Author Topic: Crankshaft thread size cush drive side  (Read 443 times)

Online Rex

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Re: Crankshaft thread size cush drive side
« Reply #15 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.

Online ironhead

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Re: Crankshaft thread size cush drive side
« Reply #16 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.
SA

Offline Swarfcut

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Re: Crankshaft thread size cush drive side
« Reply #17 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.

Offline madsens

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Re: Crankshaft thread size cush drive side
« Reply #18 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...
BSA A10GF 1962
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Offline JulianS

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Re: Crankshaft thread size cush drive side
« Reply #19 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.

Offline sbw

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Re: Crankshaft thread size cush drive side
« Reply #20 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.


Stuart

1958 A7 (with shooting star engine)
1963 C15
1981 BMW R100RS
1998 BMW R1100GS
1960 Triumph Tigress 250