Like all Chris Craft motoryachts in the late 1960s, my Roamer came with cable-drive tachometers. This boat was originally powered with gas 427 Ford engines with a 4,000 rpm WOT, so the tachs went to 5,000. Since I repowered to Cummins 6CTAs (2,600 WOT), I had the tachs recalibrated to 4,000rpm max when I had Kucian Instruments restore all of my original gauges.
When people repower these old boats, it’s typical to swap over to new gauges or even fully electronic touchpad readouts. I had the cable drive tachs restored because I like the classic look. Also, by happy coincidence, the Cummins engines I bought came with tach cable drives running off the injection pumps…or so I thought.
With my plan to splash the boat and relocate it in October 2022, having functioning tachometers is one of many priorities of mine.
The original tachometers, restored to new condition
OEM tach cable connects to the Cummins tach drive
The good news is, the cable fits the drive and it’s long enough. The bad news is that Chris Craft’s cable core has a square-drive end and it’s recessed inside the cable housing. The Cummins drive requires a keyed shaft drive, not square drive, AND it’s also recessed inside the drive. So the cable attaches to the drive, but there’s no way for the drive to turn the cable core.
Tach cable core is recessed 1/2″ inside the housing
I pulled off the starboard tach drive to get a better look at things
There’s a drive hub very loosely attached to the starboard engine injector pump
I could wiggle it around very easily. This doesn’t look good. It looked like I’d have to remove the whole front cover to see why the drive hub was loose.
I checked the port side to see how the hub was attached there
Houston…we have a problem
There was no tach drive hub on the port side. But I could look inside the hole where it should have been and see the injector pump shaft, gear, and the bolt that holds the gear to the shaft.
After staring at the front cover for a while, I decided Cummins couldn’t have been so stupid as to make the injector pump bolt visible but inaccessible without taking off the front cover. Turns out my guess was right. The round thing the tach drive attaches to is a cap that was threaded on to the front cover. I put the two tach drive attachment bolts back in, then used a wrench to twist off the cap.
A 10mm wrench between the two bolts gave me the leverage to remove the front cover cap
Voila!
Cummins tach drive hub
The body is plastic. The shaft is steel.
The part number is/was 3907757
So, the hub just sits on the injector pump bolt. It relies on the drive gear housing to hold the shaft end in place. So being loose when I removed the drive housing was perfectly normal.
I spent three weeks looking online for the drive hub. Needless to say, cable drive tachometers aren’t common anymore, and it appears Cummins may have stopped making these things a while back. Lots of Cummins part sellers have this part number on their online stores, but they were all out of stock.
I finally found a place with the hub in stock: mddistributors.com. Around the same time, I found some very short cable adapters at isspro.com that looked like they could bridge the gap between my OEM cable core and the tach drive output.
It’s always exciting when rare parts show up
Square-to-keyed cable adapters from Isspro
Looks good!
I wanted to test the setup before putting it all back together.
Drill with the trigger taped at a low speed
If I took video, the blooper reel for this test would have been extensive. I finally got the drill to stay in place and stay on long enough for me to run up to the helm and eyeball the tachometer.
Rock solid needle at ~600rpm
I referred to these tachometers as ‘clickers’ before because all of them I’ve ever seen made clicking sounds when operating. These Kucian-restored units are dead quiet.
I greased everything up and put the tach drives back on
Tef-Gel protects the aluminum drive threads from the steel cable nut
Port tach drive cable is hooked up
I routed the cables so there are no tight turns
That’s one long, loopy cable
Port side is done
Starboard tach drive cable is done
It took easily three times as much time to locate the parts as it did to install them. I hope I never have to do that again.
Next up in our 1969 Chris Craft Roamer 46 Refit: Installing Throttle Cables & Accusync Engine Synchronizer
1969chriscraftroamer46 