|The MGA With An Attitude
INSTALLING AN MGB GEARBOX IN THE MGA - GT-302
At 11:30 PM 1/16/02 -0500, Marvin Deupree wrote:
>"I read the article about installing the MGB flywheel/clutch assembly with considerable interest as I am considering installing an early MGB trans into my '56 MGA as my 'A transmission is very grunchy, especially on fast shifts. And I've got a 'B unit in very good condition just sitting in my garage."
IMHO, you may be in for more pitfalls than it is worth.
>"Have you had any experience with such a transplant?"
Never done it myself, and wouldn't recommend it, but others have done it.
>"Based on your article, the flywheel/clutch piece of the puzzle looks straight-forward enough; that leaves two other pieces to consider; 1) the installation of a 'B back plate onto my 1500 block and; 2) fitting the trans into the chassis. Do you know of any particular pitfalls associated with these two items?"
Yup. You didn't say which MGB gearbox you have (how early). The later 4-synchro unit has even more pitfalls than the earlier 3-synchro one.
>"Will the back plate bolt-in without modification and is my assumption that it will give me a proper rear seal correct?"
No, but not a worry. The MGA and early MGB 3-main bearing engines don't have a rear seal. They have a thin scroll groove machined into the crankshaft just ahead of the flywheel mounting flange. This area of the crank is a very close fit with the engine block and rear main bearing cap, such that the scroll groove will auger the oil forwards back into the engine sump when the engine is running.
You need to use the engine rear plate that is appropriate for the engine/gearbox combination:
3-main engine to early and mid 1500 gearbox with splined output shaft, use early MGA plate with low starter position.
3-main engine to late 1500, 1600, Mk-II or early MGB gearbox, use late MGA or early MGB engine plate.
5-main engine to late 1500, 1600, 1600-Mk-II or early MGB gearbox, use engine plate from MGB 5-main GB engine, 1966-1967. This was a short transition period when the first 5-main bearing engine was used with the 3-synchro gearbox with small bellhousing and small flywheel. The 5-main engine does have a rear seal, which is mounted in the engine plate. Alternatively you could use the MGA 1600 engine rear plate, and bore out the center hole to hold the crankshaft rear seal for the 5-main bearing engine.
5-main engine to 4-synchro gearbox, use engine plate from post '68 MGB.
3-main engine to 4-synchro gearbox, use post-'67 engine plate and machine out the center hole to clear the mounting flange on the crankshaft. Large flywheel does not fit on 3-main engine. Small flywheel does not work with later engine plate. This requires one of two things. Either re-machine late engine plate to use earlier starter located closer to engine block, or re-machine late flywheel to mate with 3-main crankshaft. Custom making such one-off parts is expensive.
In any case you need to use the inertia type starter with the small flywheel and the pre-engaged starter with the large flywheel, as the pinion gear enters the ring gear from the rear on the early models and from the front on the later models.
>"The Trans looks like it should fit without too much effort; I think I'll have to improvise something for the mounts and modify the stay-rod assembly,"
Gearbox rear mounts are substantially different and lower on the MGB. Installing the MGB gearbox in the MGA generally requires cutting and welding at the frame cross member to install a removable cross member with the new mounting configuration.
>"and I'll have to install a new drive shaft front U-joint yoke."
Just use the entire drive shaft appropriate for the gearbox. There are several different driveshafts. Very early 1500 gearbox has a 1 inch 10-spline output shaft. Mid 1500 (most of 1500 production) has a 1-1/16 inch 10-spline output shaft, and the driveshaft is also a different length. Late 1500, all 1600 and Mk-II, and all 1800 gearboxes have a flanged output shaft. MGA had three different propshafts. MGB had 3 different propshafts (including overdrive units). The late MGA and all MGB propshafts are only different in length. Early MGB non-overdrive gearbox uses the shortest of the three MGB propshafts (30 inches). Pretty sure the late MGA double flange propshaft is a different length than any of the MGB units, because neither Moss nor Victoria British have a part number for it. V.B. does have the splined input propshafts for the early and mid 1500, but not the double flanged unit for the later MGA. Not to worry about availability though, as the new ones are too expensive anyway. Just buy a used one, clean and paint it, and install new u-joints.
>"And since I don't want to modify my transmission tunnel which is designed to go with a low-mounted starter I believe I will need to use one of the new replacement starters which engages the flywheel from the front and doesn't project out past (to the rear of) the bell housing."
Bad assumption. The later starter has a larger mounting pilot diameter and different bolt positions, as well as a pinion gear the engages the ring gear from the opposite side. You have to use the early starter with the 3-synchro gearbox (small bellhousing and flywheel), and the later starter with the 4-synchro gearbox (large bellhousing and flywheel).
When installing the later 3-synchro gearbox (high starter position) in the early/mid 1500 you may have to modify the tunnel for clearance for the starter Bendix drive end. Installing a 4-synchro gearbox in the MGA is a royal PITA, as there is nearly zero final clearance between the bellhousing and the tunnel, and the early/mid 1500 definitely requires modification of the tunnel to clear the starter location.
Another thing not yet mentioned is that the shift lever on the MGB gearbox will be about 1-1/2 inches farther forward than the MGA units. The slight modification to the MGA tunnel cover is not particularly difficult, and in fact you may be able to buy an aftermarket part specifically made for that application. The other thing is the need to bend the shift lever back to achieve the original location for the knob, so you're not constantly banging your knuckles on the heater control panel when shifting into 1st or 3rd gear.
>"I've just started to contemplate this project and am beginning the due diligence, so I may have overlooked something or may be simplifying something that is in fact a pain in the neck to execute."
>"Anyway, can you offer any insight into this project?"
I have a few suggestions. Let's start with the non-recommended conversions. First, installing a 4-synchro gearbox in the MGA is such a PITA that it's not worth the effort unless you absolutely insist on having the 1st gear synchro, and the reliability factor of the larger gearbox. (In that case you might also consider installing an MGB 5-main bearing engine along with it). The 3-synchro units are so cheap and easy to repair that you can rebuild two and keep one for a spare for less bother and/or money than installing the 4-synchro unit. Second, installing an overdrive gearbox in the MGA will require cutting and welding of the frame cross member and widening of the tunnel, so unless you're really hooked on the O/D idea, that's also a prohibitively expensive operation, and there is never any economic justification for doing it.
Now for the better news. The MGB non-O/D 3-synchro gearbox is internally nearly identical to the MGA gearbox. All of the parts between the front and center ball bearings are interchangeable with one exception. In early 1967 (at engine number GB74720) the layshaft was changed from .645 to .668 diameter, and has four oil holes to accommodate four needle bearings. The later laygear and shaft can be installed in the earlier gearbox by enlarging the two mounting holes in the aluminum case. Shifting forks, main shift rods and all of the selectors are the same. Only the single remote control shift rod is a different length. Mainshaft (output shaft) and rear flange and ball bearing are the same. Only the speedo drive gear is different.
The front gear cases are interchangeable (BTDT), given due consideration for the later layshaft diameter. This allows for bolt-in conversion to the larger 4-bearing layshaft. used from early 1967 (mid 1967 model year) through end of 1967 production model. The rear housings for the flanged output gearboxes are also interchangeable. What this means is that you can mix and match the housing parts (and input and output shafts as necessary) to convert any one of the 3-synchro gearboxes to any other model. So you have some relatively easy options.
You could install a late MGA rear housing on any MGB 3-synchro gearbox, giving it the proper rear mount to bolt right into the MGA frame, and use the late MGA propshaft. Also use the entire MGA shift mechanism, including the remote shift housing and hand lever. You could install the early MGA gearbox front housing (low starter position) on the early MGB gearbox allowing it to bolt into the MGA 1500 with no modification required to the tunnel.
While thinking about doing these things it should dawn on you that you are effectively converting the later model gearbox into an earlier model. This may require disassembly of two units and reassembly of at least one of them, so you will be doing most of the work required to rebuild a gearbox (which isn't particularly difficult). So as a matter of ease and cost effectiveness, I would recommend that you just rebuild the 1500 gearbox that you have, and maybe use some of the internal parts from the MGB gearbox in the process.