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A-series Clutch , Gearbox and Related Components - How to make it work right.
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There is a great deal of confusion when switching A-series gearboxes in Morris Minors, Sprites and Midgets, that this tech article will attempt to illustrate why the right combination of components is essential for proper operation.
From the early 60's to mid-70's, there are numerous changes to input shafts, clutches, throw-out bearings, flywheels, clutch forks, front transmission covers, forks, pushrods, clutch slave cylinders, and engine backing plates. There are also differences in master cylinder bores which can effect the clutch performance. Because most of these components readily bolt together without complication, it is common after all these decades, to have a car that previous or current owners to combine the wrong components together. The end result results range from poor performance to complete failure.
Early type gearboxes are commonly referred to as "smooth case", and later improved gearboxes are commonly referred to as "rib case".
Beginning with the rear backing plate, there are two different styles. The early stamped steel plate was used with the 948 and early 1098 (10CG) engines. It is easily identifiable by the curve formed around the entire perimeter of the back plate. The metal thickness is approximately 1/8" These all used the smooth case type gearbox. The later style back plate is cast steel plate, which is completely flat and is a cast steel of uniform thickness (square edge). Because the rib case input shaft is .185" longer than the smooth case, it is possible to have the input shaft bottom out in the opening in the back of the crankshaft. The thicker cast plate should be used anytime a rib case is bolted to any of the engines.
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Clutch Forks |
Bugeye / Smoothcase |
Part # 2A3406 |
Sprite/Midget 1098 thru 1275 10CC thru 12CE |
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1098/1275 clutch forks |
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Both forks are the identical part number.
The top one is bent. |
Front Covers Morris/Sprite/Midget |
The next potential mismatch is the clutch for and front cover of the gearbox. There are several iterations of this cover, both for Morris Minor and Spridgets, and also, in the case of Morris Minors, right and left hand drive cars. Since all Spridgets are hydraulically operated, all variations have the slave cylinder/fork mounted on the right hand side of the gearbox. However, the smooth case fork and front cover have different fulcrum points than the rib case fork and front cover. |
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Morris Minor |
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Morris Minor 10MA (Ribcase fork) |
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Morris Minor 9M (Smoothcase) compared to Spridget 1098/1275 (Ribcase) fork. Overall length is about the same but very different fulcrum point and shape. |
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There are three different clutches and flywheels; 948, 1098CG, 1098CC, and 1275 (all iterations CC, CD CE, CJ, etc.) None are interchangeable without at least some degree of modification. All flywheels are a 4 bolt flange, except the 1275, which is a 6 bolt flange, with two offset alignment pegs. The 948 has a 6.25" clutch disc, the 1098 is 7" and the 1275 is 6.5". Because all these clutch covers are different heights, the matching throwout bearing must be used. The bolt circle of these clutches will obviously also vary, so you cannot bolt the clutch from one to another without re-drilling and resurfacing the flywheel. |
Colin also offers a roller conversion, seen below. |
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Throw out bearing photos by Colin Dodds |
All models used the 7/8" slave cylinder until the introduction of the 1275, at which time a 1" slave with flex hose was introduced. In addition, when disc brakes were introduced, the combination master cylinder bores, (clutch included) were reduced to 3/4". Along with the change in clutch fork, the slave cylinder pushrod was also changed in length. For the 948/1098, the push rod should be 2-3/8" from hole center to end. For the 1275, the push rod should be 2-11/16" from hole center to end. Under no circumstances should these lengths be altered/lengthened to compensate for a poor or non-performing clutch. Doing so can cause several problems such as premature throw out bearing/clutch failure and is one of the major contributors to a bent clutch fork. Once you have determined you have the right combination of mechanical components, you must make sure the all the fulcrum pins and corresponding pivot holes and in good order. Loose, sloppy and oval shaped holes will degrade performance by taking up the essential amount of travel you need to get sufficient release of the clutch. |
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Additional caution note on release bearing construction. |
Photo contributed by Kevin Wayne Williams |
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At some period, certain undetermined manufacturers used a rollpin to secure the carbon beaing the the housing. These seem to fail fairly early, so it may be a good idea to avoid installing these. |
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