7BA Helicoil

7BA Helicoil

This topic has 5 replies, 5 voices, and was last updated 4 November 2018 at 18:51 by Nick Hulme.

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4 November 2018 at 14:57 #379121
Bill Greeley
Participant
@billgreeley13267
I'm trying to bring a Stuart Triple Expansion engine back to life. Some of the threads for the low pressure piston cover have been damaged. One appears to have a helicoil insert. Are 7BA helicoils available?k I found a 6BA but not a 7BA. Also looking for a 3/16-20 tap and die. Does anyone know were I can purchase these parts.

Thanks
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4 November 2018 at 14:57 #26285
Bill Greeley
Participant
@billgreeley13267
4 November 2018 at 15:19 #379126
JasonB
Moderator
@jasonb
You could make up a couple of threaded bushes, probably have to be 4 or 5 BA external thread and 7BA internal a bit like these

The other option is to drill out and thread the hole to whatever you can get in maybe 5BA and then make a stepped stud with 5BA one end to go into the new hole and 7BA so it looks correct from the outside and fits the cover.

Don't hold out much hope of you finding a tap and die as the specified thread is a double start so needs single point cutting and make your own tap at the same setting.
4 November 2018 at 15:28 #379129
Mike Poole
Participant
@mikepoole82104
Tracy tools are always worth a try for taps and dies, would converting to M2.5 from 7BA be out of the question? They are very close but not interchangeable so new bolts or studs would need to be made. Helicoils are great for putting a durable thread in many softer materials especially if dismantling is a regular task.

Mike
4 November 2018 at 17:40 #379153
Neil Wyatt
Moderator
@neilwyatt
Another option is to drill oversize and use a tap-in-fit plug (possible with a little high-temperature retainer).
4 November 2018 at 18:51 #379171
Nick Hulme
Participant
@nickhulme30114
I can only find down to 6BA –

https://www.uni-thread.com/tr_ba_inserts.php
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