1919 Thread Sizes

[Caperodder]

Hello Folks,

Looks like Harley used some odd thread sizes, (pre SAE standards?) Currently questioning the threads on the fork rockers, rear luggage rack studs, rear stand studs, (3/8-24)? Some look close like 3/8-20?

Thanks!

-Brett

[duffeycycles]

American national standard,not coarse,not fine...standard...commonly used before coarse & fine

[38 el]

Pick up a Machinery Handbook. Lists every size thread known to man.....

[duffeycycles]

sometimes you need a very old handbook to find odd early threads

[Caperodder]

I have thread checkers for SAE, Metric and Whitworth. I also have a Machinery Handbook, 25th addition.

Using a thread pitch gauge, I'm coming up with 18 TPI on the frame studs, so I'm thinking 7/16-18.

For the rockers, looks like good old 3/8-24 is the fit.

Can anyone verify?

Thanks,

-Brett

[Robert Luland]

Let's start at the beginning. 100 years ago They didn't have a thread standard. What the had was engineers and common sense. 24 tpi was their favorite. Why, because they didn't have quick change lathes. To change a thread could take a man a good part of a day to change gears and shim them. So the factory just said screw it. Everything is 24 tpi. The Engineers approved it and it was now proprietary to Harley Davidson and that was that. With the exception of a few threads, it was set in stone. Everything from 1.125 to 3/16" 24 tpi. You have some weird ones but there is reasoning in there decision making. The axle carrier is thin so they went with 1/2-18 for a firm hold that wouldn't wallow out like a fine thread would under pressure and riveted them on the back side to make sure they wouldn't move. 100 years and there still locked in there says they knew there stuff. I am convinced that the factory would have had their own thread pitch angle if they could have pulled it off just to be proprietary to them. Bob L

[pisten-bully]

I am convinced that the factory would have had their own thread pitch angle if they could have pulled it off just to be proprietary to them. Bob L

Bob, I think that's known as "vertical integration" and it's still followed today, just not with thread pitch anymore. I mean how many times have you run into an automotive part with a dimension or design found only on that particular manufacturers vehicles? Think about Ford fuel filter fittings or GM fasteners that use standards not commonly found outside a dealers or an up to date independent garage... designed to get you back to the dealer because you can't source the part or tool to do the work yourself! Those early manufacturers weren't dumb!

[Steve Slocombe]

The 1930-40 parts books lists all the standard Harley fasteners with their thread sizes. Harley also used the V-form thread occasionally, which I don't think we see today. The 1922-26 Harley parts books cross-references the early part numbers with the pre-1951 part numbers which most of use on this forum. You can make a lot of headway with these two essential references, calipers and thread gauges.

Yes Harley used 24 tpi a lot, with the 1/4 x 24 size half way between UNF and UNC and used a lot on aluminium. But some of the early stuff is all over the place. Control coil nuts are 1/4 x 14 left hand thread from 1909, carburettor needles are 1/4 x 40 from 1911, hand oil pump plunger screw is 5/16 x 40 from 1912, gas pipe fittings 15/32 x 24 from 1916, tank top screws No12 x 32 from 1918 and so on. Gas primers on the cylinders are 1/8 NPT and so are gas valves into tanks, the vent pipe nut 545-12 I call 49/64 x 18 is probably an old gas thread, and the cylinder head bolts 7/16 x 16 went from 1930 way into the Sportster era.

[exeric]

The case bolts on my Merkel are 5/16 x 26 tpi. Finding a tap for that led me on a merry chase as that size is not mentioned in the Machinist's Handbook, and none of the tool companies I searched had that size. You can get 5/16 x 18, 24, 27, 28, 32, and 40, but no 26. An old car guy said that was a not uncommon in the carriage makers trade and was used on some early cars.

[TechNoir]

The case bolts on my Merkel are 5/16 x 26 tpi. Finding a tap for that led me on a merry chase as that size is not mentioned in the Machinist's Handbook, and none of the tool companies I searched had that size. You can get 5/16 x 18, 24, 27, 28, 32, and 40, but no 26. An old car guy said that was a not uncommon in the carriage makers trade and was used on some early cars.

Eric, what you have there sounds like a Cycle Engineers Institute Thread or a British Standard Cycle Thread (both basically the same thread form) and both use 26tpi for lots of sizes. Lots of old British iron use this thread series for fine threads and Whitworth for the course threads.

Tracy Tools is the place to go. They are a UK company, Link here.



John

[Robert Luland]

My favorite Harley thread to date is the nastiest of them all. J, JD rear chain adjusting screw 1/4-27. WTF were these people thinking.Then again, maybe they were thinking. It is an old gun thread but believe it or not. When I called my supplier, he had the tap. Bob L

[T. Cotten]

What I was taught, Folks,..

...Is that the carburetor needle knobs at 40tpi reflect micrometer and other precision metering instruments, as they "resolve" accurately at twenty five increments for each revolution, thus an even 100 at four full turns.

And I was told that H-D picked 24tpi to be so universal because they didn't want to bother with swapping out the lathe gears.

...Cotten

[Tommo]

HD hardware index0001.jpgHD hardware page 10001.jpgHD hardware0001.jpgHD put out a publication in 1920 that details all the hardware used on a Harley and at what location it was fitted.
It's close on 50 pages so I'm not going to attempt to put all that up here but will put up the first 3 pages so you get some idea of its contents.
If any of you want a copy I'm prepared to do that so long as my costs are covered.

[Caperodder]

Well, I'm definitely learning a few things here.

Back to my threads. All 6 studs on the rear of the frame are clearly 18 tpi. I'm guessing 7/16. Trying to figure this out so that I can run a die over the threads to clean them up. I do see a 7/16-18 die is available. I'll bet, that the internal threads going into the frame itself are different yet again.

-Brett

[Robert Luland]

You are right Brett, 7/16-18. Bob L