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Thread Cutting on Myford Super 7

Cutting a UN thread

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Tom Gullan30/09/2019 12:28:27
87 forum posts
34 photos


I’ve cut quite a few different types of imperial threads on my Myford Super 7B with success however, I now have a problem!

I’m trying to cut a 1” x 24tpi UN thread… my scratch test is way out! The plate on the gearbox states “For fine feeds reverse gear on stud ‘A’ ”.

I can’t find stud ‘A’. Can anyone help?


Martin Kyte30/09/2019 12:41:27
2017 forum posts
36 photos

The gear is the one held in place with the 'latch strap' for want of a better word. Open the cover to the gear train and it should be obvious. The black strap just swings out of the way and the gear can be removed and reversed.

If you have cut threads before you will have had the gear in the correct orientation I should have thought. You don't want fine feeds for thread cutting. Is ther not a picture on th gearbox plate?

regards Martin

Martin Kyte30/09/2019 12:45:21
2017 forum posts
36 photos



The strap is item 211 the stud is 206 and the gear is 208

regards Martin

Brian Wood01/10/2019 09:31:21
2246 forum posts
37 photos


Just to add a bit of detail to Martin's posts. The gear on stud A is a tandem 19/57 tooth compound if that helps you recognise it and as he says, it is reversed on that stud to give fine feeds for normal machining...

24 tpi is a standard pitch that the gearbox can provide, the middle choice of the set 12/24/48 tpi , just follow the picture on the gearbox plate and set it accordingly.

Regards Brian

Tom Gullan01/10/2019 10:15:38
87 forum posts
34 photos


Thank you, I appreciate your input.



Simon Williams 301/10/2019 10:16:36
519 forum posts
80 photos

Tom, good morning.

24TPI isn't a fine feed within the meaning of the phrase in this circumstance, and you don't need to reverse the 17/57 gear pair on stud A to get it. Reversing this gear gives you the range of fine feeds as marked on the gearbox label, but we're talking about a few thou per rev (13 down to 1) such as you would use in turning a parallel surface.

If setting the top lever to centre, and the sliding handle to third from the right doesn't get you 24TPI there is something else wrong. Could you let us know what thread your scratch test showed, maybe we can reverse engineer the cause from the symptoms.


Handily my picture happens to be taken from the very angle that lets the top lever obscure the 24TPI setting on the label.

What thread have you actually cut, and if it is very fine indeed is the gear on stud A already reversed?

Otherwise can we have a picture of the gear train from the mandrel down to the gearbox input gear cos' the explanation's there somewhere.

HTH Simon

Tom Gullan01/10/2019 10:27:44
87 forum posts
34 photos

Brian, Simon,

I managed to figure out what I was doing wrong. The gear leaver on the top of the gearbox was not engaged in the middle position although it looked as if it was... it was actually cutting 12 tpi instead of 24 tpi.

The reason I’m cutting a fine thread is to allow me to screw cylinder liners onto the casing of my Upshur Opposed Twin four Stroke Petrol engine. I only want to start the thread so that the die will run parallel with the liner. I’m not an engineer.... only a keen amateur!! I thought this would be the best way to ensure that the thread was concentric with the bore.



Simon Williams 301/10/2019 12:25:08
519 forum posts
80 photos

Ah Hah! That'll do it!

Many thanks Tom for the update, it's always nice to know how these things resolve themselves.

Now we're looking forward to some pictures of the EIP (engine in progress) pretty please

Best rgds Simon

Emgee01/10/2019 13:47:03
1652 forum posts
224 photos
Posted by Tom Gullan on 01/10/2019 10:27:44:

Brian, Simon,The reason I’m cutting a fine thread is to allow me to screw cylinder liners onto the casing of my Upshur Opposed Twin four Stroke Petrol engine. I only want to start the thread so that the die will run parallel with the liner.



Tom, when you are set-up for threading it may be best to continue the thread for the full length to almost full depth before using the die to clean the crests and roots, your'e sure then of a parallel thread for the full length.


Tom Gullan01/10/2019 17:21:37
87 forum posts
34 photos


Thank you.


Tom Gullan01/10/2019 17:38:56
87 forum posts
34 photos


As requested, here are some photos of my build so far:

1. The Petrol tank


2. A CAD mock-up of the casing.


3. Cylinder head.


4. Crankshafts.


5. Flywheel 01.


6. Cylinder, con-rods, and piston.


7. Flywheel 02.


8. Cams.


9. Carburettor.


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