Solutions for Chassis Pem Nuts falling out?

[nevetslab]

Among the hardware issues on servicing some aging SWR 750x and Bass 750's, I found the Bass 750 containing 'spare parts' rattling around inside. There were missing top cover screws, as well as missing right side cover screws. Once I got the cover off, I saw what I didn't want to see. Pem Nuts that had come out of the 1/8" aluminum folded chassis. Those are put in with a Hager Press or equivalent, under signifant pressure (don't recall the 'tonage spec), but I've never had any success in pressing in new Pem Nuts where the former ones had come out. I've never tried that with something like JB Weld.

Nothing comes to mind, short of Clip Nuts, though all I have in that area are for 16ga material, # 6-32. I suppose I could track down some J-Clips, though I'd rather have a machine-thread clip nut or something proper.

Any ideas?

[drewl]

I have no problem popping them back in an either tightening the flange to keep them in place or a tap of the chassis that bends back into place with a hammer.

[J M Fahey]

As a Plan B: if steel nuts and steel chassis are involved IŽd plain *solder* them in place.

IŽd even use a drop of that suppossedly corrosive flux not recommended for electronic use, which is way more active that our regular snowflake approved rosin.

Just use a 100W iron and wipe the area clean afterwards with a wet rag to remove flux traces.

[nevetslab]

In this case, having already tried to drive them back into place with a small machinist vise, it popped back out. I stopped off at a local industrial surplus hardware store on the way in, found some # 6-32 j-clips that fit 1/8" panel (rare item), though once I slipped them into place, the holes didn't fully align. But, it was close enough to drive in # 6-32 FHMS's and thread them into the J-clips.

When I ran into that problem and still in manufacturing, I had access to the Hager press, which DOES drive them back into place.

I like Juan's Plan B solution! Though here, it was black anodized aluminum, so I'll keep that plan for later!

I must have encountered a Pem nut hole that got buggered by the nut that popped out. They are hole-diameter critical, and when pressed in, the material flows around the shaped sleeve that tapers inwards to a smaller diameter, then the teeth grab the material to counter rotational force once pressed in and in use.

[drewl]

Luckily we also use the pop in ones on different products at work so I have a bunch of those in different sizes to replace lost ones.

Also, if one is missing check to make sure it isn't rolling around inside the amp, ready to short something out and blow it up!

[galaxiex]

Glad you found a solution.

I have a 1970 Traynor GuitarMate that has long bolts that go all the way thru the chassis.
The keps nuts are separate and you have to hold them in place with your fingers.
The one in back of the P/T is very hard to get at.

So the last time I had the chassis out, I MIG welded the nuts to the chassis.

If I ever strip one, I'll just grind it off and weld another.

This obviously wouldn't work for you with an aluminum chassis,
unless you can figure a way to weld a steel nut to aluminum...

[Jazz P Bass]

If you are referring to PEM broaching nuts, the ones with the little teeth, it is the hole diameter that is critical to the fastener gripping correctly.

If the hole diameter is ripped up from an old fastener, a new PEM nut will not grab.

[drewl]

Looking at the bottom plate of a Bogen power amp I made into a guitar amp years ago, I used a big ass soldering iron to solder nuts to the bottom plate to mount it in a head box.

It is literally staring at me right now.

I'm scared.

[galaxiex]

If you are referring to PEM broaching nuts, the ones with the little teeth, it is the hole diameter that is critical to the fastener gripping correctly.

If the hole diameter is ripped up from an old fastener, a new PEM nut will not grab.

Not sure if this was directed at me...

The original nuts on the Traynor chassis were keps nuts,
the ones with the little star washer crimped on like you sometimes see on speaker nuts.

I just welded plain nuts to the chassis.

[Jazz P Bass]

"Not sure if this was directed at me..."
No, it was not.
It sounded like Nevetslab was dealing with them.
He did mention that he did not know the force required to drive a PEM into cold roll steel.
It does depend on the screw size but it is somewhere in the neighborhood of 4000 lbs.

[patlaw]

When the chassis will hold the PEM, I've been able to pull them back into place with a screw and washer on the opposite side of the chassis. Thankfully I haven't had to do many.

[nevetslab]

"Not sure if this was directed at me..."
No, it was not.
It sounded like Nevetslab was dealing with them.
He did mention that he did not know the force required to drive a PEM into cold roll steel.
It does depend on the screw size but it is somewhere in the neighborhood of 4000 lbs.

Yup.......about 4000 lbs on cold rolled steel, a bit less on aluminum...2500-3000 lbs. I just looked up the details on Pem's website. Found I mispelled Haeger as 'Hager). On the second chassis having spare Pem's rattling around inside yesterday, I took a shot with a Grade 5 # 8-32 Hex washer head screw, and drove that into the Pem nut, drawing it back into the vacated hole for the Pem nut, and that did seem to hold. For how long? At least until the next repair, though felt like it was in solid. Think good thoughts putting the cover back on.....hate to find out when it's the last screw of 14 going in to have that knock the Pem nut out!