3 replies to this topic

[mcurtiz]

Upgrading my son's 82 Mini to disc brakes. One of the used calipers has a snapped-off bleed nipple (and a failed attempt to remove the nipple by the previous owner).

I have removed the corroded broken nipple remnants but the threads are not usuable.

I am going to make an (M12) insert tapped and sealed into a new larger tapped (M12) hole in the caliper. I need to tap a new hole in the insert to take a nice new bleed nipple - Does anyone know the thread/size of these nipples in an AP caliper. It looks like 3/8" UNF (24tpi).

Mark

"Freedom's untidy, and free people are free to make mistakes."
– Rumsfeld, adding "stuff happens"

[dklawson]

Broken caliper bleed nipples come up frequently on the board. I can't answer the thread question but I feel that regardless of the thread size you select, this is the time to point out a few options and suggestions.

In the U.S. you can buy (from most better automotive parts stores) a bleed nipple repair kit. It consists of a special bushing with pipe threads on the OD and a bleed nipple (with seat) on the inside. You drill and tap the caliper to accept the repair kit, install the bushing and you're done. Very tidy. My only complaint is the bleed nipples are usually very small (1/4 UNF threads). Still, they work, they are inexpensive and they are easy to install.
Sorry, this was the only online picture I could find for the repair kit:
http://www.speedwaym...2200063.F_R.jpg

It sounds like you are considering re-tapping the caliper for the next larger bleed nipple size. Keep in mind that if you re-tap the caliper for a larger thread size (or even chase the existing threads to clean them up) the bleed nipple isn't likely to run "true" and on center. The conical point will therefore have trouble closing the existing seat. There is a work-around that someone on this board posted many years back (I wish I could take credit for it but I can't!)

Drill and re-tap the caliper for the next larger size bleed nipple. (Again, sorry I can't help with the size you have now or what's larger). Bottom tap the hole but be extremely careful to NOT damage the existing conical seat in the caliper's bleed hole. Find a small steel ball (like out of a ball bearing). The ball must be about twice the diameter of the hole in the bottom of the caliper's conical seat.

The next step sounds like herrasy. Take your brand new bleed nipple and grind off its sharp conical point leaving a flat face. Drop the ball bearing into the caliper bleed hole and follow it with the flat-faced bleed nipple. You now have a bleed nipple that will push the ball into the conical seat for closure and it is not affected by the looseness of the damaged threads.

Good luck and let us know what you do.

[mcurtiz]

In the U.S. you can buy (from most better automotive parts stores) a bleed nipple repair kit.

I have seen these, however I am located in the UK and need to get the fix in place sooner than I could source from the USA.

It sounds like you are considering re-tapping the caliper for the next larger bleed nipple size.

No, I will still use a standard-sized bleed nipple. I have drilled and tapped (M12) a larger hole in the caliper aligned to the bore hole in the caliper. I have also now partially prepared a 'threaded' insert (made from an M12 bolt that will become headless once the thread for the nipple is done). A 3mm dia. hole runs right through the "bolt" centreline. I next need to drill/tap the hole around 12mm deep for the bleed nipple, in the 'bolt'. The bleed nipple will seat in the new insert, with the cone precisely closing off the 3mm bore hole, and not the old seat in the caliper. If the seal is not good despite the effort, I can add a corresponding chamfer for the cone to seat (a modded drill bit will do that).

I do like the ball-bearing idea too and will be a good back up IF my concept fails, so thanks for that. And yes, I will update the thread with the outcome.

Why don't I just get a new caliper? GBP85 / $160 for a recon caliper is why; plus I like a challenge!

Mark

"I'm not into this detail stuff. I'm more concepty." Rumsfeld

[dklawson]

Why don't I just get a new caliper? GBP85 / $160 for a recon caliper is why; plus I like a challenge!

I can fully appreciate both aspects of your decision and direction.

I was hoping someone in the U.K. would post a source for the bushing repair kit with the pre-installed nipple. Surely someone there must carry them. Regardless, you've got a very good plan and approach. With careful machining I'm sure you'll have this sorted.

Do let us know how it turns out.