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In the past, I've used a cut-off M4.5 cap screw and cut a screwdriver slot in the cut end.
There has to be a parts fiche on the net some where that will give the information.
Original Lucas signals had stems that were threaded 7/16-14.I have an Aprilia signal somewhere around here, will have dig it out and measure the set screw. They were used on some 850-Ts as far as I know.
Unfortunately, I used thread gauges at three different stores. The only one that would accept it was a 10-32, but 10-32 screws will not fit into the turn signal. That is the reason I posted the question here. I hoped someone might have had to replace one and would know.
That just means they didn't have the thread size in the gauge.
So why not just tap the hole to a size that you can easily get or already have?