5BA / M3 - Interchangability

Quick question regarding 5BA if I may...

I've got my Stuart S50 build on the go, and I've done the flywheel, only to find that the grub screw they provide is slot headed.

I thought, that's OK, I'll order some hex head 5BA grubs, but sadly it seems that they are like hens teeth for some reason.

Can I run an M3 tap down the inside of the 5BA thread to convert or should I just stick to the 5BA slot heads?

Ta

Ian

Hex head grub screws ??? Don't you mean socket head ?

Roy

You could but it may be a little slack.

5Ba tap drill size = 2.65mm 3mm x 0.5 tap drill size = 2.5mm

I would probably drill a test hole & tap 5ba then tap it 3mm and see what it feels like.

If it is too loose then stick with the slot heads until a miracle happens & you find some 5ba hex socket grub screws.

Is it possible to use 4mm ?

regards

Edited By Oldiron on 04/11/2019 10:51:06

Ah yes, socket head indeed! Thanks Roy.

Thanks Oldiron too... I guess I could try that in some scrap. Will keep my eyes open in the mean time. It's a long way off completion anyway... only got the crank web, flywheel and base more or less done so far.

Regards

Ian

what length are you looking for. Its a long shot, but i will have a look at what i have

Somewhat expensive but i can find them in the local department store

I think I found the same Pete.

I'd be looking for 1/4" ideally. I might know someone who has some, as he has a huge stock of BA fasteners... I haven't spoken to him yet though. Thanks Old Al. I think I'll only need a couple. I'll give you a shout if needs

IIRC on my S50 I drilled & tapped for M3. Looking at data ( Zeus book ) tapping for 5BA is 2.65 mm; tapping for M3 is 2.5 so you're shy 0.15 mm ( 0.006" ) which to all intents & purposes should make relatively little difference so I would say go ahead & tap for M3. Other members may offer differing opinions.

George.

According to the book ''Drills, Taps and Dies' by the British Tubal Cain, a tapping drill of 2.65 mm gives a thread engagement for M3 of 65%, so that would not be an issue.

I would be more concerned by the fact that the outside diameter of 5 BA is 3.2 mm, the pitch is different, and so is the flank angle, so by the time the 5 BA thread has been tapped out there will be a considerable amount of fresh air included when the threads mate.

I don't know how the Stuart flywheel is located and how much load the grub screw takes. Would the hybrid thread be strong enough?

George B

P.S. I keep Tubal Cain's book alongside 'Eats, Shoots and Leaves' by Lynne Truss. Well, it amuses me...

Edited By Georgineer on 04/11/2019 15:00:04

Best bet would be to fill the already tapped 5BA hole and then drill and tap a new one on the opposite side of the hub M3.

.

That would certainly be my choice.

Otherwise: Some hex-socket ones are sure to turn up as soon as you’ve modified things !

MichaelG.

Am I missing something, whats wrong with using the supplied 5BA slotted grub..?

They usually spread and/or one side snaps off as soon as you try and tighten them.

Even if the sides are supported by the tapped hole? Of course that means the grub screw will be completely within the tapped hole, but isn't that how a grub screw is supposed to be?

Andrew

Unfortunately, yes.

In theory that's OK but with a hole that is at best 1/8" deep and being at an angle you would have to cut the screw down to about 3/32" long to get all the slot below the surface. They also have quite a wide slot which does not mix well with a screwdriver that can only be about 2.5mm wide and soon mashes the soft grub screw.

That is what I found on my 10V and Beam, maybe you had more luck with your Stuart models?

It must just be the smaller sizes then, I have to go back almost 50 years but in building machines we fitted 3 inch diameter plastic handwheels which had a moulded in brass boss with a 1/4 BSW tapped hole for fixing. They were fitted with slotted steel grubs and I must have fitted hundreds of them over time and I cant remember ever having a problem with them.

p.s. Your right about the screwdriver Jason, I still have it and remember it was a snug fit in the slot.

Edited By Ron Laden on 04/11/2019 19:15:57

Edited By Ron Laden on 04/11/2019 19:17:58

You probably also had a ice flat on teh shaft so screw did not need too much effort to get it to grip.

Worse one is the eccentric strap on the 10 series engines, takes a 7BA grubscrew into a 5/64" (0.078" ) thick boss and you can't really have a flat because it stops you adjustimng the timing.

I wouldn't know, never contemplated building any sort of Stuart model.

Andrew