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MG MGA - gear box for Barney ?

The 3 syncro box is on the bench again ! to sort the stiff shifting in all gears and a very nochy 1-2. I had missed a badly worn clutch pivot, the inside of the carbon thrust was fouling the input shaft when operating. Causing the stiff gear lever shifts ? ? ?

The 1st. and 2nd hub has a lot of fore and aft movement where it is splined onto the output shaft. That is, it moves back and foreward when selecting 1st or 2nd. Being located at the rear by the the bearing and at the nose by the lock ring thrust, it looks like there should be another thrust washer at the back to stop this end float. The book does not show any additional washers. Could there be a miss match of parts somewhere, ie., a shorter hub or the inner ring of the rear bearing not wide enough ? The bearings and brass syncro and lay shaft were replaced last year. Also the clutch and a badly worn carbon thrust The box had previously been fitted with the later front cover ( with the seal ) The shifts were free before my repair. After wards the shifts were very stiff for a while and the 2nd syncro never worked properly ( very nochy ) Barney your input will be greatly appreciated ..... Thanks Sean
S Sherry

Sean -
The synchro hub is supposed to move on the splines, something around 1/8". This is the movement that activates the baulk ring as the hub slides forward pushing the ring against the taper on the 2nd gear. The 3/4 hub has a similar movement.

If the release bearing was contacting the 1st mo shaft, it might act as a brake, fighting the synchros. Or the bad pivot might mean the clutch was not releasing fully. Either would cause tough shifting.

And, are you quite certain that the front cover with seal is the correct cover, ie an MGA coil spring clutch cover and fork, as opposed to an MGB diaphragm cover and fork?

Last, are you sure that you have the correct release bearing? There are numerous bearings with differing offsets, which could also cause shaft contact and release trouble.

FRM
Fletcher R Millmore

Thanks FRM, I had just worked out why the hub needs clearance, to stop the ring from dragging ! There is a big problem with the clutch release arm. The pivot lugs on the front cover are badly egg shaped. I think the wrong bolt was fitted so that the bolt was not locked, the arm was swiveling on the housing holes instead of the bush. Also, I was a bit cavalier with the origonal job , just fitting a new syncro and bearings. I find the chanels where the three balls run on the inner huds are some what clogged and probably the springs are none to free in their bores. Self inflicked wounds ! !.
Is it advisable to lightly grind in the 2nd. syncro to the hub cone with very fine paste ? ? Thanks Sean
S Sherry

The 1-2 sliding hub internal part is supposed to move slightly fore/aft on the main shaft splines. I did have that figured out once, just can't recall why at the moment. Perhaps it's because it just doesn't matter, and it avoids certain issues of assembly tolerance build up.

The split bronze bushing on the remote selector shaft front lever is all important. Most current replacement parts are the MGB nylon type which tend to break after several years of use (temperature cycling and plastic aging problem). If this piece is missing or broken the shifting can be very sloppy and might occasionally lock in gear due to over travel of that front lever. If you need one, the split bronze bushing is available. http://mgaguru.com/mgtech/gearbox/gt120.htm

High resistance to shifting motion (aside from clutch not releasing) will usually be found in the sliding shift rods. Be VERY careful to assure that the shift fork set screws have the tapered nose sitting directly in the drill hole of the shift rod for perfect location. If the fork is slightly out of position on the rod it will interfere with the interlock detents. That can make it difficult to shift out of neutral into any gear.
Barney Gaylord

Sean & Barney-
To expand on my above remarks:
The hub must have free clearance when all parts are new, ie, at max material condition; but, there must also be enough travel to press the ring against the gear when things are quite worn, or at the end of life. That's why there is so much motion allowed. The hub is held in the correct unloaded position by the shift forks, so all Barney's comments apply. When fully assembled and in neutral, both 1/2 and 3/4 hubs should be free to move easily within the limits of the fork grooves - any preload here means that you are coming up against the interlock or other detents.

" I find the chanels where the three balls run on the inner huds are some what clogged and probably the springs are none to free in their bores. Self inflicked wounds !
Yes, dirty is evil. Also check all the detent spring holes for crud. These means dismantling the hubs and the interlock block; mark everything and beware the flying balls! I think Barney covers this.

Further to Barney's comment re the shift rod locating bolts, it is common that there is a raised burr around the hole, which makes it difficult to easily locate the fork correctly. Use a very fine file and a light touch to remove these burrs so that the forks are free on the rods.

"Is it advisable to lightly grind in the 2nd. syncro to the hub cone with very fine paste ?"/
Absolutely NO! The grinding paste will embed irretrievably in the bronze ring and become a lap, which will then eat the steel gear over time. If you have the steel ring and matching 2nd gear, the same might or might not apply, but it would be an unnecessary effort & risk. If the gear face is very highly polished and scorched/glazed looking, you could use some 400 grit abrasive paper with oil in a lathe to deglaze it. This taper face will usually be high polished and can be left that way.

Do you still have our correspondence from the last discussion? I might have copies.
And, you might want to read the MGB Tech thread re synchro rings, CR Huff currently on the board.

"...a big problem with the clutch release arm..."
Yes, common, and the sides of the slot for the arm will be worn badly too. I love it when I find one with a fully threaded capscrew in there!
Correct fix as follows, if you don't have the tools you need to approximate this as well as you can (took me about as long to write it as it takes to do it!)

I set the cover up on a mill, located as best I can on the worn hole centers, usually not too difficult as the holes wear mostly in one direction. Bore or ream the holes to minimum oversize for truth and to allow a thin wall brass bush to be pressed in with Loctite. Say the holes are now .562". Flycut the inner faces of the ears, again to minimum necessary, but enough for the bush to have flanges on the inside, say about .030 per side. Press in a new Oilite bush in the arm and file the arm flat and parallel. Measure the arm thickness and the slot width and make two "tophat" bushes with suitable flange thickness to allow about .004 side clearance for the arm; make the bush OD the same as the ear hole size - .562 in the example - you want a very light press fit, leave the holes in the bushes a few thou under the shoulder diameter of the new pivot bolt. Press the bushes in from inside the slot with Loctite, dress off any protruding end of bush, ream the holes in line to closest possible fit on the bolt shoulder .499 or .500. The ideal case is that the bolt is tight in the ears, but you must not crush the ears together and bind the arm. I assemble it and ensure the arm has .004-.005 side clearance, and measure the protrusion or otherwise of the bolt shoulder. I then machine under the bolt head or the end of the shoulder as required to be .001-.002 below the face of the ear, so that when assembled with the correct heavy washer and nut the bolt is tight and the arm side clearance is closed up to .002 - .003. This gives a snug fitting bolt and minimum arm slop and should last forever.

FRM
Fletcher R Millmore

For Barney and Fletcher. What can I say for your detailed suggestions, except that your are both now on my Christmas Card list Many thanks Sean
S Sherry

This thread was discussed between 22/07/2010 and 23/07/2010

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