Picador pulley grub screw thread

[oldchadders]

I am trying to establish the thread used on the grubscrew on a Picador pulley.
The pulley is marked 2½ inch and fits a 5/8 inch shaft. I suspect it is quite old (pre-metrication). The grubscrew which is in it has a slotted "head which is badly rusted and worn Because of the wear and relatively small size it is rather difficult to measure, but the diameter measuires 0.24" with a digital caliper and the tpi seems to be around 18 or 20 - my first guess was ¼" UNC or BSW. I tried the former and it is much too large in diameter and a 3/16 BSW is much too small. M6 seems to fit - ie it starts in the thread on the pulley, but is much too fine a thread.

 

[dave-in-england]

It is probably olde worlde 1/4" UNC

You could try tapping through the thread with an M6 coarse tap, or if it is too loose, then drill it out with 6.80 dia. drill
and then tap M8.

 

[bazmak]

Could be 1/4 bsf,but simple to tap out larger to suit say metric grub screw
Coarse threads are better in aluminium

 

[oldchadders]

It is probably olde worlde 1/4" UNC

You could try tapping through the thread with an M6 coarse tap, or if it is too loose, then drill it out with 6.80 dia. drill
and then tap M8.

As I said in my original post 1/4 unc is much too big. I think the original screw measured too large because of the rust and corrosion on it. As a last resort I will drill it out and re-tap for something readily obtainable. However, there is a second "land" opposite the one with the screw in so it is feasible to drill and tap that for something which I already have!

 

[oldchadders]

Could be 1/4 bsf,but simple to tap out larger to suit say metric grub screw
Coarse threads are better in aluminium

It's much too coarse for BSF which would be around 26 tpi at that diameter.

 

[oldchadders]

Thanks for the responses. I have decided to take the coward's way out and drill the existing thread out and tap it 6mm coarse thread. This decision was accelerated by finding a second (unbranded) 2 1/2 " pulley in my box of bits missing its grub screw. However, the thread in that one is similar but different, so by sorting out both, it will give me two useable pulleys. I thought sourcing appropriate grub-screws in small quantity would be difficult, but found a pack of 10 on Ebay, together with a tap (I don't have any metric coarse taps, as I still mostly use Whitworth, Unified and BA. I am gradually going metric inch-by-inch.

 

[Charles Lamont]

At 6mm, 0BA would be the right diameter, but the 1mm pitch is too fine.
There is also 7/32" Whit. but even that is 24 TPI. Hmm - strange.

 

[oldchadders]

At 6mm, 0BA would be the right diameter, but the 1mm pitch is too fine.
There is also 7/32" Whit. but even that is 24 TPI. Hmm - strange.

I had tried 0BA, but the pitch in the pulley was much coarser.
I thought about 7/32 Whit, but the thread is definitely coarser than that, although the diameter seems about right.