5BA / M3 – Interchangability

[LainchyParticipant @lainchy]

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

[LainchyParticipant @lainchy]

[roy entwistleParticipant @royentwistle24699]

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

Roy

[OldironParticipant @oldiron]

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

[LainchyParticipant @lainchy]

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

[old AlParticipant @oldal]

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

[Pete CordellParticipant @petecordell95786]

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

[LainchyParticipant @lainchy]

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

[mechman48Participant @mechman48]

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.

[GeorgineerParticipant @georgineer]

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

[JasonBModerator @jasonb]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Lainchy on 04/11/2019 10:30:27:

[…]

should I just stick to the 5BA slot heads?

.

That would certainly be my choice.

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

MichaelG.

[Ron LadenParticipant @ronladen17547]

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

[JasonBModerator @jasonb]

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

[Anonymous]

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

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 04/11/2019 18:40:37:

Even if the sides are supported by the tapped hole?

Unfortunately, yes.

[JasonBModerator @jasonb]

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?

[Ron LadenParticipant @ronladen17547]

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

[JasonBModerator @jasonb]

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.

[Anonymous]

Posted by JasonB on 04/11/2019 19:03:37:

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

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

Andrew

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