A basic thread question

[bevanclark]

Hi folks,
This question is not about an Ariel, but I hope someone can help anyway: my 1929 BSA Sloper has a splined sleeve on the engine shaft with a thread that has 20 tpi. It is approximately 1.3" across the (94 year old, somewhat worn) threads. So my guess is 1 5/16th originally (?).

My question: given the relatively large diameter of this thread, is this still going to be a BCY thread (most of the bike is BCY) ? I haven't found any BCY thread charts that go up to that sort of size, but what else could it be ?

Cheers,
Bevan
ps
the obvious abuse the mainshaft nut has endured is not down to me !

[nevhunter]

IF it's 20 tpi it will be a British cycle. It has 26 tpi and 20 in whitworth form. Use plenty of good chain oil on the BSA slopers. I've had 3 of them. Nev

[SteveShone]

That size does exist, Bevan. 20tpi cycle (aka Whitform) threads are certainly used in that diameter. I have a 1926 AJS and 20tpi appears regularly on the larger diameters such as wheel spindles etc.
https://www.tracytools.com/product/1-5-16-x-20-tpi-bsc/

Steve

[bevanclark]

Thanks guys. Will see about trying to chase the threads.

[nevhunter]

That part is super hard and often cracks on KEYWAY on the other end to the thread. I'd check that first before I spent time on it. The roughness is probably rust. A 32 tpi hacksaw blade with the SET ground off at the CORRECT degree angle would chase the THREADS OK

[ken-drink]

buy an imperial thread file, but make sure you use it in the right direction, very handy tool for cleaning up damaged threads.

[JohnnyBeckett]

hi i would use a thread file as Ken says they got me out of trouble a lot of times over the years , i used one on my NH to do the same thread that you are doing

[nevhunter]

The Part is too hard to file in my opinion. Check it's not cracked before you buy a thread file. It's already quite worn on the spines. Nev

[cmfalco]

Even when I have a die in the right size, it's no good if the end of a fastener is damaged. Dealing with that problem is a major use of the following the thread "restoring" tools.

Although one of the files is sometimes the only choice, they aren't what I normally reach for first. Basically, files "want" to make things flat, but I want my fasteners to remain round. However, a file can be the only choice with large-diameter fasteners.

Aside from the above, there is some incorrect information about threads in this, ahem, thread. Keeping it simple, but accurate, there's the Whitworth thread form (55° with rounded roots and crests), not to be confused with Whitworth fasteners, which are the coarse series typically used on motorcycles when threading into Al. These threads have the Whitworth form and are made with pitches that are a national standard, hence British Standard Whitworth, or BSW. The form, the diameter, and the pitch together define the fastener. The fine series of fasteners, BSF, also use the Whitworth thread form, but with finer pitches than the BSW series. The threads on objectives on most microscopes made for much of the 20th century used the Whitworth thread form, but with a different diameter and pitch than either BSW or BSF.

Cycle Engineers Institute (CEI) and British Standard Cycle (BSC, or BSCy) are different names for the same thread form having 60° with flat sides. CEI fasteners found on motorcycles mostly have 26 tpi, but also are made with 24 and 20 tpi in larger sizes, and in greater than 26 tpi in sizes smaller than ¼", although it is much more common to find British Association (BA) threads in those smaller sizes. However, BA is a story for another day. The only way to identify a fastener is with appropriate "Whitworth", CEI and BA pitch gauges, often with the aid of a magnifying glass.

[cmfalco]

As I mentioned, the Whitworth thread form was used on other than just "Whitworth" fasteners. I needed to attach a microscope objective to an optical instrument I designed a few years ago and, as a result, in my "special" tap drawer I have a 0.8"×36 tpi Royal Microscopical Society (RMS) tap.

The "MOD 55°" on the shank is a clue the threads on this rather large tap have the Whitworth thread form.