Oliver Tiger MKIII crankshaft threads.
 

Hi,

I recently received an original Oliver Tiger MKIII which has seen much use, but I believe it is a starter. The engine did not have a prop nut, so I wanted to make one on my lathe.
I measured the crank threads, pitch and diameter, and concluded it most probably is 1/4" BSF. After buying a set of taps on Ebay I made the prop nut.

My problem is, even everything seems ok, the pitch of the taps fit the crankshaft threads, and the taps are labeled 1/4" BSF, the nut will not engage the crankshaft more than just a couple of turns.
The shaft is undamaged and the threads seem in full working order. The measured diameter is approx 6.25mm.

Could anyone please advice on the correct thread type?

Thanks,
Helge.

That's right the shaft should be 1/4" BSF 26 tpi. You might have to "chase" the shaft thread with a 1/4" BSF die or die nut.

Thank you for your reply, this is reassuring. I checked other thread types, and couldn't find any with the same dia/tpi spec. So the threads need dressing on one part. I am a bit hesitant to modify the crank threads since this engine is collecable (to me at least) because of it's competition history. Maybe I should try and make a screw just a little bit oversize, and dress the threads of my newly made aluminium spinner.

Always an option! I'm confident that replacement crankshafts are still available if the worst happens. Not genuine but mostly stronger.

Well, the spinner now fits the Oliver crankshaft. Yesterday I received a stainless steel bolt 1/4"-26 BSF in the mail. Just like on the crank, the bolt only entered the spinner a couple of turns. So i filed a cutting edge in the bolt (see pic) and with gentle force it went into the aluminium spinner all the way. The amount of shavings were very little. Then the spinner threaded onto the Oliver shaft without force and with a very good fit.

Looking forward to starting the engine and feel the smell and sound of 50 years ago.

Good news! :-)