If your engine doesn't lend itself to driving a tach from the camshaft, oil pump, or distributor, then this quality 2:1 reduction gearbox may be the answer. It drives your cable directly from the crankshaft.
Just drill a 9/32" hole in the end of the crankshaft or in the head of the front crankshaft bolt* and install the (included) hardened drive bushing. A short flex cable with square ends (also included) transmits the rotation to the gearbox while allowing a reasonable amount of misalignment.
This right-angle drive is designed to be mounted in front of the crankshaft. You will need to fabricate a mounting bracket to hold the gearbox. Our Mounting Clamp (Part No. 3298-1.50) is ideal for attaching the adapter to your bracket. Housing measures just over 1.5" long (measured parallel to the crankshaft).
The output is 7/8-18 thread with .187" tang drive (accepts our cable Part No. 1385-Size for Jones tachs). The 2:1 reduction will drive the cable at one-half crankshaft speed, so it should be used with a 2:1 ratio tachometer.
* This hole size gives a proper press fit in cast iron or steel parts. Pressing the bushing into harder materials is not recommended. If you are using a hardened bolt, you will need to broach a 0.188" square hole in the bolt head.