Engine Rebuild (Part 9) - Gearbox, Differential and Bell Housing

As you might expect, before you can rebuild the engine, the gearbox needs to be split from it. As the engine rebuild neared completion, I began to think about the gearbox: with the engine now looking much cleaner, the gearbox was begging for attention. I had already given it a wash with white spirit and a pressure wash (yes -  the dreaded pressure wash!) back in 2015. 

Initial clean up - 7 June 2015

As a result, the main gearbox body didn’t look too bad – shiny even – but the top plate and the bellhousing still looked dull and dirty.

Gearboxes compared - July 2018

The gearbox I planned to use was untested before the car was taken off the road - so something of an unknown quantity in terms of wear and tear. A line of argument said I should fix EVERYTHING while I could – to remove any uncertainty. I could see that argument, but firstly it would have involved high cost for new bearings and a re-manufactured crown wheel and pinion amongst other things. Secondly I didn't have the tools or confidence to do it myself and lastly I didn’t have the money to pay someone else to do the work for me.

The process for stripping and rebuilding a gearbox is covered in Operation D.330-3 in section 2 of volume 2 of Manual 814. Overall, Operation 330-3 is LONG and complicated.....which is partly why I didn't want to dig too deep into my gearbox. So deep that I couldn't get out. Reminding myself that I wasn’t out to build a Grand Prix winner, I stuck to my plan of just removing the bell housing and cleaning the gearbox - though even that necessitated some disassembly.

After removing the clutch operating components – thrust bearing, operating fork and clutch cylinder - the first big job was to remove the two shaft housings that connect to the differential. This was just tedious due to the large number of bolts hidden away around their circumferences.

Removing the Differential Shaft Housings

The shafts are oiled by the gearbox oil entering the housings - and so the housings are a close fit to the gearbox. 

Differential shaft removed

Behind each shaft housing is a paper sealing gasket. Replacements for these are available from the usual suppliers.

Example of shaft housing and gasket

As the shaft housings were removed from each side, a large distance piece with a bevelled edge and another large shim dropped out of the aperture of the gearbox body.

Differential, housings and and their big distance spacer rings......

It is possible to press out and replace the bearings in the differential shafts – but again I had no plan (or ambition) to do full rebuilds and, if I had, it would have meant further work setting up the gearbox at rebuild time.

shafts are 'handed'

Standing the housings on their wheel stud ends like stilts, and with the inner bearings protected with rags (so that liquid did not go into them) - I gave the housings a good wash and stipple in Jizer, followed by a rinse and blow dry. They came out looking a lot better than before.

Removing the Bellhousing

With the shafts and housings removed, the bellhousing can be split from the gearbox. When assembled at the factory, Citroen fitted ‘tamper-proof’ bolts in a couple of places. Not sure why they placed them here though? These are ‘tamper-proof’ in that the heads are an unusual design and so they are difficult to remove!

Special-headed Citroen Set Screw

By chance I already had a special shock absorber socket  - just a cheap Ebay thing - and found that, with a gentle tap from a hammer, it gave a snug fit on these weird bolts. It's OEM tool number 25284 if you're interested.

This tool just about fitted the special set screws

I worked around the bellhousing until the last bolt was free and.....

.....with a tug, the bellhousing broke free of it’s mastic seal…..spilling the heavy crown wheel onto my workbench (that was a surprise!).....

.......and revealing the drive pinion behind.

Gearbox Input shaft and drive pinion

Now I could get the bellhousing cleaned.

Cleaning the Gearbox Top Cover

With the differential shaft housings, bellhousing and crown wheel removed, the gearbox was a lot lighter. I took the opportunity to pour some white spirit in the filler hole and slooshed it about to dilute and release any lumpy crud stuck in it’s dark corners. I didn’t want to be too thorough as I didn’t want to leave the gearbox completely free of a protective oil layer.

I also wanted to take the top cover off the gearbox. Mostly so that I could see what state it was in inside (I was worried that my pressure washing of two years earlier might have left me with a rusting gearbox full of water), but also because I wanted to get the top cover vapour blasted.

The hydraulic box lid has five hydraulic pistons set into it for the four forward gears and reverse. These can be removed, but I didn’t have a tool for compressing the springs and didn’t fully understand the instructions for the ‘work-around’. If in doubt, don't fiddle.......

Inside a BVH gearbox lid

I worked out that, with the clutch lock removed, the gearbox lid could be removed without tampering with these and so, more importantly, it could be replaced without the need to reset or recalibrate anything.

Lid removed - no rust

The hydraulic gear pistons operate the dogs on the shafts

Happy that the gearbox was not rusty and full of water, my attention turned to getting the lid vapour blasted. I didn’t want to remove the gear-operating pistons, but neither did I want vapour-blasting medium in them. I took a chance. I decided to box-off the pistons to protect them: I replaced the cluster of five hydraulic pipes back on the pistons connections – and then blanked off the union where all the pipe ends meet. I then made a template from a piece of think polythene (a breadboard) and, with RTV silicone sump sealant, blanked off the open side of the cover by bolting this onto the gearbox cover. 

'Blanking Plate' for vapour blasting gearbox cover

I was fairly confident that this 'sandwich' would be water tight, so sent it off for vapour blasting. The worst case scenario was that I would need to find a way to strip and clean the pistons after all. 

The result of the vapour blasting was very pleasing – much cleaner than it had been. Like new in fact! Removing the polythene plate I was also pleased to find it dry inside. The hydraulic pipes were undone. I used a vacuum cleaner to make sure any stray beads of media were pulled away from the apertures and not pushed into them.

Fitting the top cover back on the gearbox was the reversal of removal, using a thin bead of RTV sump sealant.

Re-assembling the Differential Shaft Housings and Bell Housing

After vapour blasting, the bell housing (or as Citroen call it, the 'Clutch Housing') looked great! I was looking forward to refitting it to the gearbox – only I didn’t have enough hands….… and few chances to take photos.

The process I followed for putting all of this back together is covered at step 35 of Operation D.330-3 in section 2 of volume 2 of Manual 814. I reasoned that as I had not rebuilt the differential shafts, nor anything in the gearbox, I would proceed on the working assumption that everything would/ should fit back together just as it came apart... ..seemed reasonable to me. 

The aim therefore was to correctly locate the crown wheel on the pinion, as well as locating the differential shaft distance pieces in the circular hole formed by joining the bell housing and gearbox. As an added complication, before bolting up the bell housing, I also needed to make sure that the machined surfaces on the sides of these two joined pieces were aligned - to provide a flat surface for the paper gaskets of the differential shaft housings.  To achieve this, the gearbox/ bell housing combo needed to be partly assembled and then one of the housings loosely bolted in place to align it’s two half surfaces. Easy!

Except gravity kept getting in the way: The crown wheel frequently rolled out of the open end - making it impossible to accurately introduce and set the bell housing. As added complications, the bearing distance pieces and shims/ rings needed to be located before the bell housing/ gearbox combo could be fully tightened up. Those distance pieces were buggers and popped out at the drop of a hat! Citroen have special clamping tools for holding the crown wheel in place while the bell housing is loosely attached. The design can be found in volume 2 of Manual 814. Two pieces are needed - each of a different depth - 'a' in the diagram

Citroen tool for holding the differential in the gearbox

The tools are really little more than bent pieces of bar, but serve to act as an extra pair of hands. With the bell housing loosely fitted over the crown wheel, the tools can then be removed, and the distance pieces can be fitted. 

Citroen tool in place

I probably could have made a pair of tools but, keen to press on, I roped-in my wife Gayle instead. With the crown wheel and spacers located, I attempted to hold these in place with a big wire loop around the gearbox. What a bodge! This was far from ideal and it was really Gayle that held it together – literally.

Crown and pinion wired in place - badly......

I cut the wire, slipped on the distance pieces (in the correct order) and Gayle held them in place. There were no second chances with this as the bellhousing/ gearbox join already had a bead of  sticky RTV gasket sealant on it’s surface and so wouldn’t have liked to have been moved about.

The bell housing was slid over the long bolts on each side while Gayle held the crown wheel and spacers in place. When it finally came together, it seemed to just slot into place. I had to be careful that the distance pieces did not pop out of their spaces but once the bellhousing and gearbox was joined, they seemed content to stay in place.

Bolting on the differential bearing housings

There is an odd mix of M7s and M8s (and of differing lengths), used to secure the bellhousing so I had already laid out all the bolts according to the positions they fitted.

With the bellhousing loosely bolted together I fitted the right hand shaft housing (the one with the shorter shaft) and it’s paper gasket. Once that had pulled the machined surfaces flat and level, I tightened up the bellhousing to gearbox bolts properly and removed the differential shaft housings again. 

I checked that the two housing half surfaces were still level. Satisfied, I gave the paper gaskets of the housings a smear of ‘Hylomar blue’ just to make sure they sealed well and then stuck them to the housings. (You need to make sure you get them the right way round and line up all the bolt holes). I tightened the housings up.

Bolts on differential shaft housings

Not bad. Looks a lot better.

Compare and contrast....