Transmission question

Was making a short trip around some local twisties and hills and noticed that in 2nd and 3rd gear, when going from forward torque (engine torque) to reverse torque (coasting, engine braking) the shift knob would click and move slightly. There's no hitch in the drivetrain, just the shifter making a slight 'pop' and moving, in a different direction depending on whether I'm accelerating or using the engine to slow; e.g. pulling up a grade then lifting to coast at the crest. This doesn't happen in fourth gear, and I haven't noticed it in first.

Looking for a little help from the transmission gurus. I'm guessing worn layshaft, allowing some 'flex' between the laygear and the main gear. So far, this is an annoyance, but want to know if bad news may be forthcoming. Gearbox may have up to 90K miles since last rebuild, when we replaced the layshaft, bearings and seals.

The symptom of any gear actually popping back into neutral when you lift off, is generally the fault of a worn synchromesh hub assembly. While yours aren't coming completely out of gear, that movement can be a signal of what is yet to come.

First (1st) & second (2nd) gear share the same synchro hub, as do 3rd & 4th, so the fact that you're seeing this movement on 2nd & 3rd doesn't point to just one (1) failing hub. Perhaps it's the fault of neither, and just the springs getting weaker on the selector rod detent balls? These springs could be changed with the transmission in situ, by only removing the side or top cover, depending on whether your car has a sideshift or centershift xmsn.

The rearmost needle rollers on the layshaft are without a doubt, the very first component to wear out in a sympathetically used gearbox (an idiot can wear out the synchro rings in a single day, but assuming yours is an otherwise good box) so your suspicion may well be correct. Not so much from a "flex" standpoint, but as wear takes place, the small end (the 1st/reverse straight-cut gear) moves a little further away from the gears on the 3rd-motion shaft.

Ninety thousand (90,000) miles is probably a very good service interval for theses xmsns; might be time to start collecting parts again...

A rebuild at this point could potentionallysave you a lot of money, as the further the laygear moves away from the 3rd motion shaft, the more load is placed on the tips of the teeth.

Back in the days (70s-80s) I would spend $100.00 on the xmsn as soon as I bought another MGB; that was enough money to replace all (either 3 or 4, depending) synchro rings, front and rear seals & the layshaft bearings. Likewise, I would replace the thrust washers in the differential__the phenolic ones were always shot, and it would tighten up the spider gear backlash as good as new.

Thanks, Randy ... very helpful.

I'm confused about one thing, though: my gearbox doesn't have synchro in 1st (BJ8), do 2nd and 3rd share a synchro, and 4th have its own, maybe?

I would look at the motor/trans mounts, sounds like one of them has a loose bolt.
BillM

Hi Bob, the box is just showing a little wear in this case the gear lever dances back and forth. You are good to go until it becomes excessve and may eventually jump out of gear.--Fwiw--Keoke