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August 1, 2014

Rack & Pinion

[Click the pics for a bigger view.]

The TR6 uses a simple rack and pinion assembly for steering.  Each end of the rack carries an inner ball joint for tie rods to the steering arms.

My rack & pinion assembly looked about as bad as the rest of the car on the outside.  However, as a pleasant surprise, I found the internals to still be very well lubricated, and in very good condition.

Now I normally don't dwell on my mistakes, let alone document them for the world to see, but I'll describe here a bonehead moment that occured when disassembling the rack housing.   I had removed the pinion gear, and wanted to remove the bottom bushing that it rides in.  I was somehow under the impression that the hole in the bottom of the housing was blanked off by a welch plug that had to be driven out from the inside.  Even though a little thought would have shown that this is silly, I proceeded to try to drift out the plug from the pinion side.  Even the fact that the "plug" refused to move despite considerable force didn't phase me.  Only after I finally broke out the bottom of the alloy housing did it dawn on me that the bush and the thrust bearing, which I thought was the plug, would have easily pushed out from the bottom up.

I was doubly frustrated because I thought I'd just ruined a rack and pinion assembly that was in excellent shape.  I finally resolved that as penance for such a stupid mistake, I'd go to whatever heroic lengths it would take to repair the damage I caused.  After a little thought, I came up with a plan that was simple, though it took a while.

First, I milled off  the bottom broken surface down approximately to where the bottom of the internal bore had been.  Then drilled and tapped six 8-32 holes around the perimeter to hold a stainless cap.  The picture shows evidence of another maddening lack of forethought.  The lowest of the six holes didn't have enough meat, and broke through the edge of the casting slightly.  It would still hod a screw, but not with much torque.  If I had only just rotated the hex pattern by 30 degrees, it wouldn't have happened.  To compensate, I placed two additional tapped holes, one on either side of the bad hole.  This explains the odd screw pattern.  

With that barely controlled disaster behind me, I resumed the reassembly of the rest of the pinion part of the unit.  I checked the fit of all the bushes and they were so good, I couldn't see how new parts could provide any improvement.  Likewisewith the rack bush.  The only new parts in the assembly was the O ring for the pinion shaft, an a couple of additional shims (because of my "redesign").  After painting the rack & pinion housings and replating damper cap and plug, I started to put it all back together.  The parts inside the pinion housing are put together as stock, including the thrust bearing, so the pinion shouldn't know the difference.

I even managed not to lose the little plastic plug.  I'm not sure of the purpose for this.  Pressure relief, maybe?

Next up was the tie rods with their inner ball joints.  Everything in the joints was still covered with clean looking grease, and felt tight and smooth.  The balls were bright and shiny, with hardly any evidence of wear.  The rod part of the tie rods were pretty rusty.  

Most of the ball joint parts live out their lives bathed in grease, so plating is unnecessary.  The exception is the rod part of the tie rods.  These are at least partially exposed to the elements where the gaiters don't cover.  I thought zinc plating the tie rods would be worthwhile, but didn't want to interfere with the nice surface of the ball.  I ended up selectively plating the tie rods by spraying a couple of coats of lacquer paint on the ball parts to act as a resist to the plating.  Lacquer dries fast and is easy to remove.  It worked out well.

After close inspection, I decided that as for the pinion parts, there was nothing to gain by buying anything new.  The reassembled the ball joints with the original parts (except foir the tab washer) and shims, and fresh grease, showed firm but smooth movement.

Tie rods attached to the rack.  There is a spec for the distance between the face of the ball joint lock nut and teh center of the tie rod end.  This is dificult to measure with the gaiter in place, so that's what the tape is for--it marks how far the tie rod end should be screwed on.

Last parts to deal with were the mounting clamps for the rack and pinion assembly.  One of the U bolts had broken during teardown, and I had some trouble trying to source an exact replacement.  Some places wanted me to buy the entire clamp, which was silly.  I figured a simple U bolt shouldn't be hard to make, and it wasn't, but it did take a little time.  The original bolts have 5/16-24 threads, but they must have been rolled on, since the actual rod is only about 0.280".  My bolt made from 5/16 rod is a little fatter, but finally fit OK.

After plating and powder coating, the old clamps are ready for service again.  Last pic includes one of the new poly rack mounts.

New tie rod end.  Oops, forgot the lock nut.  

The resilient mounts need to be preloaded before tightening the U bolt clamps.  There is a factory tool for this, but this is my version.

Everything bolted in.  Time to move to the rear suspension.

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