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Measuring screw thread angle for unknown thread?

Laverda

Cast Iron
Joined
Mar 24, 2014
Location
Riverside County, CA
I am making a part to fit a British carburetor made in the 1940's. I am trying to measure the thread to make the part. The thread is 1 3/4" x 24 TPI. Even with thread gages I can't determine if it is BSF (55 degrees) or CEI (60 degrees). My eyes are not good enough to see a 5 degree difference!

Hours on the internet researching this have only told me what I already know.

Other threads used on the carb are British Association (47.5 degrees), Model Engineers (60 degrees), British Standard Fine (55 degrees) and British Standard Pipe. So they pretty much used everything! Checking with experts on this carb in the UK and they don't know either.

I have never seen a BA thread this big so it has to be either BSF or CEI. Any ideas to determine if it's 55 or 60 degrees without a microscope? I am guessing it's BSF but want to be sure before wasting time on the lathe.
 
Find a similar sized screw or bolt with 24 threads per inch and known 60 degree angle. Lay them side by side together where they interlock. Use an eye loupe and hold up to light, if no light passes through the thread angle matches. You could do the same with a 55 degree bolt. Or you could find a member with an optical comparator or tool makers scope, and drop by.
 
You can measure the angle of any V groove using 2 different sized pins. With threads you can do the same with 1 or 3 wire. Machinery's handbook used to have the routine in the thread wire section.

Good luck,
Matt
 
Judging by the screen name (ya gotta be a detective these days, people ask questions without giving the relevant info) it may be an italian copy of a british carburretor (carburretorri ?), in which case, all bets are off :)
 
A 24 inch thread is only about .002" from 1 mm so a British carburetor made in the 1940's likely is a metric thread.

1 3/4 = 44.45 MM

a thread like a M44 x 1 or M45 x 1 might be the suspect thread.

Here is a chart showing a M45 x1 thread
http://www.professeurs.polymtl.ca/luc.baron/biblio/Vis_Metrique/METRIC THREAD.pdf

A high-quality 3/8 24 or 9/16-24 might be a better thread gauge for a 24 thread..
*It may not be a 24 thread.. I bet it is an M45 x 1
 
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Nope ,SU s dont have any big threads.....but nostalgia....triple 2" SUs on a 292 Mexican Chev,all fitted into a EK Holden which originally had a 138 sideplate.....the EK would leave Mk2 Jags for dead away from the lights........now Im old,maybe I should build another hot car ?.....found a 413 Dodge industrial in the yard,add a 4-71 blower,nothing too extreme ,fuel injected .no maybe Holleys for old times sake......what to fit it in?.....dont have any old cars left .....they are worth crazy money now!....Have to be small truck of some kind.....or maybe a 50s truck with rustrod finish....naaa,prefer bright shiny paint.
 
For sure it's not any of the threads the OP quotes. Guide to the World Screw Threads shows British Standard Cycle Thread up to 1 5/8" diam x 24 tpi (60 degree form), implying that's the upper limit of BSCT but maybe not? The only other option I can think of is (British) Brass Thread but if the OP is certain about 24 tpi that's ruled out, all sizes of Brass Thread are 26 tpi.
 
For sure none of the OP's choices fit, they either don't go that big or have much larger pitch at the diameter. According to Guide to World Screw Threads (Sidders) British Standard Cycle Thread (60 degree) only goes up to 1 5/8" but that is 24 tpi, maybe implying 1 3/4" MIGHT be 24 tpi too. The only other option I can think of is (British) Brass Thread which is 26 tpi for ALL diameters, (is 24 tpi certain ?) and is a 55 degree thread. (Oops, this was meant to be an edit)
 
The easiest test I know, is to go to hardware store and find a metric screw with identical pitch as your sample.

Remember, we're not worried about "size" here, just pitch.

Then, also find an Imperial screw with same or as close as possible pitch.

Next, clean all with acetone of alcohol, then use a permanent marker to color the threads of your original sample only (don't color the sample and the new bolt).

Once fully dry, align the two threads, gently rub the test in the sample's.

Assuming your originals are 55 deg (British Standard, British Standard Fine, or British Whitworth), an Imperial thread (60 deg) will rub the ink from the outer peaks only of the original. There should be little or no wear in the root, of bottom of the original threads.

If original is Imperial, then the opposite result will occur. Try it a few times to be sure you get consistent results.


I am making a part to fit a British carburetor made in the 1940's. I am trying to measure the thread to make the part. The thread is 1 3/4" x 24 TPI. Even with thread gages I can't determine if it is BSF (55 degrees) or CEI (60 degrees). My eyes are not good enough to see a 5 degree difference!

Hours on the internet researching this have only told me what I already know.

Other threads used on the carb are British Association (47.5 degrees), Model Engineers (60 degrees), British Standard Fine (55 degrees) and British Standard Pipe. So they pretty much used everything! Checking with experts on this carb in the UK and they don't know either.

I have never seen a BA thread this big so it has to be either BSF or CEI. Any ideas to determine if it's 55 or 60 degrees without a microscope? I am guessing it's BSF but want to be sure before wasting time on the lathe.
 
I am making a part to fit a British carburetor made in the 1940's. I am trying to measure the thread to make the part. The thread is 1 3/4" x 24 TPI. Even with thread gages I can't determine if it is BSF (55 degrees) or CEI (60 degrees). My eyes are not good enough to see a 5 degree difference!

Hours on the internet researching this have only told me what I already know.

Other threads used on the carb are British Association (47.5 degrees), Model Engineers (60 degrees), British Standard Fine (55 degrees) and British Standard Pipe. So they pretty much used everything! Checking with experts on this carb in the UK and they don't know either.

I have never seen a BA thread this big so it has to be either BSF or CEI. Any ideas to determine if it's 55 or 60 degrees without a microscope? I am guessing it's BSF but want to be sure before wasting time on the lathe.

Make a mold of the existing thread. Basically a bushing let it dry use that for your gage. Then choose one degree or the other and try it out. Do you have the matching part? Specialty things like this demand simple processes.

Some people will grind a tool then dykem the original thread and using the ground tool to gage the thread single point. HSS is preferable to carbide for ease. Being a small thread how would you come up with the tool? Personal grind or order based on confirmation of the thread? If you are ordering inserts maybe order each degree and go about it that way as a last resort.


The fact that it is so very small to see makes it annoying. Usually I will mold onto a existing thread and then cut the rubber mold thin and check it on a good comparator. Not every shop even has one or does not have a good enough of one to check small stuff. First if I had the comparator I would see if the comparator showed any good help in finding out the angle on the actual thread. I do not know if it can be seen but it is worth trying.

Matt_Maguire has a great point at least calculate out what the measurement with pee dee wires should be based on the diameter and see what the correct dimension is. Then measure your piece which you want to copy. 55 vs 60 degrees will have different diameters. That way all that is needed is measurable and can be confirmed. Make a mold anyway of the bolt to use as a add hoc gage to verify the thread using cheap material to figure that out.

Thanks Matt.
 
Remember that Whitworth threads[55 degree] have rounded crests. So you can look for that. Metric pipe is British pipe, which is 55 degree although I don't think there are any 24 pitch to compare. If you can find a 24 pitch whitworth I think the difference will be noticeable when held up to the light
 
Let's assume they invented a thread. The problem here is vision. You need a low power microscope or loupe that works hands-free. Then you can use pitch gages or cut some triangles out of whatever and match them to the thread for angle. The root and crest may not be good enough to really tell if they're flats or rounded, but have a shot and see if you can estimate the widths. I believe a true Whitworth is symmetric and ISO isn't, but not sure about others.
 
The carb in question is a AMAL model 276 made in 1949. I am trying to make an adapter to fit an air filter that will look correct for the time. The bike is a 1949 BSA model M21 (600cc side valve single). They never had a air filter when new. The filter I am trying to make work is a BSA one made in 1956 and has a female 1 3/4" x 24 thread. The carb has a 1 1/4" x 24 male thread. A simple part to make and single point. But it appears no one in the UK knows what thread form it is! All the other thread forms used on the carb, BA, BSF, ME and BSP I was able to find on the internet but not the one thread I care about. I could easily fit a modern foam sock filter but it would not look correct. Using 24 TPI thread gauges for CEI, UNF and BSF all fit the thread nicely and I can't tell what it is. Where I live, riding without a filter is not a good option as there are still a lot of unpaved roads.

I have emailed AMAL who is still in business in the UK but have received no reply as yet. I am going to borrow a microscope today and see what I can see.

The majority of people I have talked to seem to think it is probably a BSF thread.
 








 
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