24-01-2021, 11:15 PM
(This post was last modified: 26-01-2021, 01:33 AM by Steve Bryant.)
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
