27mm 1.25 Pitch Tap

[Jughead]

I repair motorcycles for a living, and recently has to remove a gearbox from a BMW R1150GS. The swingarm, which houses the driveshaft, pivots inside the aluminium gearbox casing. Unfortunately, whilst removing the pivots, one of them went threadbound and ripped the thread in the casing.

This necessitated making a 27mm 1.25Pitch tap, and a jig to make sure the repair was straight and inline with the opposite side. I started off with a 30mm piece of EN36.


Added some marking dye


And single pointed a 1.25mm pitch thread.



Drilled and reamed a 12mm hole down the center


Then took it over to the milling machine for a few flutes.


8 in total


Then flipped it over to cut a square on the back end


Done


Looks a bit messy after the heat treat, but nothing a scotchbright wheel cannot solve


Then made a small guide adapter for the opposite side from and old buggered pivot


And finished of with a 12mm stainless guide shaft.


So it works like this
The guide adapter screws into the LHS pivot hole and accommodates one end of the 12mm shaft


The newly made tap slides on the guide rod and is used backwards and exits the hole on the RHS, repairing the thread as it goes. This ensures that the thread is exactly in line and there is no chance of cross threading it, which is very easy with such a large (26mm) tap.






And with a little black added.

[olderdan]

Nice fix, I had to do similar for the exhaust ports of my veteral AJS.

[Frank S]

A hollow tap with internal guiding rod is a rarity indeed and a unique way to insure you started straight in the original undamaged part of the thread.
Not saying they don't exist, but I don't think I've ever seen one other than line taps for tubing fittings

[motorhead]

I have worked on a lot of different types of machinery in my life lifetime. Everything from air conditioning to Nuclear power plants on Navy surface ships and submarines to gas turbine powered generators and cogeneration plants as a civilian, and there have been countless times a tap like that would have been a dream!
I'm almost ready to retire now and I think that i just might have to try making some of those for m y sons who are in the trades.

[oldmachinist]

I always seem to need a special tap like this for blind holes. sucks to be me.