Almost a Gearbox

Tim, Dave, Jay and his son Ben came down yesterday to help me put my spare Citroen gearbox back together, and if we were lucky, mate up the new Renault 'box and Emme engine and put them into the frame. Turns out this was an ambitious plan.


We made some pretty good progress on the gearbox. I had all of the bearings, gears, and other bits on hand, along with the setup tools I had made a few years ago. By the way, the setup tools and shim/spacer kit I have is available to anyone who needs to rebuild their gearbox. Just let me know and I'll ship it out to you.


Rebuilding these gearboxes is not hard, but it is fiddly. You have to make some educated guesses for the shims and spacers for each step, put most of the gearbox togther, torque everything down, and take measurements. Then, take it all apart, calculate what your next set of shims/spacers need to be, put it all together again and take another measurement. Most of the time the math works and you don't have to make another trial. Other times you are not so lucky. We ran out of luck when setting the backlash for the crownwheel and pinion.


The first step in this process is to set the pinion height. This is the distance between the top of the pinion and the centerline of the differential. Part of the kit is a fixture that is used to accurately measure this distance. The tool has a known thickness and fits in the saddles for the differential support bearings. There is a pin that drops through the fixture and rests on the top of the pinion. A dial indicator is used to measure the distance the pin drops below the fixture. The goal is to match the sum of the fixture thickness and the distance the pin drops to the number etched into the top of the pinion. The distance is adjusted by replacing a shim in the stack of gears behind the pinion. We were off a bit with our first measurement, but calculating a new shim thickness and replacing it with the closest shim we had in the kit.




This blurry picture shows the fixture with the brass pin just resting on the top of the pinion.


Next, you need to figure out how much thickness you need in shims to set the differential bearing preload. This is the amount of "squish" on the conical roller bearings that support the differential provided by the spacers and the clamping force of the output housings. This is measured by wrapping a string around the differential hub and pulling on the string with a fish scale. The highest reading you see before the hub starts to move is the preload. In order to get a good reading, the differential support bearing races need to be clamped into the gearbox housing. These clamps are parts I had made 3-4 years ago and are part of the rebuild toolkit. We used the shims that were originally installed in this gearbox as a starting point and sure enough, the preload was within spec.



Setting the bearing preload.









With the pinion at the correct height and knowing the total thickness of shims you need to have the correct preload, now you need to determine how much of that thickness needs to be on the right side of the diff and how much needs to be on the left side to get the correct amount of lash between the crownwheel and pinion. With the pinion in place, the differential is put back into the box, with shims, clamps and output housings installed. The pinion is locked in place, and with the dial indicator on a flat of one of the crownwheel teeth, the crownwheel is rocked back and forth from pinion tooth to pinion tooth. The amount of movement is the lash. If the lash is too much, you move thickness from the right side to the left side to move the crown wheel closer to the pinion. If the lash is too small, you move thickness from left to right.








The trick is to move the crownwheel closer to or farther from the pinion by swapping out shims without changing the overall thickness of all of the shims. This is where we ran out of luck. We didn't have a large enough selection of shims to give us the right balance right to left. This has stopped us until I can order up more shims to fill in some gaps in our stock.







While we were doing this, Jay was working on swapping out some steel hardware in the motor, gearbox and trailing arm mounts with stainless fasteners. Ben made himself busy riding his unicycle, shooting up defensless pop bottles with a pellet gun and carving me a nifty, monogramed open-end combination wrench. He was moving around enough we didn't get a picture of him...






Here is the Renault gearbox, ready to be installed.