removing chrome?

['51 Special]

My pace for putting my '51 together has been slow. I had the back bumper re-chromed in 1996, and am just now getting it back on the car. Here is my problem: There is a 24.5 inch rail that mounts between the bumper guards, and on that rail was mounted a piece marked "Guide L-43," which in turn mounts the license plate and a small light to illuminate the license plate. The light is held on by two screws that are riveted to the piece. The plating company (now out of business) chromed the entire Guide L-43 piece, including the threads of those screws. There is so much chrome on the threads that I not only cannot screw the nuts back on, I cannot even get a die on them to cut new screw theads.
My question: Is there any way to get the chrome off of the threads?
This is what I have considered: If I am careful, I would be able to immerse just the threads in a solution. Could I just wire the piece to the positive terminal of a battary, dip the threads of the piece in salt water, then wire some clean scrap metal to the negative terminal and sinking the scrap into the salt water? And, I assume, watch to see that I don't take off too much material from the threads.
Any better ideas would be appreciated.
'51 Special

[raycow]

Any better ideas would be appreciated.

This is not necessarily better, but just different.

Do you know what size the original threads were? Do the plated threads still look like threads? If yes, you might want to consider tapping an oversize thread in the nuts, or maybe new nuts so you can preserve the originals in case you find a better fix later on.

Ray

['51 Special]

Ray-
I like your idea. I tried an internet sourced chrome stripping idea, using an overnight soak in muriatic (diluted 32% hydrochloric) acid, but there was no discernable change to the chrome.
The threads are clearly there, and are (or were) 10-32 NF. But, my 10-32 NF die won't start on the threads, and I don't have a 12-32 NF die. The nuts are unusual capped nuts, being 1.375" long, with a threaded hole about .875" deep. They have a screwdriver slot as well as hex sides for a 3/8th socket or flat wrench, and appear to have a rust-resistant exterior. I would really like to use these nuts as they are obviously designed to keep the threads safe in a corrosion-prone enviroment.
Do you think that, if I successfully started by running a 12-32 NF die, I could then run the smaller 10-32 NF die? (Slowly and with a lot of oil, of course.)
'51 Special

[suntreemcanic]

I have a 52 Super and uses the exact same system you described, the little screw driver hex heads and all. I bet they are available at a wrecking yard. Also did they chrome the light bulb holder? Will a bulb fit? Remove that license plate light holder and take it to a chrome shop and have it dipped in their stripper tank and I bet that chrome will come off the inside of those "nuts." Nothing can stand up to that acid.

[raycow]

Be careful with that HCl. It vaporizes continuously and the vapor can get into places you can't even see and cause nasty corrosion problems later on that will make you wish you had never started with it.

It sounds like you have already done quite a bit of homework on this problem. The next step I would suggest is that you mike the O.D. of the threads to find out how much chrome build-up you have on there.

The theoretical max major diameter on a #10 thread is 0.190, but production threads are always less than this. I just now miked a few 10-32 screws (cheap, hardware store grade) out of my parts drawer and got a range of 0.183-0.185. Precision parts will be a little larger.

12-32 is not a standard thread (12-24 and 12-28 are the normal NC and NF), although 12-32 dies do exist and you can special order them. The hiccup here is that a #12 is 0.026 larger than a #10, so you might not take off enough chrome to be able to start your 10-32 die.

Depending on your actual thread O.D., you might want to look at a metric 5 x 0.8 die, which is a standard thread. The major diameter is about 0.197 and if you work out the pitch you will see how close it is. If you can get it to start, that may be your easiest way out of this mess. Of course the other half of your problem is that chrome is pretty hard. Carbide dies are available, but are kind of expensive for a one-time job.

Good luck, and please post back to let us know what you come up with.

Ray

['51 Special]

Ray-
You always suggest posting back about how a suggested fix works out. Well, this is it.
I put the "Guide L-43" piece in a vise (padded with softwood jaws to protect the chrome), and using a very sharp small file, carefully filed down the shafts of the two chromed screws. Not a lot, keeping them as round as I could.
Because the two screws are about 1.5 inches apart, there was no room for the handle of my 10-32 NF die. Handheld, I couldn't put any real pressure on the little die, let alone keep it straight. The die is round, but, in addition to the little dimple to accept the handle's set-screw, it has a single small vertical groove on its outer edge. I used a single wrap of masking tape to hold a .25 inch piece of #12 wire in the grove. Then I dropped the die into a 1 inch six-point socket. Snug fit, and the bit of wire locked the die into the socket. A trio of washers behind the die raised it to near the front edge of the socket. With this tool I was able to get the die started on each of the filed-down screws. Once started, and using lots of oil, I gently turned the die onto the screws with a ratchet. In less time than it took to key this note up, the treads were perfect.
After doing this, I wondered if that little groove on the die is for exactly the purpose of locking it into a socket. Anybody know?
'51 Special

[raycow]

Ya know, this is why I like this board so much - because we have people here like you, who can figure out a fix like that. It was nothing short of genius.

To be honest, I have noticed that groove before, but never stopped to think why it was there. I don't think it was for the purpose you used it for though, because hex-body dies are fairly easy to find. Perhaps it was used when the die was being manufactured, to locate the blank for one of the machining operations?

Thank you for posting about that fix. It may be of great help to the next person who comes along with the same problem.

Ray