c3 number 4 body mount

[Bill Stephens]

When I loosened the number 4 body mount bolts to install new fuel lines the captured nuts began spinning so that the bolt can't be backed out of the nut. I also noticed that the reinforcement is rusted around the hole the bolt goes through. I believe the cage for the captured nut is welded to the reinforcement. Is that correct? If so, it seems that the (painful) remedy for all this is to remove and replace the reinforcements, cages, and captured nuts. I've been through the archives and seen similar questions, but nothing exactly like this.

I had just put the car on the road after three years when the gas tank began leaking, so this is a big disappointment, but I think I may be off the road until next year. Any advice would be appreciated.

[Warren F.]

Re: c3 number 4 body mount

Bill,

This exact same scenario has been discussed over on the Corvette Forum. You might browse that site and post this inquiry there as well.

[Chuck S.]

Re: c3 number 4 body mount

Bill, the captured nut may appear to be welded to the reinforcement, but the retainer for the nut is riveted to the fiberglass on the inside, and the reinforcement is actually riveted to the outside of the body.

It's true the reinforcement will have to be removed to replace the nut retainer rivets. If the car is a convertible, replacement will be further complicated by the folding top hinge support that covers the captured nut. For the details on this installation see your AIM about UPC 1, Sheet B4.

If you have a coupe, the captured nut is openly accessible from the rear compartment; if you want to postpone these repairs, a friend may be able insert a screwdriver into the retainer (if a wrench is not useful), and hold the nut until it can be tightened. Once the mounting bolt is tightened slightly, friction on the nut face will help resist the turning moment to full torque.

[Bill Stephens]

Re: c3 number 4 body mount

Chuck,

It sounds like cutting the rivets holding the body to the reinforcement won't work because the reinforcement will still be held to the body by the rivets for the captured nut retainer. I've read in previous threads that people have cut the bottom of the reinforcement off. That means cutting off the fiberglass as well, but it doesn't seem as if that provides much support anyway. The only other option seems to be to cut off the bolt. Is that how it looks to you?

Thanks for your help.

Bill

[Bill Stephens]

Re: c3 number 4 body mount

Warren,

Do you have the URL for that site? There are several corvette forums and I couldn't find the discussion you mentioned. Thanks.

Bill

[Chuck S.]

Re: c3 number 4 body mount

No, not how it appears from the AIM.

I haven't taken one apart, but it looks like the captured nut and retainer is one assembly riveted to the body with rivets inserted from the bottom up; head on the outside. The reinforcement is then simply a "cap" riveted over the bottom of the body mount well fiberglass, on the outside of the compartment floor, with the rivets inserted from the inside down (heads on the inside). From the positioning of the rivet heads, I would say that sequence is the only way it could have been assembled.

Cutting the fiberglass for this repair is much, much less preferable (read unacceptable option) than cutting the bolt IMO. Ideally, you could load up the nut/bolt with penetrating oil, let it set a couple of days, then hold the nut with a wrench, and remove the bolt without cutting anything. If that's not possible, cut the bolt.

[Bill Stephens]

Re: c3 number 4 body mount

Chuck,

Sounds like good advice. That's the way the AIM manual looked to me as well. Thanks for your help.

Bill

[Chuck S.]

PS If You Have...

Successfully moved the bolt in the nut to loosen it, then the bolt CAN likely be removed...it will just take a lot of patience working the bolt back and forth in the nut until you work your way through the corrosion in the threads. Penetrating oil will help in this exercise.

If you have managed to loosen the bolt until the nut began to turn on you, that tells me the threads inside the nut are good and you have simply moved into corrosion on the outside threads. Your next move will be to go slightly in the direction of tightening the bolt...then apply a little oil, and then back in the loosening direction, repeat as necessary, back and forth, etc, etc.

If the corrosion is so bad that the threads are completely gone, then the bolt will either twist off, are you can just give up and cut it; it won't be reuseable anyway. I hate like heck to lose original headstamps, but replacement body mount bolts are readily available.

[Harmon C.]

Re: c3 number 4 body mount

Bill
What I have found is the two rivets holding the caged nut are weak and break so it turns. The fast fix is to cut the bolt off and install a new bolt flat washer,lock washer, and nut. This is done with two people and their is no need for the caged nut. The correct way is to remove the cage and replace the caged nut and replace the rivets in the reinforcement. You can usally save the caged nut if you cut off the bolt.Be carefull as this is the way frame offs start when one thing leads to another.
Lyle

[Bill Stephens]

Re: c3 number 4 body mount

I'd dearly love to avoid a frame-off. I've been in the process of a frame-off on a 52 MG TD for almost 10 years. I bought the corvette to drive while I did the restoration. I started changing the ball joints and now three years and many hours and dollars later I'm at this point.

I think the rivets have broken because of the rust. I'm going to try to get a small needle nose vice grips on to the caged nut and take Chuck's advice, tightening and loosening with lot of lubrication. If I get it off, I think I'll use your method of putting a nut on the inside to replace the cage nut.

Thanks again to everyone for all your help. I think I know what to do now.

[Brett D.]

Re: c3 number 4 body mount

Bill
You need a 1 inch deep socket. It has to be an 8 point socket. Most industrial tool supply stores should have one. It will take two people to loosen it. My eight year old son helped me with mine. He powered the impact gun while I manned the breaker bar and socket. The 1 inch nut is merely captured by the cage. The cage is riveted to the body with two rivets. You must remove the sheetmetal body mount bracket that is installed on top of these two rivets to replace the cage/nut. If I wanted to continue driving the car I would remove what is left of the cage and install the new #4 mount hardware using the 1 inch socket. This is indeed where frame off restorations have begun.
Hope this helps.
Brett

[Chuck R.]

Re: c3 number 4 body mount

Lots of GOOD penetrant and a stout pair of vise grips will close the deal for you Bill.

If you can get the bolt to tighten back up and you have a dremmel, you can knock some of the rust off of the bolt with the little wire wheels that come with the kits and then as Chuck stated, start working the bolt in and out of the nut using lots of penetrant in the process.

Better yet, you can either grind the threads off with a fine stone or keep burning up those little cut off wheels until you can snap the excess bolt off.

Chuck