# Retap #8-32 hole for M4?



## JimG (Oct 3, 2012)

I know this is OT, and I apologize in advance.  But I thought someone in this group might be able to advise me.

I have a need to replace two binding posts (for wires) in an electronics device.  The present binding posts are #8-32 and fit into a solid copper block that has been tapped for that size thread.

The manufacturer of the posts (Pomona part no. 3770-0) has switched to M4 threads on these posts, and I can only buy the M4 version.  The existing #8-32 tapped holes will not accept the new M4 studs, and I am afraid that the present posts are too far gone to be able to repair them (the plastic covers have split and fallen off).

Is it likely I'd be able to re-tap the 8-32 holes in the copper to accept M4?  It is not necessary for these posts to be tightened beyond a snug fit, and there is relatively little load on them during use.  A small jam nut will be present on the back side of the copper block once the post is threaded all the way through the copper block.

Thanks in advance.

Jim


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## kvom (Oct 3, 2012)

The best way to test this is to take a piece of scrap copper, drill and tap 8-32, then see if the M4 tap will rethread properly.  8-32 is .79mm pitch vs. .7mm M4.  Might work if the copper isn't too thick.


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## JimG (Oct 3, 2012)

kvom said:


> The best way to test this is to take a piece of scrap copper, drill and tap 8-32, then see if the M4 tap will rethread properly.  8-32 is .79mm pitch vs. .7mm M4.  Might work if the copper isn't too thick.


The copper's about an inch thick.  I happen to have some scrap and will give your suggestion a try.  Good idea.

Thanks.

Jim


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## Alchymist (Oct 6, 2012)

IF there's room, and the M4 retap doesn't work, drill & tap for 1/4-20, screw in a copper (if available, or brass) screw, cut off flush, peen for tight fit, then drill and tap for M4.


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## kvom (Oct 6, 2012)

Alchymist said:


> IF there's room, and the M4 retap doesn't work, drill & tap for 1/4-20, screw in a copper (if available, or brass) screw, cut off flush, peen for tight fit, then drill and tap for M4.



Silicon bronze would likely be better than brass.  For 1/4-20 bronze full thread screws look here:  http://www.boltdepot.com/Tap_bolts_Silicon_bronze.aspx


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