If you install a 1965 or newer MGB engine in your MGA, keep in mind that the five main bearing engines have no mechanical tach drive. The most common solution to this is to install an electric tachometer from the 1965-1967 MGB. This is the time frame when the first 5-main engine (the GB engine) was used with the three synchronizer gearbox, generator, and positive earth. Look for Smiths tachometer model RVI2401/001. The face appearance of this electric tach is similar to the MGA mechanical tach, except it may say "Smiths" rather than "Jaeger".

If your MGA is still wired for positive earth this MGB tach can be used as original. Required wires are:
    G - power from switched fuse
    B - ground
    RW - lamp wire from rheostat
    W - from ignition switch
    W - to ignition coil
Note that the direction of the white wire loop on the back of the instrument is important as it pertains to the electrical polarity.

If you change your MGA electrical system to negative earth you would need to change the polarity of this tach to match, which involves reversing a white wire loop on the back and reversing power and earth wires inside.

Note also that there is also a fairly rare and different tach used for the MGB in this same time frame. That one has different internal construction which requires a certain finesse for polarity conversion. For those notes and pictures refer to Chicagoland MG Club Driveline, May 2005 issue.

If you might be considering using some other MGB electric tach, observe the required wiring details. My Haynes service manual for MGB shows inductive loop on all electric tachs 1964-1971 (and maybe some late 71's titled as 72's). These have:
    G - power from switched fuse
    B - ground
    RW - lamp wire from rheostat
    W - from ignition switch
    W - to ignition coil

The 64-67 models were originally positive earth and would need conversion for negative earth. Those cars had the tach-to-coil White wire connected to the - side of the coil when the + side went to the contact points. The 68-71 models can be used as original for negative earth, but these are the smaller instruments. Those cars had the tach-to-coil White wire connected to the + side of the coil when the - side went to the contact points.

1972 was a transitional model year where the early ones used inductive loop and the later ones didn't.

Late 1972 through 1976 models had small instruments with negative earth, one less wire, no inductive loop. These have:
    G - power from switched fuse
    B - ground
    RW - lamp wire from rheostat
    WB - from negative side of ignition coil (same as points wire on distributor).

The 1977-1980 models had larger instruments with same wiring as the 72-76 models.