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UNF reducing bush

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mick H20/06/2020 13:19:10
723 forum posts
21 photos

Can anyone assist in locating a 7/8 (male) x 3/4 " (female) UNF reducing bush. I am blowed if I can find one on Google.


Roderick Jenkins20/06/2020 13:33:11
1925 forum posts
503 photos

Have you tried searching for adapter rather than reducing bush?


HOWARDT20/06/2020 13:33:58
584 forum posts
15 photos

I doubt you will find one, the difference between the core diameter of 7/8 and the old of 3/4 only leaves around 0.032” in diameter. Unless you can accomodate an adaptor.

Oldiron20/06/2020 13:54:57
484 forum posts
22 photos

HowardT beat me to it. Had a look through all my books but cannot find one


Howard Lewis20/06/2020 14:25:41
3544 forum posts
2 photos

For concentric threads, IF you are prepared to live dangerously, assuming that the 3/4 is the male item, you could try making one.

Drill and tap or internally screwcut the 3/4 thread and then screw it onto the 3/4 bolt /shaft.

Turn the OD and screwcut the 7/8 thread, but VERY gently.

You may get lucky and succeed, but with only 0.023" a side, you have little metal to provide strength..

If there is space for an adaptor with the threads end to end, you could make the 3/4 internal thread, and a little way, (at least 0.0625" ) beyond it start to cut the 7/8 external thread.



Ed Duffner20/06/2020 14:56:38
810 forum posts
91 photos

A few results from Google <here> ...will need some further lookup from the results, based on the required application (air, water, gas etc).

Search term "7/8 male to 3/4 female UNF reducer". You could also try 'coupler'.


Speedy Builder520/06/2020 15:36:14
2081 forum posts
145 photos

Helicoil ??

Martin Connelly20/06/2020 15:48:20
1422 forum posts
165 photos

The standard UNF threads have 3/4" x 16 tpi and 7/8" x 14 tpi. Hard to helicoil with two different pitches.

Martin C

not done it yet20/06/2020 15:52:18
4900 forum posts
20 photos
Posted by Ed Duffner on 20/06/2020 14:56:38:

A few results from Google <here> ...will need some further lookup from the results, based on the required application (air, water, gas etc).

Search term "7/8 male to 3/4 female UNF reducer". You could also try 'coupler'.


They are adaptors, not bushes. I expect that is likely what the OP needs, mind.(see Rod’s post earlier).

Martin Connelly20/06/2020 16:19:54
1422 forum posts
165 photos

I bet "reducing nipple" pulls up some unexpected Google results wink

Martin C

Speedy Builder520/06/2020 16:49:02
2081 forum posts
145 photos

I wouldn't discount a helicoil, agreed 14/16 tip mismatch is not the best, but a compromise may work it all depends upon how much load you need to put on the insert, do you need to undo the joint often etc etc. Perhaps more info on what you are trying to achieve would help.

Chris Gunn20/06/2020 18:07:59
336 forum posts
24 photos

Could you open up the 7/8" thread to say 1", and then you will have more wall thickness to make a 1" to 3/4 adaptor? or find what adaptors you can get, and open up the 7/8" to suit. Without knowing what you are trying to fit together it is hard to advise a good solution.

Chris Gunn

Macolm22/06/2020 12:38:15
17 forum posts
4 photos

In most cases this “thread adaptor” problem is easily solved, provided you have a little proficiency in screw cutting. (I previously posted how I converted M12 threaded Morse parts for a 3/8 Whitworth drawbar). Clearly a wire thread insert works fine, so the problem is to make a similar item if the available wall thickness will be insufficient to support the cutting forces

However, we can trade wall thickness against the depth (or percentage engagement) of the external thread by truncating the inner portion of the thread form. Decide the minimum diameter necessary, and bore out the thread which needs converted to this size. Now turn up the adaptor per the sketch. Cutting the external thread is very easy to do using an ordinary V lathe tool, though preferable with a flat on the nose of the V. A pilot length that is a nice fit in the bored out mating part makes sizing easy.

Do not set over the topslide, but cut the thread increasing the depth to reach the pilot diameter. Now take cuts at this diameter advancing the topslide until the full thread is reached. You can check this using something with the same (full) thread.

Clearly it is best to cut the smaller internal thread first to allow a relatively thin wall.thread adapter.jpg

Paul Lousick22/06/2020 14:12:37
1501 forum posts
572 photos

Because of the thin wall section between the 2 threads, can you do someting like this ? Probably cannot buy a commercialy made adaptor and would have to make it yourself.


thread adaptor 2.jpg

Macolm22/06/2020 14:59:07
17 forum posts
4 photos

That`s the obvious way where there is enough room for the extra length, and including any considerations about cross section if there is axial loading. In the case of something like a drawbar thread, it needs to keep within the original profile and also take the tension force.

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