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How hard is it to tap an old steel crank?

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How hard is it to tap an old steel crank?

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Old 07-09-18, 08:13 PM
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Aubergine 
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How hard is it to tap an old steel crank?

I have a couple old French steel cranks, and I would love to be able to use them with modern pedals. I have a tap, but before I try it i’d Like to know if there are any particular issues tapping out a steel crank for modern pedals. Thanks!
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Old 07-09-18, 09:17 PM
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I'm not certain of the existing vs new thread pitch, but in general a sharp tap and plenty of cutting oil should be what you need.

Please don't tap the hole like the buffoons on OC Choppers - by driving it in with a drill. Use a tap handle and back the tap out often to clear the chips.
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Old 07-10-18, 01:13 AM
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Should go pretty easy but make sure you have the right taps (one right-hand threaded, the other left-hand threaded), a tap wrench, a vice to hold the cranks and ways to check for alignment and squareness.

The biggest problems you might encounter:
1. Hole too small to start with (check the recommended hole for that thread) - it might appear to work but will likely break the tap.
2. Tap is not aligned properly. - will make a crooked hole or break the tap. either is VERY bad for pedals. Will feel like a bent pedal axle.

So be sure to take all precautions to align the hole as good as you can (pedal axis must be parallel to the BB axis). Start the tap and place a square to check in two planes. Check often to ensure a properly aligned thread.

Do you have the taps? - they are not very common and their value can be higher than the crank-set you are trying to refurbish.
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Old 07-10-18, 08:40 AM
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9/16 X 20 is the correct thread.

You said that you have the taps...Did you get them from a bicycle store or other source? People often assume that imperial taps are the correct thread, but pedal threads are not common in the north american market other than for bicycles.

So the above notes are largely correct, but all you are doing is changing the pitch of the threads by a small amount so all of the advice about drilling etc. is needless. Simply ensure that you are using the right and left taps, insert them into the existing thread and start slowly. You will feel the tap beginning to cut within a couple of turns. Drive the tap through the crank, with goodly amounts of cutting oil, and you'll be fine.

I have performed this operation many times without a problem: don't try to use your french threaded pedals again, though, because they will not seat properly and you'll end up with stripped cranks.

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Old 07-10-18, 10:24 AM
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Thanks for all the advice! Yes, I have proper left and right pedal taps. I need cutting oil though. I have in the past used the taps to clean out threads in cranks and other things, but this will be my first try at cutting new taps into French cranks.
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Old 07-10-18, 10:50 AM
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Use plenty of cutting oil

and only advance the tap a quarter turn, back out again , and clear the chips ,

then advance another cut , repeat, don't rush it..

is best practice with thread cutting taps.



...
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Old 07-10-18, 11:02 AM
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Originally Posted by fietsbob View Post
Use plenty of cutting oil and only advance the tap a quarter turn, back out again , and clear the chips , then advance another cut , repeat, don't rush it..

is best practice with thread cutting taps...
FB beat me to it - great advice. I have converted many aluminum cranks with French Threading and it's easy and effective. Steel is a bit tougher so go slow. Also, I always start my re-taps from the back side of the crank to avoid any irregularities on the front-side threads caused by inattentive mounting.
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Old 07-10-18, 11:11 AM
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Pick up a can of Tap Magic. In the size you need, it's cheaper than chips.
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Old 07-10-18, 11:28 AM
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If the pedals are standard Fr threading, they are 14Mx1.25 thread which for the length of the pedal thread is so close to the 20 TPI of modern pedals (9/16" nominal)
that you will only have to remove about 20-25 thousandths of metal with the taps. In a pinch any oil will do though cutting oil is preferred, you can get cutting oil
at HD/Lowes in the plumbing dept. but if this will be your only threading job any lube will suffice for a one-shot job. The advice to turn the tap no more than 1/2 or 3/4 turn
and then reversing the tap at least 1/4 turn is important as this breaks the chip you are gouging out and facilitates the cutting action with standard taps.
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Old 07-10-18, 12:41 PM
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Again, thanks so much for the great advice. I will find some cutting oil for my shop.
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Old 07-10-18, 05:57 PM
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it could very well be so that you see that the diameter is too close and you cant cut it because you will be cutting in other threads. could be.

And it could also be that the arms are hardened steel. and then you cant cut it either. unless you wanna spend 10 taps on it.
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Old 07-11-18, 12:30 AM
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...I have done this maybe four or five times. It goes better if you do it off the bike by removing the cranks.

Other than liberal use of cutting oil, going slowly, and backing the tap every turn or so to remove the chips that tend to jam it, the only other advice I can give you is that the longer and more gradually tapered taps that Hozan makes and sells under their name are more easily started, and you have a better chance of getting them in straight. But I did my first set for someone with a regular, shorter set of taps from Eldi. You just need to have a metal square so you can make certain you've got them started at 90* before you drive them all the way in. It's quite possible to get the hole a little off from 90*, and then you're kinda screwed without reaming them out larger and putting in a threadsert.

Short tap:



Hozan tap:
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Old 07-11-18, 09:43 AM
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Alas, I have short taps but at least the cranks are off the bike. :-) I have a second set of steel cranks on a bike too, but I can try tapping the loose ones first.
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