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MG MGB Technical - Gearbox (again) and now I have a problem!

So I have decided to use an overdrive box, of which I have a few in stock, but all need reconditioning or at least checking.

Newest one probably has the lowest miles, and not in bad order, but I do need laygear thrusts to reduce the end float.

But guess what, they are NCA!!! (Moss)

Anyone point me in the right direction to where there might be some stock?

Thanks

Colin
Colin Parkinson

How much do you need to reduce the endfloat by?

If only a thou or two then a large hammer on the back of the casing will take care of it.

Otherwise a crankshaft nose shim between the rear thrust washer and the case.
Chris at Octarine Services

Looking at about 8 thou to give me 2 thou end float.

Dont like the big hammer idea ( I am an engineer not a butcher!!!)

But the crank shim likes like an idea!!

Colin
Colin Parkinson

I don't really like thin floating shims in gearboxes, they tend to work their way out of place,
Also-which gearbox do you have- there are two series of thrust washers for the 4-synchro MGB gearbox. For cars with pre-18V engines, the thrust washers for the laygear are round, with a spiral groove cut into both sides to allow for oil travel. These thrust washers are hardened steel, shiny silver, and are free to spin with the laygear or not, as friction dictates.

The gearboxes on cars originally fitted with the 18V engines used laygear thrust washers that have retaining tabs built into them. The front thrust washer has an angled tab that fits into a recess on the inside of the front of the main case. This indexes the thrust washer to the case so it cannot spin. The rear thrust washer has two ears that are forked and, when installed, these forked ears fit onto each side of a flange built into the main case, so that the rear thrust washer cannot spin either.

In this later style, the front and rear thrust washers are a steel/bronze composite, with the steel surface facing the main case and a perforated bronze surface abutting the laygear. The perforation allows for lubrication of the thrust washer/laygear interface, since the thrust washers cannot spin, and therefore there is constant, but minimal, friction between the spinning laygear and the stationary thrust washers...

If it's the later type then a large diameter shim between the thrust and the case 'might' be ok-----if it's the early box I believe a 3synchro box shim (still available)which is thicker can be machined to suit

William Revit

18V engines up to 1975 had the round thrust washer and the dipstick gearbox casing, only the 18V 845 engines after 1975 had the later box with tabbed washers.
Chris at Octarine Services

Willy I am using the later non dipstick box as a basis.

colin
Colin Parkinson

What thickness are the thrusts in the box--
I've heard of modified Mini thrust washers to replace the front thrust with the single tang on it they have to be opened up to fit the larger shaft size but not much and I think the tang has to have a tiddle off it's width or straightened out a bit but no big deal available in 2thou increments

DAM3180-DAM3181-DAM382 etc
Also there's the thrust on the other end of the mini cluster which is larger od and fixed size but I don't know what thickness that one is
A call to Minispares might be the go
William Revit

Front is 123 - 125

Rears a range from 120 to 133.

Colin
Colin Parkinson

To be honest the end play in the cluster isn't really an issue-
The cluster has very little end load in operation
The gears are cut in a manner that the front input gear pushes the cluster rearwards under load but the cluster is pushed equally forward when driving whatever gear is selected--it neutralises itself
More important is to make sure the bearings on the input and mainshaft are in excellent condition and are shimmed correctly to stop any endplay
The bushes in 1-2-3 gears need to be good otherwise it'll jump out of gear --as they do
------------------
Sounds like you have a range of thrusts to pick from there ---
So- are you saying that with your thickest washer combo 125F and 133R you have 10thou
clearance, or is that the parts book selection of sizes
How much clearance and what sizes were in the box as it was

Too many questions ---sorry
willy
William Revit

Willy the 2 boxes I have just stripped have the front thrust 0.123 and the same rear thrust 0.120, which is the thinnest available, 22B696. So with this set up I have .010 clearance which I think is too much. Ideally I would like to get it down to .002.

You say I have a range of thrusts to pick from, but that is the problem, non of them are available from Moss, and I havnt found anywhere else.

I take your point on the bushes, but feel that .010 end float is far too much.

Colin
Colin Parkinson

Get a 25mm id / 35mm od / .2mm shim to go behind the rear tabbed washer - https://www.accu.co.uk/704-shim-washers
Chris at Octarine Services

Thanks for that Chris. A brilliant supplier who I could have done with a while ago for another problem!!

Colin
Colin Parkinson

I do hate thin shims in gearboxes but, I guess it is the easiest available solution,
Taking into account the thrust is located and not spinning I reluctently guess it will be ok--I suppose .008"(.2mm) has got a bit of body to it

There's still the mini thrust DAM3183 it's at .130"-.132" that with some minor mods as noted earlier would give you the .008" increased thickness over your .0123" front thrust

I havn't done this mod myself but have heard of it being done, the only issue I can see is to check that the OD is large enough but as I said it has been done

https://www.minispares.com/product/Classic/Gearbox/Replacement_Parts/Shims/DAM3183.aspx?150704&ReturnUrl=/product/Classic/DAM3182.aspxBack%20to
William Revit

Thanks everyone, I have found some shim washers, and will use 1 to reduce the end float.

Not sure what they were from, just in my box of shim washers!

Colin
Colin Parkinson

Use these shims to take out the excess end float.
ATB7102 SHIM 0.004
ATB7103 SHIM 0.006
ATB7104 SHIM 0.008
ATB7105 SHIM 0.010
ATB7106 SHIM 0.012
ATB7107 SHIM 0.020
ATB7108 SHIM 0.030

Steve
S P Brown

Thanks Steve, I sorted it, but good to have the part numbers for everyone.

Colin
Colin Parkinson

This thread was discussed between 03/01/2020 and 16/01/2020

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