OT tightening bolt w/o access to head

[Hendrik]

This is OT since it is for a problem on my car; highly mechanical though!
The car frame has two (left and right) longitudinal members, which are hollow. Under the engine there is cross member. The cross member is bolted to the longitudinal members; three bolts on either end.
The heads of bolts are inside the hollow members and there is a contraption inside the member that keeps the bolt from turning or from being pushed up. Only the threaded end of the bolt and the nut are visible.
My problem is that this (invisible) contraption has failed on one of the bolts, making it impossible to tighten the bolt (the nut only needs two or three turns).
I again hope that you can help me with some clever suggestions. (The car is Fiat 124 spider, if that helps.)
Thanks a lot!
Hendrik
 

[Brownie 450]

Put a large flat washer on the protruding bolt & hold pressure sideways on the washer against the bolt until the nut is tightening. If there is room, put a lock washer on ahead of the nut. That will tend to grab the washer as you tighten the nut.
 

[JimB2]

Hi Hendrik, if there is sufficient length of thread protruding below the nut, clamp a visegrip on the end of the bolt. There maybe some damage to the threads but if you require to take the nut off later some filing of the threads maybe required.
JimB
 

[Curtisk84]

Lots of things you can try depending on the scenario. If the threaded part of the bolt is past flush with the nut you could try holding it with vice grips or pliers. Or if the threads are nice and clean, a lot of times using an impact wrench will tighten it without having to hold the bolt. Hope this helps.
 

[Janicholson]

Drill a holt perpendicular to the offending connector, (near where it protrudes) and either drill into the "nut connector" and pit a sheet metal screw in it to locate it, or wire weld through the hole to reattach it. Jim
 

[John G. Hasler]

Before grabbing the end of the bolt with visegrips grind flats on it.
 

[Super-H-Mike]

You could try oil on the threads and then hit it with an impact gun, should work if the threads are clean and not damaged. Otherwise, I would cut the bolt off and make a new one if you have access to drop it through the top. Those clip-nut type of fasteners are probably great for the assembly line, but arent so nice after years of rust and grime are they.
 

[Hendrik]

Thanks Jim.
Welding the whole assembly together is my last resort.
Wire welding the head through a drilled hole in the member is a good idea. How small might this hole be without impairing the flow of the gas (presuming one use wire weld)?
Thanks, Hendrik
 

[mkirsch]

The hole you drill for the welder would have to be big enough for the entire nozzle, or use flux core wire.

I suggest: Grind a slot in the end of the bolt with a Dremel tool, and use a large flat screwdriver to hold it.
 

[Hendrik]

Thank you all for your suggestions!
Will try the air impact or, if that doesn't do the trick, the vise grips. Only a few threads are above the nut
If still no go, will drill a hole to try and weld the head inside the beam.
Thanks again, Hendrik
 

[Janicholson]

3/16" is usable Jim