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MG MGA - MGB back to MGA trans swap?

I've had our '59 MGA for a couple of years. I discussed the 'architecture' of the drive train with the PO when I bought it. He described it as having a '67 MGB 5 main engine, and the ('67?) B transmission. It has no overdrive. I did not know enough at the time to ask about the clutch and flywheel. From my reading the spline count is different from B to A.

Why am I asking now? Well...From the time that we bought it until now, the 2nd gear shifting has gotten progressively 'touchier', and is difficult to get into second without a 'crunch', at anything above a slow roll. Reverse gear has started popping out just this summer.

The good news is that I brought home the original MGA tranny when we bought the car. My intention is to do one of two things...I want to have the car off the road the minimum amount of time possible at the end of this season (fair weather seasons are far too short in this part of the country!). One choice would be to practice disassembling and reassembling the tranny (MGA) that's currently on my bench, and then pull the engine/trans (MGB)out of the car at the end of the year and do a proper overhaul on the MGB tranny. I expect this to be shorter & perhaps less expensive than overhauling the bench tranny (MGA) and trying to fit it up to the current drivetrain at the end of the year.

If I tried to fit up the MGA tranny, from what I've read, I would also need to change out the clutch. Although I am not 100% sure of which flywheel is currently in the car, it seems like I can leave it alone. What about pilot bushings? These are probably different too, right?

I have some time to plan this out. What are your thoughts on advantages of one versus the other? I know I am bound to get the 'What you ought to do is replace the 4-speed with a 5-speed upgrade' suggestions, but while is is on my future wish list, I have no money to do that until I get two teenage boys through college.

Thanks, in advance, for your (cost conscious) suggestions.

rpb bunch

This should tell most of what you need:
http://mgaguru.com/mgtech/power/pp201.htm
Barney Gaylord

Like Barney says, you have to use the engine plate that matches the gearbox you intend fitting.
I hope you still have one around.

I have the 5 speed ford gearbox to my car and it is probably the ideal set up.
But if I were you I would fit the 4 speed as you have planned and then change the diff to a 3.9 when your budget allows.

The MGB engine is so torquey that it will easily pull the higher ratio final drive and you will probably find that you will only ever use 2nd 3rd and 4th gear once you are moving.

I thimk you will like it even better than before.

Colyn
Colyn Firth

Thanks Barney & Colyn for your answers. I have a clarification question...

My MGA trans is a high starter position. The MGB that is currently installed is a high starter (standard position). Will they swap directly without needing to change the engine plate?

Colyn...Are you running the 3.9 diff with the 5-speed setup?
rpb bunch

No rpb, I have the standard 4.3 diff in at present with my 5 speed.
I think that a 3.9 with my 5 speed would be just a bit too high, my gut feeling is that the 4.1 would be perfect.
The 4.1 diffs are really rare now so its not so easy to check this out.
Colyn Firth

I don't think so. There is a subtle difference in position of at least one bolt just above the starter. See here: http://mgaguru.com/mgtech/engine/cs200.htm
Barney Gaylord

ok...thanks Barney and Colyn...

this pretty much seals it for me. I'll need to rebuild the MGB tranny. I do not have a high starter MGA engine plate.

Cheers!

Rick
rpb bunch

...and just because I'm curious....I see the dimensional difference called out in your engine plate link, Barney. Anyone ever mess with adding an additional hole in the '67 engine plate to align with the offset mounting hole, and plug the unused hole?

It may very well not be practical...but it would help my overall understanding of options.

I was thinking along the lines of making a template for the 'A' transmission, and using the template to center punch and drill & tap the additional hole.

Cheers,

Rick



rpb bunch

It may be the starter motor is in slightly different location. The starter has to lineup with the location in the bell housing.
Barney Gaylord

yep...I see what you're observing. Once I pull it all apart, I will sort it out and report back. Thanks for all of the references from your site. This is an amazing resource. If I get real industrious documenting this project, I will draft an overlay of one backplate on another to clarify the geometry differences.

Cheers,

Rick
rpb bunch

This thread was discussed between 26/06/2011 and 30/06/2011

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