Stumped On This Thread Problem

boostin53

boostin53

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Today I decided to make a shift knob for my car, which is threaded m12x1.25. I made a mandrel to test fit the knob since it has a counter bore for a reverse lockout. The factory knob fits it well.

Made up the new knob from 6061 aluminum. It fits the madrel just fine. Take it to the car and it's a no go. It begins to thread then gets really hard to turn then just stops. Feels as if the thread pitch is wrong. But I used a m12x1.25 tap to make the threads and used a 27/64 bit for the pilot hole. Again, both the factory knob and my new knob thread on the madrel I made just fine. But the new knob will not thread on the shifter in the car. I also triple checked the shifter in the car for the correct thread pitch. Anybody have a clue what I should do???
 
Does the knob feel slightly looser on the replica you made? If not, then either your threads have a peak that the shifter can't accept or possibly there's a taper somewhere. Did you use a taper tap or plug tap? Can you use a wire brush on your female threads and see if that helps?


Steve Shannon
 
The tap I used is a plug tap. The threads have a length of 1.05" and the knob only threads on to the shifter about .25" before becoming stiff.

Both the factory knob and my new knob thread on the replica about the same. I'm thinking that maybe the root of the thread needs to be deeper than the tap provides. I just compared the two knobs on the shifter. The factory knob has a fair amount of wobble while the new knob has a slight wobble.
 
Have you checked your shifter thread carefully? 1.25mm is equivalent to 20.3 tpi. An M12 x 1.25 thread is very close to a 1/2-20 thread. An M12 bolt would probably thread onto a 1/2 - 20 nut because of the slightly smaller diameter but an M12 nut will not thread onto an 1/2 - 20 bolt.

(Edit: corrected hastily written M12 x 1.25 bolt to 1/2 - 20 bolt)
 
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to throw another consideration into the mix, there is a m12x1.5 metric pitch as well...
 
According to several aftermarket knobs for the same car, it's an m12x1.25 thread. I determined the crest of the thread was the problem and the cheap tap was to blame. I put the knob back on the lathe and took .003" off the top of the threads. Rand the tap back thru just to clean it up. Took it to the car and it now fits. It was a snug fit, but it's now on. I guess I should invest in better taps and dies haha. Here's the new knobimage.jpeg
 
Specs on the knob for anybody that's curious: 6061 aluminum, 2" o.d, 8° taper At the bottom, .875" counter bore for the reverse lockout lever. Slight radius at the top with three grooves for detail that are .10" deep and spaced .15" apart.

Next project will be to make the actual reverse lockout lever. Not really needed, but I like being able to say I made them.
 
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