Brake Master Cylinder Cap Thread Repair

I have to dress up the threads on my wildkart master cylinder which of course are aluminum.
What repair techniques have worked well for you? It’s an M10 thread if I recall correctly.

Some options…

Dress up threads with a tap.
Replace cap with tapered brass plug.
Replace master cylinder.

I actually had a tapered brass plug in there for a long time, but the head is worn. Problem is I don’t actually know the specs on it. If I can find a replacement I may just go that way and buy multiples.

Got a photo? I’m just wondering how bad it is. Try the tap if you have one, a thread chaser kit is a good investment.

Maybe you used 1/8-27 NPT (national pipe thread) plug? Also, a 1/8-28 BSPT (british standard pipe thread taper) is available.

According to original plug is 10mm.x1.25mm metric fine, or 0.394 nominal x 20.3 tpi.
As Dedo suggests it has probably been replaced with a 1/8 NPTT taper plug, 0.390-0.400 dia. 27tpi.
So alternatives are.
1.Fit a new taper plug as above.
2.Tap thread with 1/8 NPT tap and fit new taper plug.
3.Tap to original 10mmx1.25mm metric fine and hopefully find a longer plug to ensure decent ’ engagement.
4. Tap out to 7/16 UNF (0.437"x20 tpi) and fit plug to suit.
If you have the ‘pumper’ m/cyl with integral reservoir, you should be able to fill and bleed the system from the reservoir so the ’ plug 'can be loctited in place.

1 Like

I went with a 1/8” plug and it’s working splendidly so far after a day at the track. The thread for the cap in the master cylinder itself is (well was) M6.

I did put a little plumbers tape on the cap although I’m not sure if that’s best practice or not.

One thing to note is to avoid a hollow plug otherwise you’re adding to the volume of the master cylinder which leaves you you with a longer pedal and a small pocket of air in the cap.