By Corky Guenther
The steering system of the TC is a Bishop Cam arrangement also known as “Worm and Peg”. Much maligned in the TC for its free play on-center at the wheel edge and general “dartiness” for lack of a better term. Much of this is not the fault of the box itself but rather the wearing of the other components in the system. That said the box, contains a shaft with a Drop Arm on the bottom connected to the wheels by a Drag Link and Tie Rod and an arm on the top with a peg at the end of the arm riding in a groove machined in a cylinder at the end of the steering column, the Sector. An “Ancient and Honorable” system. The box in the TC is not the most mechanically sophisticated of arrangements with the steering shaft riding in an unbushed bore in the box. This is not to imply that significant effort was not expended in the design details of the Sector groove. Not the discussion here. The interface between the peg and the worm is a sliding one which means that it is subject to wear, mostly of the peg. The top of the arm rides on the bottom of the box cover another wear point. The adjustment of the interface was originally accomplished by shims placed between the top cover and the body of the box, a tedious and time-consuming process.
A solution to this is the Thompkins Kit, a replacement top cover with an adjustable needle bearing arrangement bearing on the top of the shaft, eliminating the need to remove and re-shim the cover. Some have disparaged the Thompkins Kit for removing support from the top of the arm and causing a fracture of the arm at the top of the shaft. Maybe, but for the over 100k miles we’ve driven the car we’ve had one installed. In that time there have been 2 failures of steering components. The first was a fractured Tie Rod end ball shaft which we discovered when we exited the Mass Turnpike and after stopping in line to pay tolls found that directional control was compromised. Turned out it was the one on the right-hand side so the left hand one was still connected by the Drag Link. Not sure when it snapped but surmised that even had it been on the pike at speed, the right-hand wheel would have castered happily along until we stopped. It could have happened when we moved from rest and steering input was applied, probably more likely. No way to know for sure.
The second was a fractured sector shaft arm in the usual fashion at the top of the shaft. This happened while exiting a parking lot when moving slowly and this time directional control was nonexistent. That shaft was, presumably, the original which had been hard chromed and reground during the restoration some 100k miles previous. No idea how many miles before that. It was replaced with one from Doug Pelton at “From The Frame Up” made from modern steel. The old shaft had been ground oversize presumably to allow reaming of the box to compensate for wear. So, this time Bronze bushings were installed and honed to fit. Steering effort was significantly reduced.
Now for the additional heresy. During the Spring of 2015, at about 85k miles, I modified the Thompkins kit. I removed the upper TRD – 1/8” thick hardened washer and replaced it with a TRC 3/32” hardened washer on top of which I placed a Wave Washer, 3 Waves, 0.76" ID, 1.016" OD. The wave washer and the hardened washer are contained within the machined recess of the top cover. When I adjust the contact of the Worm & Peg during normal maintenance, I tighten until contact is made (everything centered of course) and then add a further approximately ¼ turn of the adjusting screw and lock it in place. This arrangement assures lightly loaded contact of the Worm & Peg and did reduce free play “on center”. Also provides some shock absorption within the box. Not that it precluded the fracturing of the original sector shaft.
