To my other GT6 pages.

May 27, 2021

Bonnet Latches

The GT6, like some other British sports cars of the era, had a distinctive method for engine access.  Instead of a "lid" type opening over the engine, the entire front clip was hinged behind the front bumper, and swung up to expose everything in front of the firewall.  So, in effect, the "bonnet" included the front wings and grille.  When in the closed position, the bonnet is secured on each side by an over-center latch mechanism operated by an exposed chrome plated lever.

I had come to the point in the restoration where I needed to sort out the panel gaps, and for this, I needed to set the bonnet position, which would require the latches. I couldn't really remember if there were problems with my latches, so I dug them our for inspection.  They looked pretty bad at first.




Both of them seemed a little loose.  They were also both fairly rusty, and one of the levers had blisters in the chrome (the one on the left).  The pitted chrome bothered me.




These latches can be bought, but besides the trepidation I have with buying replacement parts, the price is not inconsequential at around $100.  I decided that I didn't have much to lose by trying to improve the worst of the two latches.  I recently tried a chrome powder coat that came out pretty good on this horse shoe, and thought it was worth a try on the latch lever.




This of course meant that the mechanism really needed to be taken apart, especially to clean up the rust.  It is held together with a few spacers with rivet-like noses on each end.




This is the lever with the blistered chrome.




The brackets came off the lever so I could fix the corrosion on them.




Blisters on chrome are usually caused by corrosion under the plating, and when you remove the blister and the corrosion, it leaves a pit in the base metal.  This is what some of the pits looked like under some magnification.  They were actually deeper than they appear in the image.




I decided to try to fill the pits before powder coating.  I've had some success in the past using something like JB Weld, which is a metal filled epoxy that has a temperature rating that can take the powder coat curing process.  The set filler has to be baked to make sure it will not outgas during powder curing.  Also shown are the re-plated brackets.




Well, the result was sort of OK, I guess.  There was a little orange peel, but from a distance, it looked pretty good.  The spoiler was that there were defects in the surface exactly where the pits were.  They don't show up in the picture.




I assumed that the filler was still outgassing, so I sanded the part down to the bottom of the pits.  Decent chrome plating is actually three layers--copper first to help with adhesion to the base metal, nickle next to provide most of the protection and shine, and a very thin layer of chromium on top, which gives the bluish tinge.  You can see in the pic the copper layer on top of the pot metal base, while the nickel has a slightly yellowish color.  Most of the chromium is gone.

 


After powder coating again, the result was even worse.  Besides the dull sheen, there were still defects at the sites of the original blisters.




I then assumed that the problem might be localized porosity in the base metal that would account for the defects in the powder coat as well as the original blisters.  Contaminants in the pores caused corrosion under the chrome, and also ruined the powder coat, maybe through outgassing.

I had two more goes at it, first sanding more, then finally trying zinc plating the part before powder coating.  Both failed.  Now I understand better why platers don't like to chrome pot metal. 

Reluctantly, I finally threw up my hands, admitted defeat, and ordered a new latch.

It was not a total loss, though.  During the process, when I still had high hopes, I stripped and replated the other parts of the mechanism.




Then re-built the latch except for the lever.  The original special rivet/spacers got replaced with separate rivets and spacers.




I now had a renewed latch without a lever, and a complete latch with a good lever, but rusty mechanism.  The next step was obvious.  Move the good lever to the rebuilt mechanism.




Then, the new latch came in.  I often don't have high expectations for replacement parts these days, but in this case, I was pleasantly surprised.  The new latch appears to be first-rate (new one on the left).




The only thing left to do was to re-do the catches that the latches mate to, along with their nut plates and fiber pads.




This was a piddly little job, But it required more thinking than I planned on.  Cost was around $100, but in a way, I got two latches for the price of one.

Update, May 28:  A comment on one of the forums about wear on the plastic roller prompted me to compare the original roller with the one on the new latch.  While the new roller was a nominal 5/8" OD, the original one was quite a bit smaller at around 0.57". 




Whether this was due to wear or design, I don't know, but it seemed pretty easy to fix.  If I'd had some white Delrin or Nylon, I'd have used it, but my only choice without ordering something was black Delrin.




This makes me feel better, even though the colors don't match.  (I think I like the looks of the black one better.)




This was a piddly little job, But it required more thinking than I planned on.  Cost was around $100, but in a way, I got two latches for the price of one.
 
Comments to Ed at elhollin1@yahoo.com

To my other GT6 pages.