caution on caliper refurb

[Chris B N]

Hi Guy's
post re caution on caliper overhaul.
3 weeks ago did a full caliper strip down, clean, new seals etc and repainted Matt black with high temp paint with red NSX detail. Look the dogs B------- as they say.
A week later going down to Gloucester for a family get together
as I was pulling into the Trowel services on the MI ( wife needed a pit stop)
at around 15 mph going into the parking area touched the brakes and
got a hell of a bang behind me
Parked and investigated, the offside upper rear caliper mounting bolt ( one of 2 fastening caliper to hub) had disappeared and the caliper thrown forward into the wheel. Fortunately no damage other than losing adhesive weights on the inside of the wheel. Definitely very lucky, could have been a disaster at speed.
No idea why. The calipers were remounted using original bolts torqued to the correct 80 ft lb.
the only explanation I can come up with is possibility of dirt under the flange on the bolt or maybe new paint not fully hardened scraped up under the bolt flange which then shrunk as it heated just enough to release a little tension, vibration during use causing the bolt to loosen.
Since then new bolts on the caliper and all 4 calipers lock lighted and re torqued up again to 80 ft lb.
Both before this event and since the caliper overhaul has mad a significant improvement to brakes.
Would appreciate any comment or thoughts to why other than my possible/plausible explanation.

[Kaz-kzukNA1]

Hi, Chris.
First of all, I’m glad you (and the passenger) were safe and as you wrote, you were extremely lucky that it didn’t happen at higher speed.

From the torque figure, the bolt you are talking about is the one at the calliper bracket.
It’s corrosion resistant bolt and to be torqued at 110Nm that is almost the same as the wheel nuts.

I have never seen this calliper bracket bolt getting loose on any of the NSX even on a corroded or painted bracket. In fact, without the breaker bar, I can't even loosen it.

Everyone has different procedure and I don't want to interfere with your work so I just want to share my experience.

During the brake services, I always torque the bolts in a specific sequence and also use red, yellow, black, white paint markers to show that the bolt was torqued to the spec.
And then, once the brake overhaul service was completed, I’ll double check for the presence of paint marker on the specific bolts as well as using ‘one finger torque check method’ on all of the bolts to make sure nothing is loose. You just use one finger on the tool so that you won’t be able to apply big torque but enough to ‘feel’ for any movements.

Addition to this, I keep two set of torque wrenches for each ranges. This is to keep one set while the other set is being re-calibrated. I re-calibrate them once a year but probably not required if you don’t use them frequently. Another reason why my wife travels to Japan twice a year as she is carrying my tools…

As a side note, if your calliper moved forward and also you lost quite big bolt, I strongly recommend checking your brake hose, parking brake cable and surrounding area for any damages. If the lost bolt hit the wheel or driveshaft, it could have thrown at high speed against something around that area.

On the rear, I always disconnect the brake hose bracket to swing the calliper/hub away during the driveshaft overhaul so I don’t know how far you can move the calliper forward without stretching the brake hose to the worrying level….. You don’t want to drive your NSX with brake hose waiting to fail.

Kaz

[Chris B N]

Hi Kaz,
thanks the comments, everything was checked for damage etc when the new bolts were put in.
the fact that it came loose at such a slow speed was not only lucky but the reason there was no damage other than the slight scrape on the inside of the wheel.
It is worth noting that as I have spoken to other people/mechanics ,several said they always use lock tite on the caliper bolts
Chris

[Chris B N]

Hi KAz,
should have mentioned have new goodrich hoses on and as these are smaller diameter the hose moves a little easier where it passes through the hub
as the caliper hinged forward it only moved about 2.5 cm ( the clearance between caliper and inner wheel so hose was not stretched.