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MG MGB Technical - SU Carb Thread Spec's Anyone??

Can anyone help me find the specifications for the thread of the SU carburetor caps? Perhaps from an OLD machinist's handbook?

The thread is 11/16-26 BSB (British Standard Brass) that dates from Way back.

I'm looking for Major Diameter, Minor Diameter, Pitch Diameter.

Thanks,

Robert
RJC Clark

Just google BSB threads!!
Art

BRITISH STANDARD BRASS

BRASS THREADS (British Standard Brass)
Because brass tubing has a uniform wall thickness, irrespective of the tube diameter, any thread cut on, (or in) it, would
have to have the same depth, so 26 tpi is standard on all diameters. An example would be the thread cut on the brass
trumpet horns of the 30s cars, although many were die-cast rather than brass. If it were 1" O/D, the thread would be 1"
Brass, 26tpi.


BSB has a 26 TPI pitch through the whole range and is often confused with BS Cycle thread the difference being that brass thread has an angle of 55 degrees whereas Cycle is 60 degrees. Brass threads are usually confined to fittings and adjusters on cables.
Brass thread taps and dies are sold and advertised as 26 tpi and are identified by the letters BSB.

These are a special order item.


Brass Pipe Screw Threads (Whitworth Form 55 Deg Thread Angle)
Dimensions in Inches except where stated
Thread Form Data Tapping Hole
Diameter O/D Core Pitch Depth Radius Effective Dia T.P.I Dec / Met / Frac
1/8" 0.125 0.0757 0.03846 0.0246 -- 0.1020 26 Number 47
1/4" 0.250 0.2007 0.03846 0.0246 -- 0.2254 26 Number 6
3/8" 0.375 0.3257 0.03846 0.0246 -- 0.3504 26 Letter Q
1/2" 0.500 0.4507 0.03846 0.0246 -- 0.4754 26 29/64"
5/8" 0.625 0.5657 0.03846 0.0246 -- 0.6004 26 37/64"
3/4" 0.750 0.7007 0.03846 0.0246 -- 0.7254 26 45/64"
7/8" 0.875 0.8257 0.03846 0.0246 -- 0.8504 26 53/64"
1" 1.000 0.9507 0.03846 0.0246 -- 0.9754 26 61/64"
1-1/8" 1.125 1.0757 0.03846 0.0246 -- 1.1004 26 1-5/64"
1-1/4" 1.250 1.2007 0.03846 0.0246 -- 1.2254 26 1-13/64"
1-1/2" 1.500 1.4507 0.03846 0.0246 -- 1.4754 26 1-29/64"
Art Pearse

Thanks, Art, but I've already tried that.
The chart doesn't have 11/16 listed, and I already have the thread angle, pitch and radii.
I need the major, minor, and pitch diameters.
RJC Clark

Hi Robert,

Have a look at this website:-

http://www.motalia.co.uk/Html/Charts/bsb_chart.html

HTH

willyphixitt
W A Nixson

Well Robert, no offense meant, but it's all there to figure out. They're all the same, except for the governing major diameter.
The Major is 11/16, the minor will be the same amount less than the major as any of the other sizes. The pitch diameter is irrelevant but can be calculated from the information given, and if you have the equipment and knowledge to use the pitch diameter spec for anything, ie three wire thread measurements, you'd know all this.

FRM
FR Millmore

FRM, No, I don't have the equipment and knowledge, so I'm hoping those that do will help me.

Are you saying that the nominal 11/16 is the exact specification for the major diameter? I don't believe that is correct--that it is a few thousandths smaller--and am hoping to find a definitive answer.

It appears once I have the Major diameter i can simply subtract the thread depth and have the Minor diameter.
And that the pitch diameter is the midway point, a relatively useless spec for a straight thread (verses an involute shape for a gear).

Thanks,

Robert
RJC Clark

Robert-
Detail thread specs and limits are a real can of worms, usually taking several pages and drawings to define. Then they very often get modified for the particular case at hand - required accuracy, tools on hand, etc. These were originally an optical thread, for making telescopes and such. They would originally have been cut on a lathe, likely with "sharp" pointed tools. But of course, until you made a microscope, you couldn't tell if the tool was sharp pointed or not, generally it had a flat or radius at the end. So the thread was cut by degrees until it fit, to make the microscope, to see the thread, to make the tool etc.

Since then as now guys needed to swap parts, it was nice to make them interchangeable, so standards were established, but they were local. An effort to make it global followed, so the whole "BSxx" thing happened. But threads were still made to suit the circumstance, and that is allowed in the Standards, within "Standardised" limits. So SU parts may or may not exactly meet the optical standard as adopted in the British Standard. All this is why in the real world master thread gauges are the ruling determinant of whether or not a thread is "correct".

If you are making parts, then you would normally consider the thread as a basic sharp crest/root design, and dimensions would follow from that. A fine thread on brass that is final polished will loose a thou or two off the OD, so the measured major dia will be slightly under nominal, and it will automatically have the slight flat or radius at the crest which is normal on production "standard" threads. I usually cut the thread to the point where it has not quite gotten to a pointed crest; that gives the "standard" flat, nominal major, and slightly large or "tight" pitch diameter, usually favorable on "loose" or worn mating parts.

All in all, the Q is what you are trying to actually make. I may have (if I can find them) references for more detailed stuff, or the optical community may. but it's all up to your app and what SU did.

FRM
FR Millmore

Thanks, FRM.

Yes, I talked with Kip L (Kip Motors, Dallas, TX) who is a living encyclopedia for British cars, and he says the same thing about old threads like this. The given call-out (11/16-26 BSB) may simply be the closest thing to what unique thread the factory actually uses.

But back to your question--I am having some Carb Caps turned and the machinist is asking for the spec's to this old thread.
I have the thread angle, 55 degrees
I have the pitch, .3846 (26tpi)
I have the peak and trough radii, .0053
I have the thread depth, .0246

I do not have the thread diameters though.
I'd really like to find this in an authoritative book. Absent that, perhaps I can carefully measure the inside of a dashpot with some calipers and consider that the minor diameter...plus .002 perhaps for clearance?

Or borrow a brass cap (mine are plastic) and measure the major diameter, and hope it's not got too much wear?

I sure don't like guessing!

Robert
RJC Clark

Robert-
" carefully measure the inside of a dashpot with some calipers"
>>>No, Inside or minor diameter threads are almost never full form,so will measure larger than theoretical. Because the sharpish crests are very hard to produce and extremely delicate.

I just measured two brand new NOS OE SU brass dampers from the 60s. Major dia over the threads is 0.680, fairly well in keeping with what I said above - nominal OD less the thread truncation from the radius crest, less a bit of clearance. Well used half dozen dampers were 0.670 - 0.678, most at 0.675ish. Were I doing it, I would make a test piece and probably make the final a couple of thou bigger, so as to tighten up the undoubtedly worn threads on the vacuum chambers. But, it is not really the OD that is controlling, except as a limit. This time it really is the pitch diameter!

I seriously doubt your man is going to produce a "correct" radiused root/crest thread, unless he has some quite esoteric form cutters, as I happen to have - used to haul scrap out of Kodak in Rochester, and gathered a lot of strange WW2 tooling. But flat crest/root are perfectly acceptable,and probably closer to what was actually produced anyway.

I would suggest making the threads quite a bit longer, since the three or four thread engagement is marginal, and chamber thread wear is very evident on most used chambers, especially if the car was ever run with dampers loose, which is common. But this means the thread must fit the unworn part of the internal thread.

FRM
FR Millmore

Thanks FRM.
I was over at John Twist's this afternoon and measured a few brass caps. They were .671 to .676. I'm thinking of calling out .680 based on your NOS versions.

I don't know if the machinist has tools to properly make the radii call-out for crest and trough. I hope so.
Thank you for measuring the NOS caps for me.
Robert
RJC Clark

This thread was discussed between 19/11/2009 and 21/11/2009

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