Q: My 66 'B' has an inoperable tachometer, it doesn't register anything. I read that an MG converted from 2-6 volt batteries to 1-12 volt battery will affect the tachometer to where it doesn't work, which is my situation. How do I get it to work? Thanks for your help.
A: The switch to a single 12 volt battery should not affect any tachometer, but regardless, the '66 MGB should still have a cable driven tachometer and not the electronic one. I say should as changes may have been made over the years. Determine this before you start diagnosing anything, though from what you've said, I believe you have the mechanical unit.
That you seemed to think it was an electrical problem, if it is the mechanical type, you obviously have not lubed the cable at any time, so that the cable itself broke and would be the most likely suspect and the first thing I'd check.
Checking the cable is relatively easy. The drive and cable connection are at the back of the engine by the flywheel. This may be difficult to access, as it is behind the heat shield and intake/exhaust manifolds. If you can get to it, unscrew the end and see if the tip is broken off or the cable can be removed from the housing. The lug that holds the cable into the casing is on the tachometer end, so if it pulls out from this end, the cable is probably broken. If the cable is not obviously broken at this end, check the drive. If you can see or somehow feel the end of the drive with the carbs and manifolds in place, turn the engine over and see if the drive turns. If it doesn't, you've got a bigger job than just replacing the cable.
If you can't get to the drive end, remove the tachometer from the dash. To change the cable, you ultimately would have to anyway. It takes a bit of contortions from below to do this, but it is easier with it out to check and replace (hint: remove the heater vent control so you don't cut yourself to ribbons on the raw metal at it's back) and unscrew the cable connection. See if the tip is broken off. Check if the tachometer itself is seized or binding. Either would tell you if the issue is at this end. If all good, then try to turn the cable hand. If I does, it's probably broken. If not, try running the engine and see if it turns. If pushed all the way down and not turning, there is a problem is at the other end. See above to check, even if it now means possibly removing the carbs and heat shield to get at the drive.
The cable is the relatively easy one to replace and is still available (Victoria British part no. 6-734, about $19 or Moss part no. 331-200, about $23). There seems to be only one size of these, so no measuring needed. Tough part there is that it is on the block below the intake and exhaust manifold, so may be difficult to access the drive end.
Most of the drive parts are available as well from either Moss or VB, though neither has the drive pinion (steel part) or seal (an o-ring, though this should be easy to replicate from any parts store kit), but they do have the drive gear (the breakable part), housing, etc.. The bad news is the drive gear is on the end of the camshaft, back of the engine and not at all readily accessible.
If the tachometer itself is seized and broke the cable, it can be rebuilt. New units are not available from the usual sources, but they can be repaired or found from aftermarket rebuilders or used part sellers (like from British Miles in PA or on eBay).
The alternative would be to go with a conversion to an electronic tachometer, either an OE MGB unit or an aftermarket piece. See above about sources, as they don't look to be available new either. The tachometer is usually wired in series with the primary lead to the distributor, so if the connection is shorted or broken, the engine won't start. If the tachometer is not working, this usually happens, so it's one of the clues that you have the mechanical type, as your engine still runs. Also make sure you get the correct ground (earth) tachometer if you buy a used electric one.
Hope that helps you. Keep me posted on what you find.