Rear subframe stay bracket bolt removal issues

[DatsunBanana]

Trying to swap the rear subframe on my R34 and can't remove the L-shaped stay brackets between the front subframe mounts and the sill/chassis. I'm in the UK so rust is a problem. I've used loads of WD40 and then Tunap 103 for days beforehand. I backed off the bolts forward a bit, backward a bit repeatedly for ages to try and avoid snapping. They have all moved a few full turns. The one I've spent most time on has come loose and I can turn with fingers, and it will pull down and push up 2-3mm. But it won't come out. I can tighten it back up and loosen it off again repeatedly but it still won't come out. I assumed there's a captive nut inside but if that's broken loose then I can't understand why I can always tighten the bolt back up - I'd expect it to just spin both ways. Any ideas what's happening here? I can't visualise what's inside there to cause this.

[GTSBoy]

Grind them off, drill them out. Repair any rust on the other side. Start again from scratch.

[DatsunBanana]

Yeah, I'm pretty sure that's how it's going to end up. Mechanic suggested I cut a hole in the chassis/body to access the inside. I can cut the hole for access but will have to get someone else to help if any welding is needed afterwards. Things I'd like to know are:

Assuming there's a captive nut inside, how's it fixed? And how can it be tightening but not fully loosening?
If I cut a hole to access/replace a captive nut, is there a good place to cut for this one? Does it have to be at the side, or is there somewhere I can cut from above inside the car?

[GTSBoy]

4 minutes ago, DatsunBanana said:

Assuming there's a captive nut inside, how's it fixed?

Mostly likely welded.

4 minutes ago, DatsunBanana said:

And how can it be tightening but not fully loosening?

'Coz it's f**ked.

4 minutes ago, DatsunBanana said:

Does it have to be at the side, or is there somewhere I can cut from above inside the car?

You have the car right there. Get someone to tap on the underside while you sit in the back with the seat out. You'll find it faster than we will.

[MrStabby]

Techniques for removing stuck bolts are: hitting the bolt head with a hammer, heating the bolt to red hot then letting it cool and working it back and forth like you are already doing. AFAIK the idea is to try to crush whatever material is stuck between the threads (and heat also melts locktite but that's almost certainly not relevant in this case).

[MBS206]

TBH, it sounds like the threads on the bolt are fubar in a specific section. Most likely as you've been working it back and forth it has torn threads out, as you loosen the bolt, this section enters the nut, now you're spinning a round shaft inside the nut, there's no threads here.

 

Use something similar to a claw on a hammer that will fit either side of the bolt head and try to apply downward force while also undoing it.

The idea is to attempt to catch a thread, or just rip the f**ker out. Caution on ripping out, as it could bend/maime/mangle what the captive nut attaches to.

If the captive nut is actually okay, you could do the dirty part of cutting the head off the bolt, then cut a slit in the end of the thread that's now exposed and then screw it up and into the area the captive nut is until it falls out the other side. However, this does risk the remainder of the bolt rusting over time and damaging near the captive nut and also making it rust. However, so can cutting pieces of panel work out to access this area if you don't reseal it properly.

[DatsunBanana]

Thanks everyone for the replies and suggestions. Got the seats out (hoping I could find some existing grommets but no such luck). By tapping and measuring etc. I could figure out where I could drill through if needed. But first I borrowed an inspection camera and managed to go through factory holes in the chassis rail and could see that the captive nut was holding steady which is why it could retighten. So it was indeed a stripped section of thread, so I applied downforce by levering the bolt head with a screwdriver and went slowly back and forth until it came out. Camera helped a lot cos I could monitor that the captive nut was holding tight.

Now I just have one very seized main subframe nut to tackle 😅

[GTSBoy]

10 hours ago, DatsunBanana said:

Now I just have one very seized main subframe nut to tackle

Soak that one in Penetrene. Leave, repeat, etc. Then heat.

[joshuaho96]

15 hours ago, DatsunBanana said:

Thanks everyone for the replies and suggestions. Got the seats out (hoping I could find some existing grommets but no such luck). By tapping and measuring etc. I could figure out where I could drill through if needed. But first I borrowed an inspection camera and managed to go through factory holes in the chassis rail and could see that the captive nut was holding steady which is why it could retighten. So it was indeed a stripped section of thread, so I applied downforce by levering the bolt head with a screwdriver and went slowly back and forth until it came out. Camera helped a lot cos I could monitor that the captive nut was holding tight.

Now I just have one very seized main subframe nut to tackle 😅

If you can get heat to it somehow that and penetrating oil will do wonders. Induction bolt heater, whatever it is.

[MBS206]

Best kept secret for seized bolts is crayons...

Heat the sucker up with a torch, and push/melt crayon into it. Works a wonder.

See if I can go 2 from 2 for you on how to get your bolts out...

[DatsunBanana]

I didn't know about induction heaters but checked them out and they look awesome but too pricey for the occasional job like this. And I'm too nervous to take a flame anywhere near there as the petrol tank is so close, although I was intrigued with the crayons suggestion 😁

I filled a coke bottle lid with penetrating fluid and raised it on the jack so the nut was submerged in it all night. Then went in with a tiny cold chisel and got it almost to the point of splitting but not quite (I didn't want to damage the bolt threads). Then I hammered a socket back on and gently worked it until I felt movement. And it's off 😀