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?
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.
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.
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.
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!
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