Trying to remove the front subframe, I get to step 25 "remove the front subframe rear mount retaining bolts." The passenger side comes off without a problem using an impact wrench. The driver's side spins but doesn't unthread. I put a ratchet on there to get a better feel and I poke my finger into a hole in the unibody above and I can tell the bolt isn't threaded to the unibody frame, it is threaded to a nut that sits in the unibody frame that has some sort of anti-rotation stop that has failed. I can cut the head off this bolt (the repair manual says remove bolts, it never says to do this by unscrewing them) and get on with the subframe removal, my question is what do I do then? I see absolutely no way to get at that nut inside the unibody rail. The car won't go safely down the road without the subframe bolted in...so some failed little nib of metal has doomed the car to the scrap heap? How are these nuts installed inside the unibody frame?

 

I'm not a mechanic, but I've seen this sub frame problem on the threads here where the so called 'captive' nuts that connect the studs from the other piece come undone. Just from what I've read here, I would approach the dis assembly of the sub frame ready to re-weld the captive nuts to the frame. What a pain in the ***, huh?

I think there's a thread where a member here cut thru the floor beneath the driver's foot well to get to a broken captive nut that anchored the sub-frame....appears to be a recurring problem as this car ages and undergoes rebuilds and what have you.

Why are you removing the sub frame?

 

I was trying to replace front control arms (the ball joint ones) that necessitate taking those rear sub frame bolts out. Experienced the EXACT same problem. This attached post (with pictures) shows what I had to do. We opened it up (EXTREMELY hard) as the metal encasing where the "captive" nut is, is double layered with like super hardened 2 layer steel. I assume for strength, but cutting the access hole was a nightmare. Got it large enough to get a small pipe wrench in the hole. Then spent literally hours futzing on and off to get it to bit to spin the bolt out. Install was nearly as difficult.

https://www.jaguarforums.com/forum/x...l-arms-186936/

 

You can either lift up carpet and gain access to the captive nut, you will see a grid type of square with the nut visible through it, or make a cranked spanner to get into the box section from underneath, there are details online to fabricate this cranked spanner.


To me it seems a bit brutal to cut a section out of the box section.


Paul.

 

Thanks all for the advice. As far as the motivation... this all started with a P0420, rear catalytic converter below threshold. I can't get the car inspected in Maryland with the check engine light on and apparently have more time than brains so decided to do the job myself.

I estimate so far I have an equal number of bolts that had to be cut off, bolts that the heads torqued off, and bolts that actually unthreaded. The engineers in Coventry must be working on a secret formula for rust.

 

I followed Paul's advice and had the bolt/nut off in about an hour. I pulled up the outer edge of the carpet and rolled it up under the brake pedal to get it slightly out of the way. There is a second layer of carpet/noise damper/heat shield under that, I pulled that up toward the dash which ripped it so there go my chances at the Concours d'Elegance. At that point you can see the square grid under a layer of sound coating/ weatherproofing tar.


I went at that with a dremel and a cut off wheel little by little until I decided I needed to cut out the 3 left steel grid sections. That fully exposes the nut.



It is a 21mm nut and requires a deep socket because the bolt is long. I didn't have this socket but was happy to find a deep 13/16" socket worked fine.

I think the nut has an eccentric flange because when spinning the bolt from below with the nut free to move and the rest of the subframe undone the entire subframe moves as if it was spinning on a cam. Also I could spin it by hand for about 180 degrees of rotation while the rest required a long wrench. The unibody rail has a box section welded over the flange which you can see in the picture. This gives the nut some freedom to move for alignment. I don't know if the problem is the welded box gives or the flange gives or if there wasn't enough eccentricity to start with. I might try peening the welded box with a punch before I put it back together.

Thanks again to everyone for advice. My next thread will be about why the carpet foam and the floor under the carpet are sopping wet when the car has been under the carport out of the rain for 6 weeks.

 

Damn Paul, where were you when I was working on my car, lol? Yeah, it was brutal and a gigantic p.i.t.a. There was/is no access that I could see from under the car to get at the nut. The section where that nut is, is entirely encased in body steel.

 

When I bought my car approx 9 months ago, the "dealer" had to replace the dual mass flywheel/clutch/release bearing, and they cut the floor to gain access to the nut that had become un captive.
As I don't trust the dealer doing the job right, I looked in to it, at the same time I had to replace all the extra damage they did to the car by cutting corners.


I have checked the alignment of the subframe with the alignment holes in the frame and box section, and as expected they don't quite line up, so I am going to make myself the fabricated spanner very soon, so in future I can re align the subframe without lifting the carpet.


Today (Friday), I have just had the car mot'd, and passed, so I can relax a little and slowly fettle some minor points at leisure.


The sills are solid from underneath, so I need to remove the outer sill covers and give the inner sills some tlc.


Btw Dell, thanks for your previous and ongoing service and tech information that you regularly post for people, very helpful.


Paul.