Thread substitute

Hi All. If plans for an engine require a 3/8" 26tpi external thread on a shaft is it a problem to use 3/8" BSF instead?

Help much appreciated.

Andy

Andy,

3/8" BSF is 20 tpi

... If the connecting parts match, I can't see it being a problem.

MichaelG.

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Edit:  Alternatively, you might consider 3/8" UNF, which is 24 tpi

... as used on many drill chucks.

Edited By Michael Gilligan on 20/01/2014 21:17:17

Thanks Michael

3/8 x 26 is what's know as 'cycle' thread neither taps or die's are hard to obtain with most ME suppliers selling them.

There are two 26TPI series of threads.

BSB (British Standard Brass) with 55deg thread and the BSC (British Standard Cycle) with 60deg thread.

Barry

Andy,

What model and which parts need the 3/8" 26tpi thread ?

Dave

Thanks all. It's the crank shaft for the Wyvern IC engine. The shaft is threaded at both ends and secured with nuts to stop end play.

Use the UNF 24 thread, unless you find a cheaper die in cycle thread. Unless you can cut the thread on the lathe. If you have never done thread cutting, now is a chance to try, grind up a HSS tool, and try it, even if you just turn the lathe by hand, that's if you have a screw cutting lathe of course. Find some scraps, and practice. Ian S C

Well-said, Ian

Another possibility [ Having had a just quick look at the Wyvern illustration ] might be to make the crankshaft components from existing High Tensile 3/8" UNF Bolts; which should be readily available from Automotive parts suppliers.

MichaelG.

I hadn't thought of the bolt idea. I certainly can screw cut the thread - it means digging out the imperial conversion kit on my metric Denford lathe which I have never used.

Good advice

On looking at the drawing, I would do a modification to the crankshaft, one of two ways, first way, extend the shaft say about 3/4" beyond the nut, and reduce the diameter to 5/16", or 8 mm to take a pulley. Method two; make a pulley, bore and tap to suit the thread on the shaft, and use that instead of the hex nut. The motor will look more realistic with a means of driving something, maybe a little generator, or a model lathe, anything, motors are made to do work, or at least look like they might. Ian S C

Thanks Ian

I was just looking a Rod's Wyvern build and you won't be able to use bolts for the crankshaft as they need to be a larger dia on the bearing surfaces.

Pic of the sort of Pully Ian is suggesting here or if it too late you could always go for one that fits to te flywheel spokes.

If you don't want to fit your imperial conversion kit why not make the reduced ends 10mm dia and screwcut a M10 x 1.0 fine metric thread and make the nuts to suit.

Edited By JasonB on 25/01/2014 16:57:37

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Oh Bother !!

Shoulder screws may be available in a suitable size, but it's probably not worth searching for them.

MichaelG.

Edited By Michael Gilligan on 25/01/2014 18:00:15