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MG MGA - Broken third motion shaft?

A friend of mine was driving his 1600 A and there was a loud bang from the gearbox zone - he had 1st and 2nd gear but no 3rd or 4th to get home! As the history of the car was unknown (only had a couple of years) we decided to remove the gearbox suspecting the worst.
From inspection through the side cover it appears the third motion shaft is broken or the synchroniser splines are stripped - you can block the synchroniser (3rd and 4th) and still turn the output shaft at the flange easily. From the metal bits in the oil it appears it is a broken shaft (small spring fragments and metal slithers) - gears all look good!
Doesn't seem to be many reports of this sort of failure in the archives, is it a rare occurrence?
Mike Ellsmore


Isnt it strange how some posts get ignored?
Ive never seen or heard of a main shaft breaking like that...must have been some fluke as in my experiene that would be an uncommon site.
Steven Devine

An update on my mate's gearbox - yes, the third motion shaft was broken, through the peg hole in the spline section of the shaft for 3rd and 4th synchroniser/thrust washer. Judging by the lack of comments this is a fairly rare experience and now that the we have opened the box we know why - the DPO or his mechanic had bored out the 1st motion shaft and installed a brass bush rather than use the needle rollers - as this bush wore it it would have put cyclical stress on the main shaft until kboom! Layshaft was badly worn also at 1st gear end. Luckily it looks like he can source a replacement or donor gearbox.

Mike Ellsmore

Another photo - you can just see the brass thrust washer splines at the break.

Mike Ellsmore

The offending brass bush in the end of the 1st motion (input) shaft.

Mike Ellsmore

Our local MG gearbox expert passed the following comment on to me after seeing this post -
"Have seen and replaced over the years many Mk 1 main shafts broken in the same place, even when fitted with the correct needle rollers. A very weak point where the hole is drilled for the peg lots of metal fatigue after 60 years of use. More prone to breaking when the cars are raced under a lot more stress.

A brass bush is usually fitted when the first motion shaft bearing surface has gone through the case hardening, same with the cluster gear a cheap fix."
Maybe we drive them harder downunder!
Mike Ellsmore

This thread was discussed between 15/03/2014 and 18/03/2014

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