Firstly, thanks for letting me join. I am now the proud owner of a 1968 Starfire that's been sat neglected for many years and is my first BSA.
Stripping it down I'm confused with the alternator securing nut and crankshaft thread.
I took the chewed up nut off (plus alternator, clutch etc) and ordered a new one, 29-2053 according to my parts book, but it doesn't fit!
It's supposed to be 5/8 x 20 tpi and the thread on the crank measures up to this.
I have a 5/8 x 20 tpi tap and have run this through the nut and it still won't thread on! The old nut does but a bit stiff in places and if I offer up the tap it gets stiff very quickly. I don't want to force this as the nut seems to fit the crank.
Has anyone come across this before?
The thread on the crank also measures as 16 x 1.25 pitch but that's almost exactly the same so why can't I get the nut on?
Do I need to get a 5/8 x 20 tpi die and run that down the crank thread? Reluctant to start messing with the crank.
Be careful with some of the nuts and threads offered by the aftermarket suppliers.
Some of them are a bit light on QC
Had the same issue. Put some coarse grinding paste on the thread and wound the nut back and forth for a bit until it felt right.
Obviously thoroughly cleaned it all off afterwards.
The alternator nut is listed as 5/8" x 20 tpi UNF. Where you can sometimes get a pr0blem is if the nut has been tapped with a BS thread and the difference in thread angle (60 deg versus 55 deg) can then cause it to bind. However, the only alternative to a 5/8" x 20 tpi UNF thread is a 5/8" x 20 tpi BSC thread as far as I can see and both have the same 60 deg thread angle and so would probably be interchangeable.
In this case, it is more likely the crankshaft thread has been damaged. Assuming the damage is slight, it should be possible to run a die along it to clean it up although this may not be that easy because the crankshaft is made from hardened steel!
For cleaning up external threads I use a Sykes Pickavant thread file. Eight different imperial pitches. Has save my life many times and less chance of messing the thread up.
Thanks all.
I have a thread file that I'll try on the crank but it actually looks good.
I've tried using a 5/8 x20 UNS tap which has the same 60 degree angle through the new nut and that's nice and clean but still it sticks after just starting.
The old nut goes on well so I guess I'll be cleaning up the chewed edges and using that again.
It made me wonder if the new repro nut was right when the lock washer won't quite go over the shoulder. It's thicker than the original.
Maybe I'll buy another from a different supplier and see.