminants such as sand or metal fragments may have gotten trapped inside? Test is to verify you can screw in normal bolt by hand all way in without requiring too much force.

7. Also good idea to verify calibration of torque-wrench. I test them every couple of years just to be sure.

 

Have you run a tap down to clear the threads?

 

Should really confirm there's something messed-up in threads of bracket before running tap through. Better to use thread-chaser in this case.

 

Even with damaged threads, surely the bolt will bind up and stop turning and the torque wrench will click. That won't break a bolt. Maybe if something was well off square so the bolt head wasn't parallel to the surface it was tightening against which would create a sideways force on the shaft.

 

Very odd. Are you putting anything on the threads? Lubricants throw off torque readings.

Ciao,

JZH

 

This is 8mm reduced-shank 10.9 bolt. Reduction to root-diameter of threads at beginning has some benefits:

- gives bolt more "springiness" to handle loads, extra stretch reduces damaging first couple of engaged threads where highest loads occurs
- similarly, it can absorb more displacement/movement before getting into plastic-deformation region
- handles vibration and fatigue better than straight-shank bolts
- only downside is it overcomes ultimate strength at lower torque

Torque range for this bolt is about 30-40 NM. Threadlocker is used for extra security since Honda's specs are at low-end of range.
Torque required to break this bolt is in 40-50 NM range, about 15-20% lower than straight-shank bolt.

Breaking Honda OEM bolt at normal specified tightening torque leads me to believe there's calibration issue with torque-wrench.
I'd measure its calibration just to be sure...

Without any conclusive evidence and data, I'd consider converting this to stud.

 

Hi all the issue is resolved! It turns out that 27 years of compressing the fork lugs (on which the caliper mounts) had reduced their thickness. The bolt/caliper are tapered so that they will not screw all the way through the caliper mount. What was happening was that the reduced thickness meant the bolts had insufficient purchase on the lugs before running out of thread at the end of their taper.

I fitted two thin washers and it torqued right up!!

 

Better to use thread-chaser in this case.

 

Would if I had one. I have every other size, but my bro borrow the size I need and yup broke it!

 

M8 x 1.25 is the one you need. Cheap as chips, fortunately.

The only "unusual" thread used on VFRs is the M10 x 1.25 ("normal" is M10 x 1.5), which is used for every M10 bolt on the bike, except for the fork internals and brake calipers, where you will sometimes find the even more unusual M10 x 1.0 thread.

Ciao,

JZH

 

Hey, good job figuring this out!!!

Odd that bolt was bottoming out and not clicking torque-wrench though. Strange.