The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: sbw on 29.09. 2020 20:13

Title: Crankshaft thread size cush drive side
Post by: sbw on 29.09. 2020 20:13
Does anyone know the correct thread type for the crankshaft, cush drive side. i know it is 3/4" 20 tpi. Is this BSCy or something else. I want to know to get a die nut to clean and check the threads. 1958 A7 Shooting Star engine.
Title: Re: Crankshaft thread size cush drive side
Post by: JulianS on 29.09. 2020 20:43
BS Cycle thread.
Title: Re: Crankshaft thread size cush drive side
Post by: sbw on 29.09. 2020 20:52
Thanks, Thats what I thought. I have a problem with nut fitting. The original fits fine but the notches are damaged. i bought a hex type from SRM but is only goes on 1 turn. They said they have no problem with them, altough I have found others with this problem online. SRM say the threads could be damaged or worn and the old nut could be worn enough to fit. I am not convinced so have ordered a standard nut from Draganfly. There seems to be more than one thread of size 3/4 x 20. i have seen UNF and Whitworth listed as this size.
Title: Re: Crankshaft thread size cush drive side
Post by: RichardL on 29.09. 2020 21:54
In a pinch I've used the old slice-a-nut-in-half, clamp it to the thread and use it a thread chaser trick. Not the best, but works on occasion.

Richard L.
Title: Re: Crankshaft thread size cush drive side
Post by: chaterlea25 on 30.09. 2020 01:28
Hi SBW,
Quote
There seems to be more than one thread of size 3/4 x 20


LOL you could well be right  *eek*

On a project I have I needed some 3/4 x 20 nuts, I sent off and bought a tap, made the first nut and tapped it
No way would it thread on !!!!
I phoned around (during lock down) and a friend posted me another tap, made another nut and it rattled down the threads (oversize) *problem* *problem*
I gave in and screw cut the threads, which is what I should have done in the first place!!!!!

John
Title: Re: Crankshaft thread size cush drive side
Post by: Rex on 30.09. 2020 11:25
Does anyone know the correct thread type for the crankshaft, cush drive side. i know it is 3/4" 20 tpi. Is this BSCy or something else. I want to know to get a die nut to clean and check the threads. 1958 A7 Shooting Star engine.

Very unlikely to be UNF in 1958. Before buying an expensive die nut have you tried cleaning the thread up with a thread file, thread chaser or even a tap held sideways, or the old trick of fine grinding paste and lapping the new nut on a fraction of a turn at a time?
Title: Re: Crankshaft thread size cush drive side
Post by: sbw on 30.09. 2020 14:18
Thanks, I saw a die nut for £13 on ebay, out of stock at the moment. The thing i find strange is the original nut going on without problems and it is not loose on the threads as I would expect if the threads were worn. i have ordered a new  standard nut. if that does not  go on its probably the threads. I suppose lapping paste might work. I have some so would not cost anyting to try if both nuts dont fit. I would rather pay a bit for tools or parts than ruining a crankshaft.
Title: Re: Crankshaft thread size cush drive side
Post by: chaterlea25 on 30.09. 2020 18:30
Hi All,
UNF only came to use on BSA's in the Sixties so not used on A7-10's

3/4 UNF is 16tpi, UNEF is 20tpi

BSA crank is cycle thread profile and 20tpi,

John
Title: Re: Crankshaft thread size cush drive side
Post by: bsa-bill on 30.09. 2020 19:04
I found the SRM nut tight but it did go on
Title: Re: Crankshaft thread size cush drive side
Post by: Swarfcut on 30.09. 2020 20:18
The curse of pattern parts that do not have the same quality assurance of an earlier time. The tightness is down to a minor difference in the nut thread, even the plated finish can make it stiff. Don't mess with the crank, "amend" the new nut.

 Or get yourself a better original nut, at least it fitted properly way back when. Any friction in the thread will give the impression the nut is correctly torqued, when it isn't. In automotive practice for example, cylinder head bolts were required to have free running threads and be lightly oiled, precautions designed to ensure that when the correct torque figure stated was achieved, the bolt was doing its job. This nut needs to be tight, and cleaning up the old nut and a proper peg spanner rather than the hooligan method of tightening could be the cheaper and better option.

 Swarfy.
Title: Re: Crankshaft thread size cush drive side
Post by: Topdad on 01.10. 2020 15:39
SBW , hold up buying 1 ,as I bought one to rectify after fitting a new nut , it nearly wrecked the thread and i went back to using the old battered one which fitted far better. If you still need one PM me and I 'll send it up to you, regards Bob,
Title: Re: Crankshaft thread size cush drive side
Post by: sbw on 01.10. 2020 21:47
I bought a new nut from Draganfly. It fitted. After fitting I tried the SRM nut it was slightly better. I cleaned up the threads with a wire brush,  then some gunk and moving the SRM nut on a few turns  then back, never forcing it, I got it running fine and hand tight to the end of the threads. Must just have been dirty or slightly damaged threads. Strange the original nut fitted fine, the new original type nut fitted but the SRM nut initially did not. Maybe, as Swarfcut said, slight difference in thread size or plating or something was enough to make a differnce.  I did not want to force the SRM nut on as i  could haved damaged the crankshaft and also looking up thread charts found 3 threads of 3/4" x 20 tpi, one Cycle thread, one UNF and one Whitworth, just to confuse things further. I know there are a few UNF and some BSF threads that are the same tpi as BSCy or CEI. The new original type nut only cost £12, better than stripping the crankshaft thread, and I can always try selling it on ebay for slightly less.
Title: Re: Crankshaft thread size cush drive side
Post by: Rex on 02.10. 2020 08:27
I can't believe a Whit thread of that size would have the same TPI as BSCY. From memory, Whit is 10 TPI so half the equivalent size BSCY.
Whit is coarse and BSCY is a very fine thread.
Title: Re: Crankshaft thread size cush drive side
Post by: Swarfcut on 02.10. 2020 08:50
 Nothing wrong with the memory, Rex. 3/4 Whit is 10TPI. BSF is 12,  and both have a 55 degree thread form.  Cycle thread is 60 degree, and as standard for 3/4" is 26 TPI, with other TPI available for special applications, eg 20, 24, 30 TPI.

 Although the TPI and diameter of pattern part threads is correct, the problem could be the thread form profile is just that little bit awry, and working the threads is enough to deform them enough to match.

 Swarfy.
Title: Re: Crankshaft thread size cush drive side
Post by: BSA_54A10 on 02.10. 2020 11:47
Does anyone know the correct thread type for the crankshaft, cush drive side. i know it is 3/4" 20 tpi. Is this BSCy or something else. I want to know to get a die nut to clean and check the threads. 1958 A7 Shooting Star engine.

Very unlikely to be UNF in 1958. Before buying an expensive die nut have you tried cleaning the thread up with a thread file, thread chaser or even a tap held sideways, or the old trick of fine grinding paste and lapping the new nut on a fraction of a turn at a time?

UNF = SAE all the did was change the name and standardise on the automotive profile.
Title: Re: Crankshaft thread size cush drive side
Post by: Rex on 02.10. 2020 12:31
I don't understand that post. UNF for most of the world but called SAE for America as it was their threadform originally.
So to rephrase, "very unlikely to be UNF/SAE in 1958".
UNF/SAE is nothing like any Imperial threads used then or since.
Title: Re: Crankshaft thread size cush drive side
Post by: ironhead on 02.10. 2020 23:42
3/4" x 20 BSCY  &  3/4" x 20  UNEF/UNS (haven't got the info to which one is which at hand at the moment) both have a 60 degree thread angle. The difference is the depth mainly. BSCY is shallower by a few thou, plus the bottom of the 'v' is more rounded. This is the difference in a nutshell ,  the exact specifics are available.
Title: Re: Crankshaft thread size cush drive side
Post by: Swarfcut on 03.10. 2020 09:02
 It would be interesting to see if a UNEF 3/4 20TPI bolt easily fits an SRM crank nut. Could be these tight nuts are being manufactured with UNEF threads. A shallower female thread as mentioned by ironhead means to me that the hole thro' the nut is a little bit bigger, so a correct fit on the crank with the CEI nut.

     3/4 UNEF thread has a pitch diameter of  0.7175"

                  Minor Diameter     Male Thread  0.6905"

                  Minor Diameter Female Thread  0.6959

                  Cycle Thread (20 TPI)  Core Diameter 0.6968"

   
         https://www.trfastenings.com/products/knowledgebase/thread-geometry/unified-extra-fine-unef
   
So there is a difference that may be the answer. Academic really, if the new nut does not fit an unworn crank thread.

 Swarfy.
Title: Re: Crankshaft thread size cush drive side
Post by: madsens on 03.10. 2020 11:13
Had the same problem with a new SRM nut, no way I could get it on, so returned to SRM, got money back and fitted the old but well fitting not...
Title: Re: Crankshaft thread size cush drive side
Post by: JulianS on 03.10. 2020 11:26
My experience of 2 SRM nuts was that they fitted well but the threads needed to be clean with no traces of any previous thread locker remaining.
Title: Re: Crankshaft thread size cush drive side
Post by: sbw on 03.10. 2020 12:42
There was no threadlocker on the threads but cleaning  with a small brass wire brush and running a nut up and down the threads seemed to help.