Transmission rebuild and Limited Slip Differential

The one mechanical failure of a serious nature I suffered was the loss of second gear. I was driving at 30 mph when I suddenly couldn't shift into second without a horrible grinding sound. I drove home using 1st and 3rd and emailed the Lotus Esprit mailing list for advice. Their prognosis was not good, either a worn out synchro or damaged second gear. Both would require removing and rebuilding the transmission. So that's what we did.

In the process I found that I had chipped off a tooth from second gear and had a worn 2nd gear synchro which presumably resulted in the damage to 2nd. Since the tranny was apart I decided to order new synchro rings for 1st and 2nd, a new 2nd gear and (while I'm at it) a limited slip differential. I got the LSD from Quaife, not the Lotus part that is fitted to the Sport 350 and Sport 300. It was cheaper than the retail LSD from Lotus.

As it turns out the Quaife unit is not exactly a perfect match. The basic dimensions are correct but there were to main problems.

1. Mechanical speedometer worm drive gear

If you have an electronic speedometer V8 and newer then this does not apply to you. The speedo cable is driven by a gear that sits on the end of the differential inside the transmission case. It mates to a another gear  that makes a seal with the transmission case and has the speedo cable attached.

for the Quaife LSD there was no provision to attach the speedometer gear to it. On the standard Lotus open differential there is a shoulder and circlip to hold it in place.

We machined the plastic gear to fit snugly on the end of the differential and bonded it to the end using a high power epoxy designed for repairing gas tanks. You can see the speedometer gear (yellow) on the new differential in the picture below.

2. Output shaft bearing clearance.

The right hand side of the transmission allows for the adjustment of the bearings on the differential. The Quaife unit however was too short, and so a shim at to be manufactured and inserted between the end of the differential casing and the right hand output shaft bearing.

Before reassembly we thoroughly cleaned all the transmission parts.

Right hand case half for transmission. Note we follow the manual :)

Left hand case half for transmission. Note the selector forks that slide gears between 1-N-2, 3-N-4 and N-R respectively. 5th gear resides in the tail cone area at the rear of the transmission. The fifth selector selects 5-N. A sixth gear can be added with the current selector mechanism to provide 5-N-6. Lotus USA sold one such ex-racing transmission a few months back. Also note our preferred brand of hand cleaner.

This is the tail cone of the transmission. The selector shaft (what your shifter is attached to) has been removed wile it was cleaned. Note the pen stuck in our epoxy mix from the LSD speedometer gear. I was amazed at how much it hardened overnight.

Here we have both primary and secondary shafts and the crown wheel and differential together before putting both halves back together. You can see the bearing on the differential (this is the one we had to shim by about 3mm to make it fit).

Here our transmission guru, "Hawg" (I was initially reluctant to call anyone a Hog) applies the gasket sealer before we put both halves back together.

Here the transmission is completely assembled with the selector shaft protruding from the side. We also replaced the output shafts.