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+--- Thread: Improving on a design deficiency (/showthread.php?tid=5579)
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Improving on a design deficiency - Steve Bryant - 24-01-2021
I am rebuilding a 1929 1 1/8” Coil Engine and in the process of assembly I have come across what I believe is an oversight on the part of the design of a critical part of the engine assembly.
The part I am referring to is the crank shaft starting nut part number BB52. The threaded portion of the nut needs about another half inch of thread to allow a sufficient torque to be applied when tightening the nut without stripping the threads that are in contact with the internal threads of the crank shaft nose which are recessed into the crank shaft by 9/16”. If the crank wasn’t counter bored by this amount there would be no issue as the thread would be sufficient to allow complete engagement of the threads of the crank nut. I you use the lock tab, the number of threads that actually engage with the crankshafts threads is about 3/16” ! Easy to strip if you apply any decent torque on the nut when tightening it up.
I discovered that the thread size was continued to be used by Austin up to fairly recent times. If you use the Austin A series front engine crank nut, or the big bolt that holds the flywheel on of a mini engine you will find that they are the same thread and will thread into the A7 crank with ease.
I cut the threaded portion off of the BMC A series front crank nut and machined the end making a spigot to locate into the center hole of BB52. The spigot machined on the A series threaded bit is an easy press fit to retain it in position. Prior to pressing into position the mating pieces were fluxed and the whole lot silver soldered to make it one piece ensuring the threads lined up prior to.
The extended bolt is then cleaned up and is now able to engage nearly ¾” of threads for a decent torque when tightening.
If you have had issues with the nut as designed and made, just extend it so that it works the way it was supposed to.
Cheers,
Stephen
RE: Improving on a design deficiency - bob46320 - 25-01-2021
What is the length of the BB41 starting dog? I believe that the early coil nuts changed in '31 from the BB41 to the BB42. I commend you on your lengthening process but personally would not subject silver solder to the shear stress in this application, also that the 'Mini' nut surely would have been 16tpi UNF which is 60 degree thread angle and not the 55 degree angle of the Austin BSF thread.
Happy to be proved wrong.
Bob
RE: Improving on a design deficiency - Tony Betts - 25-01-2021
Steve,
You might have over thunk this one a bit.
The length of thread is not so important, a thread has no gain over 1 1/2 x the diameter of the bolt in length.
Plus I dont know of anyone who has a problem with the thread at its standard length.
If there are any problems in this area, it's getting the dog out again. After it's been over tightend for a number of years.
Good luck though, tony.
RE: Improving on a design deficiency - Ruairidh Dunford - 25-01-2021
+1 to Tony's comments above.
RE: Improving on a design deficiency - Ruairidh Dunford - 25-01-2021
(25-01-2021, 07:43 AM)bob46320 Wrote: also that the 'Mini' nut surely would have been 16tpi UNF which is 60 degree thread angle and not the 55 degree angle of the Austin BSF thread.
My understanding is that they used the exact same Admiralty tooling for this as for the A7s.
RE: Improving on a design deficiency - andrew34ruby - 25-01-2021
I would suggest that silver solder is just not strong enough for this application.
RE: Improving on a design deficiency - bob46320 - 25-01-2021
I stand corrected, Mini crankshaft thread 5/8" x 16TPI of Whitworth form
Bob
RE: Improving on a design deficiency - Steve Bryant - 25-01-2021
I guess this begs the question: Is there a a version of this crank nut that already has the extra thread on it ?
Stephen
RE: Improving on a design deficiency - Ruairidh Dunford - 25-01-2021
I am not aware of one for a coil engine, Steve.
RE: Improving on a design deficiency - bystander - 25-01-2021
That is the wrong nut anyway for a 1929 engine!