Rear Subframe captive nut threaded

[PDF Racing]

Hi all, I am rebuilding a Mk1 Cooper s race car. one of the rear n/s captive nuts which hold the rear subframe against the nearside inner cill has threaded. Normally I would tap and helicoil and particularly as the car can’t now be chopped to access these nuts.
Is this what others do ? Helicoil
Thanks

[mk1]

A helicoil works fine in this position. It is not uncommon for this to be an issue.

[Peter Laidler]

I don't understand the question. What do you mean by 'threaded'? It is already threaded. Or do you mean CROSS threaded? Or do you mean STRIPPED of thread? There's not a lot of thread in that captive nut to helicoil but you could carefully MIG, redrill and tap too. But at least you have got the old bolt out.

[PDF Racing]

Thanks both,
It has very little hold on the thread and does up but any poundage and it just pops back a thread.
Helicoil would be the preferred option. Was wondering if this is common and what the majority of people do to get round it.
Thanks

[MiNiKiN]

As Peter said, there is not a lot of thread length from a start, so helicoil might work.
In the past I too drilled out the old captive nut and welded in a standard nut. If you can get one, get a double length nut, fit it such it protudes a couple milimeters, weld around and grind it flush. Make sure to exactly position that nut, otherwise fitting the subframe will be a right PITA, i.e. not possible.

[PDF Racing]

Great advice

[Spider]

These holes are only threaded in to a 1/4" piece of flat bar that has both holes that welded to the body - or at least that's how they were in our local production.

Not to undermine Mark's helpful suggestion, Helicoil will likely work OK as long as it's just one hole, but with the plate being so thin to start with, it's little wonder these threads strip in the first place, but it's right on the lower limit for a Helicoil.

Along with the other great suggestions here are a heavy duty version of Rivnuts available. I get mine from a supplier who's based in Pershore, he has Metric (of course), UNC and UNF types.

[PDF Racing]

Hi Spider
Rivnut is a good idea if heavy duty and able to take the hardened steel bolt.
I also had considered whether to drill a 1 inch hole into the plate within the rear n/s storage compartment. This way I could put a nut on and when the cars running have someone spot weld the nut.
Could you possibly send a link to the HD Rivnuts as I couldn’t see any link or details.
Thanks

[MiNiKiN]

Are these then countersunk riv nuts? Because the ones I know protrude by at least 2mm

[Spider]

Could you possibly send a link to the HD Rivnuts as I couldn’t see any link or details.
Thanks

https://memfast.co.uk/

What makes these work well here in particular is that it's not just going in to sheet metal but also the sandwich plate (that's part of the body).

[PDF Racing]

Thanks I will call the company and discuss as I want to ensure the rivnut is the correct tpi for the bolt.
If I can get a heavy duty load bearing rivnut it will be a cleaner quicker permanent fix.
If I just practiced with my welder and was competent I would drill out and place two nuts together welded in place. I am not so this is my next best option and quite neat. Thanks again

[Spider]

Thanks I will call the company and discuss as I want to ensure the rivnut is the correct tpi for the bolt.

This will be them;-

https://memfast.co.uk/shop/vclose1.asp? ... 6138003713

They may try and sell you an installation tool, which if you are doing a lot of them is the best way to go, but for one here and there, it's a bit expensive.

You can 'crimp' them using a High Tensile Set Screw, a decent Washer and a quality Plain Nut, Grease the thread on the Set Screw first. Wind the Nut on some way, fit the washer, then screw on the Rivnut, be sure it's fully engaged. Fit the Rivnut in to the Hole (if fresh drilled in steel paint it), then with 2 spanners, use one to hold the Set Screw Head fast and tighten the Nut with the other. It will need a bit of grunt, but def, doable.

[PDF Racing]

Hi Spider
I have a gun which I use for inserting alloy rivnuts for dashes. Never thought of using it for this....not sure why
Cheers

[GraemeC]

Be careful with your existing setting tool - if it wasn't designed for steel rivnuts then the mandrels aren't always as strong as you might hope.....