21 replies to this topic

[RobWill116]

Hi all.
I brought a 7.5" Cooper S set up a while ago and I'm at the stage of bleeding the system. On one of the caliper bleed nipple threads it appears that two threads have been cross threaded but the rest are perfect (not quite sure how this was done! :S).

But would it be possible to just clean out the thread by using a tap? If so, does anybody no what thread size tap I would need?
If there is any better way to sort this without drilling or using helicoils.

Thanks

[Cooperman]

You can't take any chances with the bleed screw threads as they are safety-critical. You need to fit a Heli-Coil. Size is 3/8" UNF. When drilling make sure you don't hit the base of the bleed screw hole as it has to be the correct taper angle.

[RobWill116]

Thanks for quick reply. So I need to drill the thread out and fit a heli-coil. Wouldn't the thread be stronger if I cleaned out the two threads? They are central and all the others are perfect so would take the pressure. Just want to double check before I go and start drilling things!

[Cooperman]

When you Heli-Coil a brake bleed screw hole the drill must go in very accurately so that the bleed screw will tighten down into the taper at the bottom without leakage.
The correct tapping drill for the 3/8" UNF will remove the damaged threads completely.

[RobWill116]

Sorry to keep asking questions, just inexperienced in this area! So I need a 3/8" drill bit, and then a 3/8" heli-coil?

[Cooperman]

No, the drill bit for a 3/8" UNF Heli-Coil is specific to the Heli-Coil outside diameter thread size. You also need the Heli-coil tap and insertion tool. What you do is contact Namrick Limited who sell the kits on-line.
Then you drill with the tapping size drill in the kit, and using a tap wrench you tap that hole to thread it with the tap provided. You then wind in the Heli-Coil with the insertion tool and snap off the tang on the inner end of the Heli-Coil. Then you fit the bleed screw and 'jobsagoodun'.
Actually I think the thread inserts sold by Namrick are called 'Re-Coil' kits, but it's the same thing.

[dklawson]

I disagree.

Yes brakes are a safety related area. However, you are MUCH less likely to have a good outcome by trying to drill and tap for a Helicoil than by chasing the threads that are already there on the caliper. The existing tapped hole is already concentric with the seat. To fit the Helicoil properly you will need to fixture the caliper accurately so everything remains "on center".

Measure the brake bleed nipple for the thread size. From memory I believe it is 3/8 UNF. There are three styles of taps, taper, plug, and bottoming. Get a new, sharp plug tap and chase the threads in your caliper. Once the damaged threads are re-cut, you are done. Do not go deeper than you have to. Make sure you flush out all the swarf.

As footnote #1 my advice is to put a little anti-seize compound on the threads of the nipple prior to installation.

Footnote #2 is to suggest thoroughly degreasing the area around the nipple once the brakes are bled so you can monitor for any leaks around the nipple.

And footnote #3, when you fit these calipers, remember that the bleed nipples go to "UP" at the top of the caliper, not at the bottom. You won't be able to bleed the calipers with the nipples pointed down.

[RobWill116]

I've ordered some new taps, got a set of all three. I will clean the threads out and see how it goes, if it appears to leak I will look at the heli-coil route.
Thanks for all the advice.

[mini 4o]

Hi
A new caliper from mini spares is only £84, is it really worth risking your life and mini?
Just try to stop at 30mph using only the hand brake, knowing the foot brake works.
John

[dklawson]

is it really worth risking your life and mini?

Bleed nipple problems are not really quite as dangerous as say... re-using old seals or friction materials. The issue, the failure mode, is leakage. The nipple isn't going to "shoot out" under braking and leave you with a sudden loss of stopping power.

Let's take one quick paragraph to do some math. The nipple is 3/8 UNF. Let's say that the nipple does not seat so ALL the brake fluid pressure is felt by the major diameter of the thread (very unlikely). Further, let's say that your pedal effort creates 1000 PSI in the system. 1000 PSI x (3/8 squared x Pi ) / 4 = 110 pounds. That is almost nothing when there are three or four threads in engagement. So.. this is NOT going to be a catastrophic failure. Again, the failure mode is leakage and you will know within moments following a static test if you have a problem. The nipple is either going to seat or it is not.

Still don't like the idea of chasing the threads? There are alternatives you can pursue at home. See the product in the link below. I have used similar on VW calipers rather than replace them. The product does work and is a better solution than a Helicoil because the whole nipple/seat assembly is inserted as one. This eliminates the possibility of concentricity errors.
Nipple repair kit: http://www.levineaut...ederrepair.html
Someone in the U.K. is bound to carry a similar product.

[mini 4o]

hi doug L
Ok that’s your take on the problem,

But in the UK if you are involved in fatal/ or near fatal traffic accident, the police take the car for inspection,

oh look brake fluid loss or no bleed nipple = brake failure, who’s to blame, the police always look for blame.

John
Foot note
thanks for the bodge link

[dklawson]

Applying the logic that repairing a brake part is unacceptable because of liability reasons... you would only allow professionals to install the pads/shoes and replace any worn brake components. Extending that argument, the shop selling brake parts could be held liable so they shouldn't sell pads, shoes, rebuild kits, or replacment cylinders. I hope it has not reached that point in the U.K.

That link is not to a "bodge" product. It is a much more practical DIY repair than trying to Helicoil a bleed nipple hole. If it were considered a bodge by your MOT boards or motor vehicle regulatory agencies I am sure the kits would not be legal for sale in the U.K. The following links are for two such items that are for sale in the U.K.
http://www.ebay.co.u...c-/170683443953
http://www.hellopro....fr-societe.html

From the OP's opening post: " it appears that two threads have been cross threaded but the rest are perfect"
I cannot stress enough that the failure mode is leakage, not shooting the nipple out. If you Helicoil the caliper the nipple is not going to be concentric with the seat and it will leak. I can all but guarantee you that if you took a caliper meeting the OP's description to a machine shop, they would chase the threads just enough to restore the fit of the bleed nipple.

[peter-b]

there is a repair set for them

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[Cooperman]

The last time I had this sort of problem the existing thread really did look very 'grotty'. I considered trying to re-tap, but as the caliper was already in two parts I decided to Heli-Coil. It did work well with no leakage when fitted and it gave me confidence that the job was right. With hindsight I guess I could have tried re-tapping/thread chasing, but if it had leaked it would have all had to come to pieces again and the caliper be re-split for drilling and tapping.
Whichever way you do it, so long as it doesn't leak when the pedal is pushed down as hard as you can then it's fine.

Edited by Cooperman, 30 September 2011 - 10:15 AM.

[RobWill116]

Thanks for all the options given! As I first stated, only two of the threads are cross-threaded and the rest or fine. So I am going to firstly clean out the threads with a plug tap and see how it goes. If afterwards it does show any sign of leakage, I will then look at Uncle Cooperman's suggestion of using a heli-coil and also Tech Doc Admin's suggestion of the nipple repair kit and decide whats best option for me.
Thanks for all you help. I will post the outcome of cleaning the thread out!