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WING BOLTS At Top To Windscreen - TT-109

At 07:36 PM 10/17/2006 -0600, Robert Muckenfuss wrote:
Original type wing bolt >"I noticed that on the header rail holes where it fastens to the windshield it had one old wing nut and the other side had a bolt in it. .... I ordered new wing nuts. I tried to take the old wing nut and bolt out but the bolt wrung off."

Oops. If you have a steel bolt there which has broken off, you will first need to pursue bolt removal and thread repair by standard practice. See here:
Removing a broken bolt - and - Removing a rusted bolt

>"I can't see an assembly for this in a catalog and am afraid to take the convertible top material apart so I can see how it goes together."
Wing bolt in assembly
There is a thick wood rail for fastening the rag top material. The wood rail is screwed to a steel strap which runs across the back of the wood rail and is welded to the steel frame at both sides. The steel frame includes an "L" shape steel foot extending under the wood rail, and having a hole which seats over the attaching pin on the windscreen frame. On the back of the steel strap is welded a short steel slug which is drilled and tapped with thread to hold the wing bolt.

When assembled there is a thin wall copper bushing with a collar flange which is inserted through the steel foot from the bottom and extends upward into the wood rail for mechanical reinforcement. The wood and steel rail assembly is wrapped with fabric. The copper bushing may be installed either before or after the fabric is wrapped. It makes a neat finishing ring if the fabric is installed first and the bushing inserted last to cover the edge of the fabric around the hole, but then you have to cut a hole in the fabric on top to bend the tabs holding the bushing in place.

The wing bolt has an odd thread. I believe it is BSF 5/16-22. The 22 threads per inch falls in between 5/16-18 UNC and 5/16-24 UNF (or SAE), so a common Unified thread bolt will not work here. The wing bolt is originally chrome plated over brass, and the threads may strip after decades of use. If it is immediately replaced with another wing bolt of original design the assembly may be okay, as the female threads in the steel frame are less likely to suffer damage.

In times past the original type wing bolt may have been hard to source as a replacement part. If some DPO tried to tap it out or force install a Unified thread bolt, the original BSF thread would be damaged beyond use. A steel bolt might be cross threaded or jammed in the odd thread pitch, and with a tight fit it may rust on place with long term storage.

A common home repair for this problem is to tap it out to 3/8-16-UNC or 3/8-24-UNF and install a Unified thread bolt. I did this some years ago using a fine threaded bolt. I also welded a bit of round steel rod across the hex head bolt for a makeshift wing bolt.
welded wing bolt made from threaded rod and a standard wing nut
More recently I wanted to buy 3/8" wing bolts from McMaster Carr, but these are only available in coarse thread. I could have drilled out the thread to 3/8" bore and tapped it to install coarse thread Heli-Coils. Being cheap I didn't want to buy a coarse thread Heli-Coil kit, and I prefer fine threads anyway to make for easier tightening with fingers. My ultimate solution was to use 3/8-24 fine threaded steel rod and weld on a wing nut for a better looking makeshift wing bolt. It looks much better now (even though they are not plated), but not exactly concours with the oversize threads and different style wing head.

Heli-Coil kit If the broken replacement bolt was 5/16" diameter, there may be enough material remaining in the hole to tap out and install a Heli-Coil insert for thread repair. See: Heli-Coil Thread Repair. You can buy a Heli-Coil kit with the correct threads from British Tools & Fasteners.

This of course assumes that you can actually find a Heli-Coil repair kit with the proper original thread type. Otherwise you might buy a Unified thread wing bolt (probably coarse thread as fine threads may not be available), and install a Heli-Coil to match. A standard commercial wing bolt may be galvanized (zinc hot dipped) or plain raw malleable iron (not plated).

If the female thread is already stripped out larger than 5/16", and you insist on getting back to the original thread to use the original wing screw for concours show, then you have to replace some material in the hole. You would need a thick wall insert like a Keensert or Key-Locking threaded insert. See here for instance:
http://www.mcmaster.com/#threaded-inserts

To accommodate an odd thread type you can get a solid insert, then drill and tap the insert to suit (assuming you have the right tap). See here:
http://www.specialty-fasteners.co.uk

I'm not sure though about going to M12 external thread to get a 5/16 tapped hole. There may not be enough material in the rag top bow fitting to hold the M12 male thread. In that case you may have to cut off the original tube and weld on a new piece of bar stock to drill and tap to original thread size. Find BSF taps here:
http://www.britishfasteners.com

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