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Cutting French threads

Old 06-17-21, 01:03 PM
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TullySteve
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Cutting French threads

I decided to turn my beat up 1972 Moto Grand Record into a 1x with a new (probably hand-done) paint job. It's been hanging in the garage for over 10 years. The fork tube threads apparently got gnarled either by the Mafac brake bracket, or spacer, or something else. When backing off the top headset adjustable cup, I'm running into a jam and can see the bad threads. I dare not go further without advice. LBS does not have French taps & dies to re-cut threads. I'm in Syracuse NY area. Some have said to go ahead and start slowly working the cup off with a bit of WD40 and the harder steel of the cup may power through. I'm skeptical. If the fork & headset won't work, the project will just be abandoned. Any experience or advice welcome.
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Old 06-17-21, 01:35 PM
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harbor freight has a set of needle files for less than 5 bucks. I need magnifiers, but you should be able to straighten out the threads enough by hand to get things to screw together properly.
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Old 06-17-21, 02:41 PM
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So the fork/headset is still in the frame? If so can you thread the top race down enough to have the damaged threads cleat the race by a bit? If not then a die won't do anything since it's not going to "see" the threads until that top race gets out of the way. If you can access the threads with a triangular or thread file I would dress the damaged threads. Either way when the top race is removed (however much effort that takes) the race's threads usually are not affected.

Once the fork is out a real chasing with a die is easy to do. The shop I work for, Full Moon Vista, has a French fork die available to them. PM me if you need this done. Andy
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Old 06-17-21, 03:01 PM
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TullySteve
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Originally Posted by Andrew R Stewart
So the fork/headset is still in the frame? If so can you thread the top race down enough to have the damaged threads cleat the race by a bit? If not then a die won't do anything since it's not going to "see" the threads until that top race gets out of the way. If you can access the threads with a triangular or thread file I would dress the damaged threads. Either way when the top race is removed (however much effort that takes) the race's threads usually are not affected.

Once the fork is out a real chasing with a die is easy to do. The shop I work for, Full Moon Vista, has a French fork die available to them. PM me if you need this done. Andy
Yes, the fork and headset are still in place. I believe a die would "see" the damaged threads with the top race back down where it belongs. The difficultly occurred when screwing it off by 1/4 inch or more. Good advice about a file, lube. Thanks. Will reach out if needed.
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Old 06-17-21, 03:12 PM
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I think some judicious filing would be a better choice, I do not like forcing things or using nuts or bolts as dies or taps.
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