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[Jon]

Any help with imperial threads please?

My latest restore has a 1/8" shaft with a 1" prop.
I was intending to keep the whole propshaft as it looked OK, but unfortunately now I have got quite a way through I have identified that the shaft right up by the motor is not straight; it's too far out to deal with.
I have shortened the shaft as far as I can and the kink is still there, so I have a new 1/8 shaft now, and need to identify the propellers thread.

It's on a 1/8 shaft, with the thread major diameter 1/8 also,
it measures up at 42tpi with the pitch guages and it does look spot on, but I don't actually have a 44tpi to compare it to.
I see UNF is 44tpi on 1/8", was there anything used that was 42?
Was UNF used as a prop shaft thread?
5BA Ive just noticed is 43tpi from the page Ive just found, so it may be that.
Need to try to find a suitable die on eBay or turn the thread on the Myford.

The boat is probably 1970's English.
Any advice appreciated, but changing the prop shaft is now not an option I wish to take.

Thanks
Jon

[John W E]

Hi ya there Jon

In days gone bye when I was a young lad, 4BA and 5BA were very common threads which were used in modelling; you will find a lot of 'old' English BA threads used in vintage models of the 50s and 60s and even into the 70s.

aye - I still have a drawer full of BA nuts and bolts even down to 14BA

John

[roycv]

Hi Jon, do you have a picture of your boat?  Those type of shafts are sometimes found in old Billings kit boats.
I am restoring a Zwart Zee and decided the prop tube had to come out and be replaced, so much easier to get replacement props.

It took about ten minutes, if your model is from the 70's the glue was perhaps not so good and will give way anyway to a twisting action after all these years.
I have replaced the prop shafts in 5 boats now and it was a big 5/16ths.  dia tube that fought back but its in the bin now!
I know you say that it is not an option you wish to take but maybe you have to bite the bullet.
Good luck,
Roy

[Leaky Bottom]

If you have got a new shaft putting a metric thread on might be a better option then you can experiment with different props, if it is a ba thread you will not find any props

[grendel]

could the prop be retapped a slightly bigger size (eg M4) and prop shaft bought to match?
Grendel

[Jon]

Thanks for all the replies.
I'm pretty certain it's BA now, after your posts and speaking to a couple of people.
The reason I didn't want to replace the tube is that I have just put it back in and painted the hull.
Me being me, I fitted the tube quite well, so would not enjoy the task of removing.
If it came to it I would make new bronze bushes and fit a 3mm shaft with a 4mm thread on a stepped end, I really don't think there is enough room for a 4mm shaft, and M3 props seem to be as rare as BA!
I'll take the plunge and get a BA die and see how it goes.
Other option was to machine the prop plain and locktite the shaft in, but I'd prefer a thread.
Rgds
Jon
 

[Jon]

Just to tidy this up with the final answer.

The shaft thread was M3.5, The shaft was 1/8", and the prop thread was 5BA 
M3.5 is 42tpi or thereabouts.

And I wonder why it was so hard to work it all out!
So replacing the shaft, threading it to 5BA, and attaching the prop with threadlock seems to have done the trick.

Jon

[zooma]

Back in the 60's when I first started with r/c model boats most prop shafts were either 2BA or 4BA.


Shortly afterwards metric sizes were used for prop shafts - usually 4mm or 5mm and this caused some confusion as the propellers in BA sizes were slowly being  phased out.


We found that the metric sized propellers simply screwed on to the old BA threaded shafts with no problem at all so the phasing-out of the old BA system made little or no difference  - thank goodness!