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MG MGB Technical - Laygear & Layshaft Disaster


67 B with O/D.
Following an extremely unpleasant "screaming" from the gearbox one day in 1st and 2nd on the way home from work (3rd and 4th with or without O/D were as smooth as ever) I decided to pull the gearbox out - with the cheerful assistance of my wife as helper!

I have not had to work on a gearbox before so was looking forward to the challenge of finding the issue. The short of it was approximately a tablespoon or two of shiny metal flakes in the bottom of the gearbox, a well worn layshaft with both rear oil holes completely blocked with metal (4 bearing type) 22 H1301, 2 complete needle bearing assemblies completely obliterated in fact turned into a tablespoon of flakes! Both shims had a nasty amount of scoring off centre, approximately 1mm deep where the gear must have run well off centre.

First gear ring with pitting evident in the centre of the teeth and I am sure that won't be the end of it. The input shaft and third motion shaft seem to be in reasonable shape, teeth all look clean and sharp, the main bearings are smooth running however feel as though they may have a little too much movement possibly up to 0.5mm or so. With costs of replacement parts these days and scarcity what is the general consensus on rebuilds? Is it financially viable to look at individual item replacement any more?

I have also stripped the O/D unit and it appears to be in good shape, the friction material is in good shape, the bearings run nice and freely and no pitting or wear visible on any teeth etc. The filter and magnets have most certainly done their work to keep as much metal out of the innards as possible. They were fairly choked with debris!

Anyway open to any and all opinions on this one.

Thanks all.



C Brennan

The wear on shaft and 1st gear outer are pretty typical on these boxes - you will need to check the inside bearing surfaces of the laygear - if worn then the laygear is scrap!

New laygear are about £100 in the UK, shaft, bearings and 1st gear outer will add another £100+

The laygear thrust washers are unobtainable new so you will need to find good used ones.

With a worn laygear, I would suggest a recon box is the way to go - but check the quality of the recon - some are not what they should be!
Chris at Octarine Services

Thanks Chris, yes the inside of the laygear is an absolute shambles - it would appear that the bearing cage has fused to the internal walls of the laygear!



C Brennan

Tha's ugly!
Only thing worse is the next step, where the shaft breaks and blows the case apart.
This is why it's highly recommended to examine the layshaft any time the box is out, like for a clutch. 50,000 miles is enough for these boxes. Replacing the shaft and bearings when the first pits appear makes the gears last a whole lot longer. Same for A series boxes.

FRM
FR Millmore

I have actually been able to source all of the parts here, so I am going to have a go at rebuilding. Thanks for the interest guys.
Attached one last pic of what happened to the thrust washers also!
cheers,

C Brennan

I think you'll find that the thrust washer should have that groove in it.
Dave O'Neill 2

...assuming that was what you meant?
Dave O'Neill 2

Dave,

You are completely correct, I have just realised this - it just did not look exactly 100% given the other damage in the area! Oh well, the things you learn!!

Cheers,
C Brennan

Thanks for posting the photos. Brings back memories. As I recall when I went through this on my '66, my tranny case was scored, at the layshaft rear hole. You've probably already checked that. Now, I can't recall how that might have happened. You'd think the layshaft would be pinned some way. In any event, it was bad enough that I was concerned that the layshaft would not properly center. Reasoning that changing cases would require adjusting a lot more clearances, and therefore more new parts, I opted for the rebuilt unit.
Terrence Goodell

This thread was discussed between 12/08/2007 and 13/08/2007

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