That mainshaft nut again! - A box damage question this time.


If this problem has been left until no gears are available rather than just an iffy second gear, does this do any damage to the internals?
If not then I'm guessing the nut can be persuaded back up the damaged (by now?) mainshaft thread & then welded in place?

& no it's not me that has left things that long! -- as if !! Smile


If left it can damage the gearbox internals, but in reality it is most unlikely as most people tighten them before they reach this stage. It will damage the bearings first, followed by the gears, then synchromesh rings.

It is not recommended that the nut be welded for several reasons, the first being the fact that the shaft is suspended in taper bearings, so will wear over time. This will prevent any future adjustment from taking place.

Heat from welding will damage both the bearings and the hardened shaft, and in some cases can distort the shaft. If you want to lock it solidly then you have a simple option, obtain another nut and machine the locking flange off, fit this first and tighten, put the normal nut on behind this, tighten, and lock by tapping the locking flange securely into the keyway. This then gives a locknut assembly, further locked with the locking flange.

Thxs assassin. As no gears

Thxs assassin. As no gears can be selected at all I am assuming the worst but maybe this otherwise fine example can be rescued easily after all....?

I thought the mainshaft thread would be a mess, hence the crude idea of tacking the nut on, but your words of sanity & wisdom are heeded Smile

One of the benefits of an

One of the benefits of an engineering degree, bearing electrical conduction is not a well known subject outside engineering circles.

Spot heating of hardened shafts from welding also causes localised expansion and contraction to a machine finished shaft, this causes many other potential problems. Unfortunately i try to keep my posts simple so most people can understand them.

thanks assassin. yes, i'd

thanks assassin. yes, i'd imagine even a slight deformation of the shaft would drastically shorten the life of the bearings and other rotating parts, however i doubt the heat from say a one second duration weld would penetrate more than about 10 mm into the steel. given this is right at the end of the shaft i think the chances of deformation becoming a problem are slim. whilst further hardening of the material close to the weld is also possible, given the tip of the shaft isn't under too much load i doubt this would lead to failure, but who knows, i might prove myself wrong!

an engineering degree is a useful thing to have but there's no substitute for practical experience. i'm very grateful to you and the other experienced regulars on this forum for taking the time to share this.

by the way any comment on the slight play in the mainshaft? did anyone else notice this?



All gearboxes or transmission systems have some degree of endfloat, this is normal, but it is when their is excessive endfloat that it becomes a problem. Measure the endfloat, if it is more than aboutabout 0.040" then it may be indicitive of a more serious problem internally.

Totally agree with the practical experience, i too have served a full apprenticeship, and worked for many years in a hands on capacity. It provides me with hours of amusement when working with other professional engineers, particularly when i highlight their hypothetical solutions will not work in reality.
One other nice one is when people, usually contractors, assume because you're the boss with numerous qualifications, you cannot do anything practical. Great when i go on site and solve a problem, then get their tools and do the job myself, most of their expressions are priceless.

Just done mine, and welded

Just done mine, and welded it. the old nut had lost most of it's thread and the staked section had stripped a couple off the end of the main shaft.

For what it's worth i don't think a small spot weld from a hobby mig will do much damage to the mainshaft as the heat would be conducted away fairly quickly before anything heats up enough for damage. if however you were to use a bloody great mma welding rod then yes i suppose this could be possible. i did read somewhere about high current not doing bearings much good so do make sure to connect the earth croc around the nut itself.

the way the nut fits on the end face of the (cylindrical section) of the nut sits flush with the end of the 5th gear cog (part 80 on page MT-10 of the manual), so a small bead of weld will sit on the threads and protrude from the end face of the cog making it very easy to remove with a hand file if necessary.

actually i had one question i'm sure assassin can answer, once i had torqued up the nut (with an extension bar!) there was still a few mm play back and forth on the main shaft - is this normal? i bloody well hope so! it all seems to work fine.


Thanks for that...

Useful info on bearing damage.

All my welding knowledge has been taken from an old book by 'The Welding Institute' from around 1974.

Here's another topic on welding the gearbox nut.

BTW, the gearbox is now another 50000 miles down the road with no further problems.