Lotus Elite Differential Box Repair

Written by Jess Marker.

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Originally published in Club Elite Newsletter Issue 1 (Dec 14, 2005)

 My Elite, #1312 has had an annoying “thump” or “clunk” emanating from the differential area since we bought the car in 1999. Lots of time and effort, not to mention money has gone into eliminating it. It occurred usually when shifting and also when crossing bumps, such as railroad tracks or just plain potholes. We had over the last 5 years made it a major point of inspection during our prep sessions for doing the Copperstate 1000. We had, among other things, changed the rubber cones at the top of the springs, changed the springs, changed the rubber differential bushings, and tightened up everything we could think of. It also had an annoying throttle steer that occurred when you let off on the throttle, it would move the rear end to the left, and reapplication would twitch it back to the right. It was very unnerving when taking a corner.

 Our last outing was the 2004 Copperstate, which included a run down the Coronado trail between eastern Arizona and western New Mexico. It’s a brutal 60 mile two lane road with 400 major turns or switchbacks. (Highway dept. statistic.)   We made it, but by the time we got to the bottom, the clunk was back as well as a loosey-goosey feel to the rear end. I checked it out, shook the diff assembly that did seem to move a lot, both laterally and longitudinally, but it appeared to be safe so we continued on. We finished fine but with some concerns about the rear end.

 On our return, I took the car to Randall Fehr in Seattle, determined once and for all to get the thing right. Randall took some time inspecting and disassembling the diff assembly and found, finally, the problem. Three of the four tapered holes the diff bolts into in the diff box were either damaged and/or had rather unusual repairs.

 My car is an early Bristol body car, complete with the iron straps. Based on the condition of the holes, Randall is of the opinion that the damage was probably done before the factory strap repair was accomplished.

 We bought a replacement diff box from David Mousely, planning on replacing the box. This is a major piece of plastic, extending across the centrelines of the mufflers forward ahead of the current box. It bonds to the floor of the spare tire well in addition to the bevelled seam that goes around the entire replacement box.. It would be a major task to remove the damaged box, grind the taper all around the piece and then bond it back it. We decided to proceed.

 Luckily, Randall never rushes into anything. He continued to study the problem and ultimately came up with another solution, which would avoid the large graft of the new box.  The plan was to fabricate a template from the new box to ensure absolutely perfect alignment of the differential. He then took molds of the tapered holes to fit on his template. Grinding the taper on the repair area to ensure the strength of the repair was a major task. Randall rigged a “surf board” to lay on while he ground the repair area and used mirrors to grind the top of the box, since it is behind the spare tire in the boot. (I encouraged Randall to cut the spare tire area out and then repair it, but Randall absolutely refuses to destroy an original piece if it’s at all possible to avoid it.) In any case, once the job was completed, it is virtually undetectable. The clunk is gone and so is the throttle steer.

 The attached photos show the previous repairs, some excess parts found in the boot and some of the work Randall accomplished. Kudos to Randall for his professional, expert repair.

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Photo 1:   Shows the lower bushings and the early repair that amounted to two tubes with sheet metal welded to form a flange. The flanges were backed with some mastic and pop riveted to the diff box. Unfortunately, the pop rivets were to too short and were just loose in the fibreglass.

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Photo 2:   Another view of the lower bushing area. The repair tube and flange are visible on the right of the picture removed from the car. The left side is as it was when repaired originally.

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Photo 3:   Shows the beginning of grinding the repair area and establishing the featheredge for replacing the damaged area.

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Photo 4:   More grinding and prep.

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Photo 5:   Mirror looking at upper repair area under spare tire cover.

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Photo 6:   Template with tapered plugs to locate diff mounting points.

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Photo 7:   Using plugs for molds to get the shape of the tapered holes. The one on the right is only the front half sticking through. Glass was laid up around the plugs, liberally coated with mold release, then when cured, the mold is unbolted and removed.

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Photo 8:   Repair completed. It looks identical to original. Note the correct double taper for the rubber bushings.


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Photo 9:   Spares found behind the thin flat panel at the back of the diff box. They were removed and discarded.


Jess Marker   This email address is being protected from spambots. You need JavaScript enabled to view it.    All photos and text from Jess Marker - Elite #1312   Washington, USA

Copyright © 2005 Jess Marker  All rights reserved.