Friday, October 26, 2012

What's in the Stand: Tapping Stronglight Cranks


Stronglight is a venerable French bicycle parts manufacturer that has been making high quality stuff since the 1940s.  I have seen more of their cranks than anything else, though I did get to see an absolutely immaculate Stronglight bottom bracket at one point. Fifty years of service and every bearing surface was still smooth as could be. They still make parts, and most of their stuff these days is of the carbon variety, but I tend to enjoy their older stuff more. They can still make an attractive alloy crank though, and the Stronglight website has all the info you could ever want and need about their current product range.

Stronglight cranks offer interesting challenges to bicycle mechanics. First off, they are French. Proudly French. And a lot of older Stronglight cranks bear that distinct mark of French manufacture, non-standard thread pitch and sizing. Second, original Stronglight cranks can have some weird bolt-circle diamters which can limit your choice in replacement chainrings.

As far as the thread pitch goes, French cranks used a different thread pitch for their pedal spindle than everyone else. French pedals are tapped at 14mm x 1.25mm (0.55" x 20.32tpi). Standard English pedals are tapped at 9/16" x 20tpi. The downside to this is that you can not fit standard, modern pedals into an unmodified French crank. The positive spin here is that it is relatively easy to retap those threads and make a modern pedal fit. All you need is the right taps (you friendly local bike shop should have them), and the experience and knowledge to use the tools effectively.

The bike in question, and the inspiration for this entry, had French threaded cranks and the owner wanted to use modern pedals on these older cranks. This is a perfectly reasonable thing to expect, and he brought it into the shop so that we could complete the work.
I'm obnoxiously opinionated and vocal about taps, and I will most likely get on your ass if I see you using one incorrectly. It's a personal thing, I can't explain it, but there it is. For some deep, unknown reason it really irritates me when I find dirty taps or dies. Not to wax overly philosophical, but I feel that it has a lot to do with respecting the tools that you use. If you genuinely love the tools that you use, if you care enough about how a particular tool fits in your hand and you went out of your way to buy good quality equipment, then take the time to take care of it. But enough of that.

Tapping cranks is fairly straightforward, but you do need to make sure that you are using pedal taps. They usually come as a pair since the two crank arms have opposite threadings. Park Tool makes a really good set for about $50. The job is the same as any other tapping job; go slowly, use plenty of cutting fluid, and make sure that you're clearing out the chips by backing out 1/2 turn for every 1 turn you go in. That and making sure you clean up afterwards. I like to spray taps with Speed Clean aerosol degreaser, and I have heard of people putting them in an ultrasonic cleaner. Either way, you don't want the tool to be put away covered with cutting fluid and metal chips.

Stronglight cranks, once they had moved to a cotterless design, also used a different thread pitch and size for their crank extractors. Most other companies stuck with the Campagnolo specified 22mm crank extractor, but Stronglight decided to use a 23.35mm crank extractor. If you want to get those Stronglight cranks off, you need the right tool. Harris Cyclery, former domain of Sheldon Brown, is one of the few places that still has these available. In 1982 Stronglight switched over to a 22mm design and things became easier.

The only thing that remains to be said about taps is that you need to be darn sure that you're using the right tap for the job. They come in all sizes and all thread pitches and it's pretty easy to mix up 5mm x .8mm and 5mm x 1.0mm. If it's close you can fix it by using another tap, but you could have also taken the time to find the right tool that first time around.

3 comments:

  1. Great details!

    If you know of anyone looking for a set of vintage NOS TA Pro Vis triple cranks, let me know. I picked them up a while ago, but never put them on the Ebisu that I sold a while back.

    Photos here

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  2. A very well-written post. I read and liked the post and have also bookmarked you. All the best for future endeavors.


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  3. wow, great article! recently became custodian of 70's/80's? Peugot PX-10. looks like someone cross threaded the left pedal a bit. Now if I can find a mechanic with the similar skillset as you.

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