The key to this pedal rethreading is that the various diameters (thread, pitch, root) are indeed quite close, and that the threaded hole is fairly short.
The TPI is 20 vs. 20.32 -- but this discrepancy *accumulates* over the course of the operation. What this discrepancy means on a crankarm, which is only 1/2" or so thick, is that there will not be a huge amount of material removed from any thread. However, if a crankarm were 1.5" thick, the tap would be pretty much completely obliterating the thread at the far end of the hole (because it would now be a full 1/2 of a thread off of "same" (peaks of the old thread at the valleys of the new)).
Yes, I know...the first few threads take all of the load, so it wouldn't actually matter.
I wonder what a threadforming tap (as opposed to a threadcutting tap) would do in this application? Not that anybody probably makes them in this oddball size (especially the left-handed one).
Scott Minneman San Francisco, CA USA
-----Original Message----- From: classicrendezvous-bounces+minneman=onomy.com@bikelist.org [mailto:classicrendezvous-bounces+minneman=onomy.com@bikelist.org] On Behalf Of Greg Reiche Sent: Monday, February 14, 2011 3:20 PM To: randy dugan; classicrendezvous Subject: Re: [CR] French-to-English Pedal Tapping - Drill or No?
Randy,
The sizes are so close, drilling and muscle aren't necessary. Only the proper cutting fluid is.
9/16" is only about .011" bigger than 14mm, and 20tpi is only .02mm more than 1.25. You'll be surprised how little metal is removed.
Greg Reiche CyclArt, Inc. Vista, CA USA
-----Original Message----- From: classicrendezvous-bounces+shop=cyclart.com@bikelist.org [mailto:classicrendezvous-bounces+shop=cyclart.com@bikelist.org] On Behalf Of randy dugan Sent: Monday, February 14, 2011 2:59 PM To: classicrendezvous Subject: [CR] French-to-English Pedal Tapping - Drill or No?
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