Occasionally (fairly often actually) someone may have in their MGA (or in their possession) a three main bearing engine where the tachometer drive mount has not been machined in the block. MGA and early MGB engines were machined to accept the tach drive. Engines or blocks without this machining may come from lots of other places, as very few other applications used a mechanical tachometer. Five main bearing engines are not candidates for this feature, as those blocks do not include the casting boss required for this attachment.

Click for larger image.

Click for larger image.

Alignment setup for machining is to fixture the block with 30 degree tilt to the right, then tilt the boring head 9 degrees forward. Or, fixture the block with 30 degree tilt to the right, then elevate the front end of the block to 9 degree angle, then bore the holes straight down.

For a skilled machinist there is more than one way to skin a cat. For instance, you can set the engine block down flat and vertical and orthagonal (straight across) on the machine table. Then rotate the table 17.84 deg, and tilt the boring head 31.24 deg, in which case it gives the same machining axis angle without tilting the block. If there is one more mechanical axis on the boring machine, you can leave the block orthagonal and rotate the machine head 17.84 deg to make the compound angle on the boring head axis.

The one dimension I am still missing is the gear center distance (distance perpendicular to the shaft center lines). So far I don't have facilities at my disposal to measure this directly. Any machine shop that can machine a part this size should be able to make this measurement by machine setup. The measurement may be taken from another block with original tach drive in place, or by fixturing the gears (cam and driven spindle) at the appropriate angles and setting the spacing for good running mesh. The first time someone may do this, I would appreciate it if they would retain that dimension number and send it to me to add to this drawing.