1996 6CTA mechanical tach drives

I’m doing a massive refit on a 1969 Chris Craft Roamer 46, and part of that project involved repowering to Cummins 6CTAs. I’m trying to keep cool old parts where it makes sense, so I was very pleased to find a set of low (400) hour engines with mechanical tach drives. I sent all of my gauges off to be rebuilt, including the OEM cable-drive tachs, which I had recalibrated from 5000rpm max to 4000. They came back perfect, rechromed, new faces, new glass, and .1 hours on the meters. The original drive cables are in excellent condition. But when I tried to connect the OEM Chris Craft tach cable to the Cummins tach drive, two problems became apparent.

1. Chris Craft’s drive cable ends in a 1/8″ square drive core that’s inset 1/2″ inside the cable. But the Cummins tach drive uses a sort of key drive that requires the core to extend out 1/2″ – 1″ beyond the cable end. But the bigger problem is:

2. On the starboard side, the tach drive works fine, so all I need is some sort of adapter between the square drive core and the key style cable end Cummins used. But on the port engine, the drive doesn’t work because the coupling is missing between the injection pump driveshaft and the the tach drive gearbox. Worse (assuming the part is still available, which I haven’t been able to find online) it appears as if the front cover might have to come off to install the part IF I can find it.

The pictures tell the tale.

Anybody have any thoughts on this?

Thanks in advance.<br />Q