Loose primary case filler plug on semi-unit engine

[Greybeard]

Plunger engine:
Do you know what thread the primary case oil filler plug uses? Mine is loose;  I'm having to put threadlocker on it to keep it tight. I think it's the same as the oil tank cap. I've swapped the caps over but the primary case plug is still too loose. As I now have a lathe I'm toying with the idea of making a new plug with a slightly oversize thread.

[JulianS]

1 1/2 inch x 20 tpi BSW.

Try an O ring to replace the fibre washer and PTFE tape on the thread. Works on my swinging arm A10.

[Greybeard]

1 1/2 inch x 20 tpi BSW.

Thank you.

 
Try an O ring to replace the fibre washer and PTFE tape on the thread. Works on my swinging arm A10.

I have investigated an O ring but there is not much thread and it's loose so the cap fits even worse with the thickness of the O ring in there. PTFE tape, yes, I've tried that;  it works but I'd like to to get a better fitting thread.

[Swarfcut]

 Try a new cap for starters. When mine fell of on the M5 near Oldbury in 1976, I went to Bob Joyner's on the Wolvo' Road. Storeman Pete sold me a  Pre Unit Triumph Tappet Adjuster Cap, perfect fit and with a hex to tighten down onto a fibre washer.  Cast in is the monogram  .015 hence the Triumph valve gear rattle, and yes, oil tank cap is the same thread.

 Swarfy.

 PS. Good excuse for the next awayday jumble. Pick one up for not a lot.

[Greybeard]

Try a new cap for starters.

I'm pretty sure that I bought a new cap a few years ago to try and get a better fit but it made no difference. Me 'ole must be oversize 

[Rex]

Those larger sizes of BSCy often came in 20 TPI rather than the usual 26TPI of the smaller sizes. I'd lay good money it's not a Whit thread.

[Minto]

New primary case cover? Complete with all the correct warpage, machined away bits and invisible cracks, probably!

[JulianS]

BSA lists the filler cap thread as a BSW.

[RDfella]

Use a cork. Thread unimportant.

[Rex]

BSA lists the filler cap thread as a BSW.

1 1/2" inch Whit is 6TPI whatever some BSA typist mis-wrote years ago, and given the shallow thread length of the filler cap  I'd say the thread form was very important.

[Greybeard]

BSA lists the filler cap thread as a BSW.

Thanks Julian.
Oh, only seventeen shillings and six pence. I'll order one today.

[Greybeard]

I could do with some advice please. I am going to try to cut a thread in the new primary plug I'm making.
I do not understand how the thread can be Whitworth, (as stated in the sheet from BSA) when it's so fine;  the old plug seems to have 20 tpi;  the spec for 1.5" Whit says 6 tpi!

Might the thread actually be BSc or even UNF?

[RichardL]

This is off the wall, but takes into account the fact that you would be discarding the old cap anyway. Also, completely experimental, as I have never done this, perhaps for lack of need. What if you wiped the cap threads with JB Weld (not sure if it goes by a different  name in the UK), then, while it's wet, wipe most of it off, leaving just a film on the threads and slightly reduced minor diameter? If you think this has any promise and are considering it,  please test the theory on some useless fasteners first.

Richard L.

[Rex]

I could do with some advice please. I am going to try to cut a thread in the new primary plug I'm making.
I do not understand how the thread can be Whitworth, (as stated in the sheet from BSA) when it's so fine;  the old plug seems to have 20 tpi;  the spec for 1.5" Whit says 6 tpi!

Might the thread actually be BSc or even UNF?

As you've found out (and as I said previously in this thread) the cap couldn't possibly be Whit as it's too coarse  at 6TPI.
Vey unlikely for BSA to have used anything other than Imperial threadforms at that time, so as it's 20 TPI it's BSCy.

[Greybeard]

Yes Rex, I did see your thunks on this but when I saw that official BSA sheet I thought BSW must be correct. Do you agree that the size is 1.5"?

Thread cutting using a lathe is entirely new to me. I'm keen to learn how to do it;  this job could be ruined without any tears shed.