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MG MGB Technical - cluster gear

Hi all
While working on my gearbox I noticed that Moss catalog lists two cluster gears and two shafts for 3 synchro tranny (early / later MGB). What would be the difference? Dear I hope that they are both for 3 synchro tranny, one for 3 bearings, and the other 4 bearings?

Martin
sharpeys

Martin if I'm correct the later 3 syncro boxes had a larger diameter layshaft and 4 roller bearings in cages. Therefore the laygear had a larger bearing diameter. When I rebuilt my late 67 box I used the larger layshaft and had my machine shop enlarge the holes in the gearbox housing. It's far easier to install the roller bearing in cages than the earlier loose ones.

Andy

67 B Roadster
74 B GT
60 A Coupe
55 TF
Andy Preston

Martin-
The most desirable of these three-synchro transmissions is the version that was introduced in the month of March in 1967 starting with engine number 18GB 74,720 (Standard transmission) and 18GB 745,29 (Overdrive transmission). It makes use of a larger-diameter (.668” / 18.967mm) layshaft (second motion shaft) (BMC Part # 22H 571) and an accompanying laygear (BMC Part # 22H 1301), a modified distance piece for the layshaft (second motion shaft) (BMC Part #22H 672), and four sets of caged needle-roller bearings (BMC Part # 22H 471) in order to support the laygear instead of the three uncaged sets of needle-roller bearings (BMC Part # 3H 2865) of the earlier design, all of which enable the transmission to absorb heavy abuse better than the earlier smaller-diameter (.643” / 16.3322mm) layshaft (second motion shaft) (BMC Part # 1H 3305) with its lesser three- support bearings. This heavier duty layshaft (second motion shaft) assembly was originally developed and introduced as a competition part by the racing department and subsequently was standardized on the mass production cars in order to meet the homologation rules of racing associations. The need for a stronger, more durable layshaft (second motion shaft) as well as an increase in support from its needle-roller bearings in order to better cope with the greater power of the 1800cc version of the B Series engines compelled this redesign. In addition, the circlip that retained the uncaged needle-roller bearings of the laygear did not allow the fine metal particles created by wear to be flushed out of the laygear assembly, thereby accelerating the rate of wear. In some cases of hard abuse, these components could wear out in as little as 50,000 miles, although with normal use they could be expected to endure for a good 70,000 miles of reliable service. However, when wear became excessive, the loose needle-roller bearings would destroy both the layshaft as well as their bore inside of the laygear, resulting in expensive repair work. When the transmission is in fourth gear, the power goes through the transmission in a straight line. That is, the power goes through the transmission from the input shaft (first motion shaft) directly to the mainshaft (third motion shaft) which is locked directly to it, without transferring through the gear on the layshaft (second motion shaft). Because the gear on the layshaft (second motion shaft) is idling, its bearings are not under a load. If the layshaft is badly worn, the bearings will then make a metallic “fizzing” sound. A set of double-paired caged needle-roller bearings (BMC Part # 22H 471), along with a larger-diameter layshaft (BMC Part # 88G 400) and spacer tube (BMC Part # 22H 672), were seen to be the solution to this problem, and were introduced on engines from 18GB 74,720 (Standard) and from 18GB 74,529 (with Overdrive). This configuration is sometimes referred to as the “four-hole” layshaft (second motion shaft) because of its four holes for feeding lubricating oil to its four sets of caged needle-roller bearings that support its larger-diameter .668” (16.9672mm) layshaft and spacer tube, while its earlier, smaller-diameter .643” (16.332mm) counterpart (BMC Part # 1H 3305) and spacer tube (BMC Part # 11G 3026) is sometimes referred to as the “three-hole” layshaft (second motion shaft) because of its three holes for feeding lubricating oil to its uncaged three sets of support bearings (BMC Part # AAU 3252).

It is possible to modify the earlier three-synchro transmissions (found with the 18GB 74529 and earlier engines) in order to install this stronger, larger-diameter layshaft (second motion shaft) assembly by boring out and line reaming the layshaft (second motion shaft) mounting holes to .6693” +/- .0005”. (17.0002mm +/- .0127mm). The best, and easiest, way to do this is to use an “end bell reamer” that is used for reaming the holes in the end bells of electric motors. This is a piloted, adjustable reamer that is adjustable (over a small range) to give the proper diameter and clearance. To keep things in line, it pilots in one hole while it cuts the other. They are commonly found at shops that rebuild electric motors, and the technicians there should do the job for a nominal fee. You will, of course, need to use the corresponding later laygear in order to fit onto the larger-diameter layshaft (second motion shaft), along with its front and rear thrust washers, caged needle-roller bearings, and its larger-diameter distance piece.
Stephen Strange

Martin:

As Stephen states; but more on laygear types:

There were 3 types of 3 synch laygears (not including close-ration types)

* 22H0054 which uses 3 layshaft bearings (MGA/MGB original)
* 22H0931 which uses 4 layshaft bearings for larger
layshaft.
* 22H1301 which uses 4 layshaft bearings for larger
layshaft. Same as 22H0931 EXCEPT it uses a different distance tube & cir-clip (22H1282/22H1284)
Rich Boris

Rich-
Do you know at what points the laygears were changed?
There were actually three different four-synchro laygears with three different first gear ratios. I've never been able to track down the part numbers for each of them. The ratios were-

1968-1974 3.440:1
1975-1976 3.036:1
1977-1980 3.333:1
Stephen Strange

Stephen

Search thru the following data to determine change effectivity points.

http://blog.earlymgb.com/2012/05/22/resource--factory-microfiche-to-sept-1976.aspx
Rich Boris

Rich-
Thanks. I have the same info in my Bentley manuals. Sadly, although the first gears on the mainshaft for the 1968-1974 and 1975-1976 are listed, only one laygear is listed. This may or may not infer that there was no difference in ratios, but I suspect that the failure to list the part number for the laygear of the 1975-1976 models is a simple omission. Info on the 1977-1980 3.333:1 ratio first gear and its laygear remain as elusive as info on the holy grail. Wht do the parts lists always stop in 1976? The car continued in production into 1980!
Stephen Strange

Stephen:

I am not familiar with latter model MGB info; but; you may want to research 1st gear info regarding MGB; MGC & MG V8 which all used the 4 synchro gearbox.
Rich Boris

Rich-
I've already thought of that. The gear ratios of the gearsets used on the 1968 MGC model without an Overdrive unit are the same as those for the 1968-1974 MGB. However, for the Overdrive unit-equipped 1968 model, as well as for all of the 1969 models, the gear ratios for first, second, and third gear are unique to the MGC-

1st 2.980:1
2nd 2.058:1
3rd 1.382:1
3rd Overdrive 1.133:1
4th 1.000:1
4th Overdrive 0.820:1
Reverse 12.098:1

Of course, The gear ratios of the MGB GT V8 were also different-

MGB GT V8 w/ LH-type Overdrive:
1st 3.138:1
2nd 1.974:1
3rd 1.259:1
3rd Overdrive* 1.032:1
4th 1.0:1
4th Overdrive 0.82:1
Reverse 2.819:1

*Due to the stress limitations of the transmission design, in the Rover V8 application the remote control tower always incorporated an Overdrive inhibitor switch (BMC Part # 13H 2154) in order to preclude the use of the Overdrive in third gear.
Stephen Strange

This thread was discussed between 16/11/2012 and 21/11/2012

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