Continuing the number-based titles... 1,2,3,4 Overdrive!

Time to fiddle with the gearbox. This had been working very well, but there was a lot of play in the first motion shaft bearing, it could be wiggled well over ¼”. So it seemed logical to strip the whole gearbox down for inspection, cleaning and replacement of the dodgy bits.

The first motion shaft bearing housing was removed, along with the selector tower, the overdrive and the rear extension. I spent a bit of time playing, to determine exactly how everything fitted together and worked. Disassembly continued, until the entire thing was in pieces. Despite having not done a great deal of gearbox work before, if enough photos, drawings and notes are taken, things are much easier, gearboxes are very simple really.

Gearbox casing cleaned and scrubbed... The overdrive has been removed, leaving the rear extension and main casing.

Yours Truly. Purpose of this photo- remember to find out what this little bracket does, before reassembly. (it turned out the be the bracket that a clip is bolted onto. The clip holds the overdrive and reversing light switch wiring in place).

The rear extension has been removed now, and the selector rods are being withdrawn.

The big nut allows the third motion shaft to be disassembled. The big, double roller bearing seen here is just about indestructible, and has very little force exerted upon it from any direction.

Removing the bearing from the first motion shaft proved difficult, due to the lack of a ring spanner of the correct size, and a vice not big enough. A hilarious afternoon of going backwards and forwards to Matt’s house then mine ensued, as we attempted to use his man-sized vice, but forgot various vital tools. Eventually, the reverse thread nut was removed, the bearing replaced and the first motion shaft was ready to refit.

The baulk rings were in good condition, however I fitted some with even lower mileage. I also fitted a slightly less mashed reverse gear, not that there was much between them. All other gears were in perfect order. The second motion shaft was in a bad way, with severe damage to the shaft and one of the needle roller bearings. This looked to have been cause by water in the oil, rather than just severe wear, but thankfully I had a spare second motion shaft, and a new needle roller bearing was ordered from Moss. Being Moss, they sent one with a missing needle roller. Brilliant. The one part I was relying on to get the gearbox back together, and they send one with a fairly obvious manufacturing defect. I am glad I only use them for bits I cannot source elsewhere… Thankfully, I managed to find a spare needle roller bearing, and I fitted that.
The second motion shaft had a little bit too much play, and for some unknown reason, the selective thrust washers necessary to take up this play are no longer made. Why? This is a MGB four sync gearbox, common and popular? Oh well, the endfloat required was 2 thou, there was 7 thou, and 5 thou crankshaft thrust washers thankfully fit.

The outside of the gearbox had been cleaned with the paraffin gun before disassembly, and now the inside was cleaned as well. All components were thoroughly cleaned before reassembly, bearings were washed in meths and all was good. The gearbox was put back together with new gaskets, the selector rods refitted with the interlock, and the overdrive was bolted on the back. Botheration.

Ahh, the gearbox has been cleaned and reassembled. it is a thing of utter beauty. The small gears in the bottom right are the reverse gear, behind them is the second motion shaft, up top is the third motion (output) shaft. Mechanical genius and awsomeness, I need to rebuild another as an ornament.

Lovely syncro hubs, first gear is nearest the camera, and fourth is furtherst away.

Don’t you hate that sickening moment as you gently tighten a UNC bolt into alloy, and it suddenly goes all loose? I wasn’t over-tightening it, honest. That meant that the overdrive had to come off again, the rear extension on the gearbox had the corroseponding hole drilled out to take a ½” bolt rather than a 7/16”, and the overdrive had to be drilled and tapped to take a ½” bolt cut down in length.

At the same time, the overdrive valves were disassembled to check no tapping swarf had got inside, and all was in condition. The overdrive then went back on, and all was well.