LT77 Gearbox problem
Discussion
I have an MGB V8 which has an LT77...C gearbox. The gearbox was rebuilt by a main supplier many years ago but has only done just over 2K miles since then. I'm starting to get what feels like dog buzzing if I pull the gear lever back lightly whilst driving in 2nd or 4th. At first I thought it was the syncro getting weak but the gearbox is otherwise ok and it does not baulk on changes unless I pull the lever for too long, ie once it's fully in 2nd or 4th. The buzzing frequency varies with road speed, not engine. Any ideas?
Might be worth reaching out to this guy who knows his stuff: www.pistonheads.com/gassing/profile.asp?h=0&me...
Thanks for the suggestion. The lever doesn’t buzz in normal driving unless I pull it back gently in 2nd and 4th and the buzz is actually a hard dog-on-dog feel, like when you try to select reverse too soon after revving the engine in neutral. So I’m thinking it could be a gearbox internal issue.
Have you checked the gear lever alignment [see info here https://www.gomog.com/allmorgan/remoteissues.html] rather than thinking the issue is within the box itself? Could there be drive vibration leading into the gear lever between the lever and the body, with the lever coming in contact with the gearbox tunnel lever hole? There's not much room between the gearbox remote/lever and the tunnel and vibrations have a tendency to travel + how was the remote and/or lever modified to fit the BV8?
Some good points there, thanks. I had a look underneath but I couldn't see anything obvious. It's a Supersport chassis, which I believe is based on the original BL V8 chassis, and it's the standard gearbox, so there should be enough clearance (I didn't build the car but it's been done to a high standard). I'll try a test drive with the gear lever surround removed. Your link didn't work for me but I've seen some info online for setting up the lever and issues with the extension housing bushes. I think I'm just going to have to bite the bullet and take the engine and gearbox out and go from there.
Before engine/gearbox removal check all gear remote rubber bushes and crown ball/spring and adjustments. Do this first replacing bushes with yellow poly jobbies + white nylon linkage bush/bushes and adjust/grease mechanism. Also worth checking of gearbox mount bushes. I mention this as gearboxes do move under acceleration/deceleration, this could lead to hidden contact with frame/tunnel and vibration extended through to the leaver. This is a small amount of remedial work that will give clean bill of health before engine/gearbox out 
Well I finally got the car on quickjacks and, sure enough, the gearbox extension rubber bushes were falling apart. Thanks V8250, you were right on the money! So glad I didn’t take the engine and gearbox out. The gear change is back to its old self now, slightly reluctant when cold but fine once warm and no nasty dog-on-dog knocking if I pull back on the lever whilst in 2nd or 4th. I didn’t make any changes to the lever adjustments. One clue to the problem was that the knocking took longer to develop in cold weather, presumably because the bushes were a little harder in that condition.
The bushes can be replaced without removing the gearbox, although access to the top bushes is tight. They came out easily but getting the new ones in was a bit tricky. This was not helped by the extension shaft spigot (locates in the gearbox selector shaft ball joint) popping out every so often. I think later boxes had a circlip to prevent this. The top bushes for mine included two flat rubber washers, in addition to the stepped bushes, but the new bushes were a little thicker, so only one flat one was required to get a suitable compression. Hope this helps anyone who has similar issues.
The bushes can be replaced without removing the gearbox, although access to the top bushes is tight. They came out easily but getting the new ones in was a bit tricky. This was not helped by the extension shaft spigot (locates in the gearbox selector shaft ball joint) popping out every so often. I think later boxes had a circlip to prevent this. The top bushes for mine included two flat rubber washers, in addition to the stepped bushes, but the new bushes were a little thicker, so only one flat one was required to get a suitable compression. Hope this helps anyone who has similar issues.
Fin35 said:
Well I finally got the car on quickjacks and, sure enough, the gearbox extension rubber bushes were falling apart. Thanks V8250, you were right on the money! So glad I didn’t take the engine and gearbox out. The gear change is back to its old self now, slightly reluctant when cold but fine once warm and no nasty dog-on-dog knocking if I pull back on the lever whilst in 2nd or 4th. I didn’t make any changes to the lever adjustments. One clue to the problem was that the knocking took longer to develop in cold weather, presumably because the bushes were a little harder in that condition.
The bushes can be replaced without removing the gearbox, although access to the top bushes is tight. They came out easily but getting the new ones in was a bit tricky. This was not helped by the extension shaft spigot (locates in the gearbox selector shaft ball joint) popping out every so often. I think later boxes had a circlip to prevent this. The top bushes for mine included two flat rubber washers, in addition to the stepped bushes, but the new bushes were a little thicker, so only one flat one was required to get a suitable compression. Hope this helps anyone who has similar issues.
This is great news, these type of faults are often quite difficult to track down. As note, keep an eye on the lever ball cap...if the gearchange starts to feel a little stiff it's nearly always this. And another +1 here for using Castrol Syntrax in a 2wd LT77/R380 'box - it's now called Transmax + Motul gearbox oils are very good.The bushes can be replaced without removing the gearbox, although access to the top bushes is tight. They came out easily but getting the new ones in was a bit tricky. This was not helped by the extension shaft spigot (locates in the gearbox selector shaft ball joint) popping out every so often. I think later boxes had a circlip to prevent this. The top bushes for mine included two flat rubber washers, in addition to the stepped bushes, but the new bushes were a little thicker, so only one flat one was required to get a suitable compression. Hope this helps anyone who has similar issues.
Edited by v8250 on Saturday 18th February 11:05
Thank you both for the oil recommendation. Checking the receipts I got with the car the gearbox rebuild doesn’t specify the oil used but it shows purchase of Yellow Lubegard. So I’ll drain and fill with Syntrax or whatever it’s called now. It’s got to go up on the jacks again anyway to see what I got wrong with the Hall effect speedo sensor - probably the gap is too big. Good to know about the gear lever issues too. Never a dull moment with old cars!
Gassing Station | MG | Top of Page | What's New | My Stuff