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January 12, 2016

Seat Tracks

[Click the pics for a larger view]

By modern standards, the seats that came with the TR6 were pretty simple.  The only adustments are a tilting seat back and a sliding track mechanism that allows the seat about six inches of travel fore and aft.  The tracks are made from heavyish sheet metal, with zinc plating that only delayed the inevitable corrosion from damp carpets.

My tracks were in decent shape, but pretty much covered with red and white rust (iron and zinc corrosion, respectively).  I had the seats out once in the '80s, and sprayed the bottom tracks with some red primer then to try to slow the rusting process.  I must have also thought it was a good idea to force some grease inside the tracks, but it probably did more harm than good.




The upper part of the mechanism which attaches to the seat rides on a pair of lower tracks that bolt to the car's floor pan.  Each seat is actually supported on four rollers that ride inside the bottom tracks.  Each roller has a pair of rubber tires.  The blue tires probably once resembled O rings, but time had not been kind to them.  They were now distorted, misshapen, and a sloppy fit on the rollers.

  


Measuring the old tires, rollers, and tracks, I was able to surmise the approximate original dimensions of the rubber parts, but looking through tables of standard O rings, nothing was very close.  The closest standard part was a -311 O ring, so I ordered a bag.  The 9/16 inch ID of the new rings seems to be close to correct for the rollers, indicating how stretched out the old rings were.  The  new rings were too fat though, which made the OD too big when mounted on the rollers.




I briefly considered casting some custom O rings in the right size, but decided to try modifying the -311 parts first.  

Machining rubber is tricky.  Some try freezing it and using regular lathe tools, but I thought keeping it cold and hard enough would be a hassle.  Others report that abrading rather than machining works well.  I gave this a try, and finally  hit on a method that worked OK.  I spun a ring mounted on a roller, and used either a file or sandpaper mounted on a hard flat backing to wear away the rubber.  I experimented with final OD dimensions, and settled on 0.850".  Once I got going, each one only took a few minutes.




Next came cleaning up and derusting the metal parts of the tracks.  The spring loaded slide lock lever is riveted to the assembly, and I didn't feel I could properly strip or refinish the piece with it in place, so I removed the rivet.




I normally derust metal parts with phosphoric or muriatic acid.  In this case though, much of the original zinc plating was intact, and the acid derusting process would attack the zinc.  The parts were too big  for my plating tank, so I needed to preserve what zinc was left.  In this situation, the best method is to use a chelate deruster.  Chelates are harmless to plating, and even most paint, and pretty much only attack metal oxides, especially rust.  The picture shows a tub with enouth solution to cover the upper track assembly.  The marbles and the big hunk of black plastic are in there to raise the level of liquid so I didn't need so much.  I used Evaporust, but there are other good products, too.  




It's a slow process, measured in hours, not minutes, but pretty effective.  The slightly darker mottled areas are where the zinc was corroded away.




Here's one other piece of education I got from taking the first track apart.  There are little dimples on both the upper and lower tracks to limit how far the rollers can travel.  They also make it a real challenge to separate the lower and upper tracks.  I ended up drilling out the dimples on the lower tracks, which made disassembly much easier.  I'll replace the dimples with screws later.




I really wanted to powder coat all the parts, but the upper track assembly was too tight a fit in my oven, so it was just primed and painted.  Smaller parts were powder coated.  The original parts were just bare metalic color, but it seemed to me that parts under the seat really should be blacked out, so I went with black.




Rather than put these sort of ratty looking plasstic covers on  the ends of the locking levers, I opted for ten or so coats of Plastidip.




These are the adjustable plates at the rear of the tracks that the seat tilt locks catch on.  




Ready to put the slide lock levers back on.  Since I destroyed the original rivet, I used a shoulder bolt instead.   




Almost forgot--I had to replace the little limit bumps I drilled out to make disassembly and assembly easier.



This was an enjoyable, if pretty involved project.  The cost was minimal--some O rings and some hardware--probably under $15.



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