Randy,
Many 9/16-20 special pedal taps have/had a long taper area to help start the threads. Some of them even have/had a ground-in reamer to enlarge the hole to the proper size.
14mm = 0.5511"
9/16" = 0.5625" = 14.288mm
You're only removing 0.0114" (0.288mm) total or 0.0057" (0.144mm) per side which isn't much.
The thread pitch is so close that it's not an issue.
The big thing is to find the proper cutting/tapping compound. Are the cranks steel or aluminum?
Tapping compound for aluminum makes a world of difference on aluminum cranks.
Pedal taps are usually made of low quality carbon steel rather than high speed steel. They get pretty chewed up cutting steel crank arms.
Most pedal taps that I've seen at bike shops are already chewed up!
I just looked at the TAP-6 Pedal Taps that Park sells. They range from around ~$30 to ~$50 a set and don't appear to have much starting taper or lead on them.
I think that there were some taps sold under the Eldi or Elde brand that weren't as expensive.
Chas. Colerich Oakland, CA USA
randy dugan wrote:
> hello,
>
> when re-threading a set of french-threaded cranks to accept 9/16" pedals, do you
> drill the old threads out using the recommended drill size, or do you just
> muscle the tap in there and go for it?
>
> thanks,
> randy dugan
> van nuys, ca usa