Noisy 3rd gear

Hello folks

I am recommissioning a 1961 99 which has been out of action since 1976. I have been on a shake-down run which was mainly top gear and 60mph and all went well until I changed down into 3rd before arriving home to hear a very dry sounding scrappy-screeching from either the box or primary drive, a bit like the noise a car clutch thrust bearing makes before it gives up. I messed around up and down the lane, the noise only manifests itself in 3rd, all other ratios are quiet. I have only changed the oil in the box and removed the outer cover to renew the gear/kickstart oil seals, no other work and it was not noisy at the start of the run nor in the few miles I have run her up and down the lane prior to this. The gear selection coming back down can be a bit hit-or-miss in that it sometimes won't go from 3rd to 2nd until I have slowed down a bit more than I would expect but I don't know if this is linked. No other issues. It ran really quietly at speed in top so I suspect the gearbox rather than primary drive. My question is - are there any parts of the gearbox mechanism linked to 3rd other than the cluster itself which may bethe source ofthis noise? Could it be the selector or kick-start or something other than the actual gear? Any suggestions greatly appreciated before I dive in with a hammer and Haynes manual.

well there is a brass thrust washer in between the 3rd and 4th gears it could be this as got damaged in some way , its a case of open up the gear box and having a look and see what happing in there if you get some discolouration in the oil this would tell you, well hope for the best, Yours Anna J

Thanks Anna. I have dismantled the gearbox and both the lay shaft and main shaft third gears have serious tooth wear. Both are through the case hardening and the lay shaft gear teeth are badly worn. The rest of the gears are fine with maybe slight wear on fourth gear but looks serviceable. Next question - is this quite normal or would this sort of wear suggest something else amiss? I aim to simply replace the gears and rebuild as everything else seems ok. There was no swarf in the oil for instance. Once again, any advice greatly appreciated. Many thanks, Bruce.

Hello again is there a brass shim washer in between the 3third and fourth gear As there should be one there yours anna j

Previously anna jeannette Dixon wrote:

Previously bruce_lindsay wrote:

Thanks Anna. I have dismantled the gearbox and both the lay shaft and main shaft third gears have serious tooth wear. Both are through the case hardening and the lay shaft gear teeth are badly worn. The rest of the gears are fine with maybe slight wear on fourth gear but looks serviceable. Next question - is this quite normal or would this sort of wear suggest something else amiss? I aim to simply replace the gears and rebuild as everything else seems ok. There was no swarf in the oil for instance. Once again, any advice greatly appreciated. Many thanks, Bruce.

Hello again is there a brass shim washer in between the 3third and fourth gear As there should be one there yours anna j

Anna,

This is NOT correct. The AMC gearbox, used since 1957, did not have the bronze thrust washer on the mainshaft between 3rd and 4th gear because the end of the mainshaft was threaded to enable the mainshaft to be bolted to the small mainshaft ball bearing.

The bronze thrust washer was used only on laydown and earlier type gearboxes.

regards

The bronze thrust washer was used only on laydown and earlier type gearboxes.

This is correct...the later post 1957 AMC type gearboxes did not have such thrust washers inside.

I have dismantled the gearbox and both the lay shaft and main shaft third gears have serious tooth wear.

This is bad news and usually points to a gearbox that has done many miles but not had regular oil changes. Once the case hardening is worn through, the gears (same with the camshaft) will not last very long.

I set my torque wrench to the 70lb/ft noted in the Haynes manual for the mainshaft nut and managed to strip the threads

This also is not good news as possibly the threads on the end of the shaft have worn thin. A new nut might help here but it may be a case for a replacement shaft. You must be able to torque up to 70 flbs or the nut will keep coming lose. Some people fit a spring (star) washer under the nut to help keep it in place.

sometimes bronze gets called brass but i am sure Anna J meant bronze when she mentioned a brass thrust washer please dont use brass for bushes or shims in any moving part of your gearbox regards Nick

Hello again

I worked out that I didn't need the thrust washer - My box didn't have one nor did the spare AMC box which I raided for the pair of gears. I also checked the manuals which didn't show one in the diagrams and the clincher was RGM's website which listed the washer but noted pre-57. Anyway I rebuilt the box using the pilfered gears and it's fine now so fingers crossed for an MOT sometime this week.

Incidentally - I set my torque wrench to the 70lb/ft noted in the Haynes manual for the mainshaft nut and managed to strip the threads, thankfully from the nut and not the shaft. My wrench might be at fault though I did test it in the vice and it clicked off ok. Do we know if that particular value is correct or a bogey number? I seem to recall mention that the Haynes manual has history of some incorrect torque values. I ended up tightening it up using my socket ratchet and the "that'll feels tight enough" method which seems to work well for most occasions featuring big nuts and bolts.

Thanks for all your help.

Bruce

well Bruce I did think thay sill used its bronze washer in-between 3rd and 4th gears but they must of done away with it before then , anyway my bike passed MOT on Saturday morning with flying colours and there was a crowded around to see my bike of all things and they where all bikers too, they all like the look of my bike. and said it looks cool , anyway I do hope your Gearbox is now OK but do not use a EP oil as it attacks Bronze parts , a SAE 50 is OK or Hypoly 80 Morris or Millers or Castrol these are all about the same , yours anna j