Group 4 Rev Counter

[Guy Mayers]

So, this has been going on for a while now, the new dash has been installed for a year or so but the rev counter has never worked. It's an original Group 4 mechanical one that I was lucky enough to be able to buy from Gerry a few years ago and it resulted in me spending a small fortune building up the rest of the gauges and converting the car to left hand drive.

The issue is how to drive the gauge. I looked at an electrical drive that Vince used for his speedo but it wouldn't react fast enough for a rev counter so i was back to finding a mechanical route. The simplest way was to drop a Dino in but the budget wouldn't stitch to that so I had to work out where to pick this up from.

First step was confirming that the gauge needs to run at half crankshaft speed. So I needed to sort out a method of driving a cable from a camshaft. And that's where it started to get complicated...

Guy

[Guy Mayers]

So, having ruled out a direct attachment to either of the camshafts or the cam wheels and zeroed in on the oil pump drive. You can see the driven gear in this pic and, as the 12v is the same base motor as used in the Alfa 75 and GTV6 this is where their distributors are and distributors turn at half crank so we've got a winner. If I could work out how to attach the cable to something...

Guy

[Guy Mayers]

So I acquired an Alfa 75 distributor and driven gear with the thought that I could swap the driven gears over, use the offset key and the shaft and base of the dizzy with a square drive machined into it and attach a threaded boss to it for the cable.

And when it arrived and I had the two gears on the bench it quickly became apparent that the two gears had different pitches so there was no way I was going to be able to use that without changing the driving gear too.

Bugger.

Guy

[Guy Mayers]

The next fly in the ointment was the discovery that the 164 oil pump shaft doesn't rotate at half crank speed. For some reason Alfa changed it for the transverse installation to 70% of crank....

So that meant that I had to get a gearbox made up to drop the revs to what the rev counter needs to see to read correctly.

Fortunately this also solved another smaller problem, not having to turn the cable through a tight curve.

Some progress there then but the next one took some thought...

Guy

[Guy Mayers]

So, I needed to get a spare driven gear and extend it up through the top plate. And you might have also noted the nut on the top of the driven gear? A bit of research at AHM turned up a replacement top plate with an integral bearing and a hold down shaft for the Alfa 75 for owners who drop the dizzy and replace it with electronics. So I bought one of those kits and it looked promising until I realised that the shaft was turning the wrong way and would unscrew under load from the cable.

So, after a lot of thought and discussions with an engineer willing to take this on we ended up with set up below. A top hat (silver soldered to the driven gear). The bottom of the extension has to be hollow to take the threaded section on the top of the pump shaft and the top part was machined for an interference fit for a plastic shaft recovered from the spare gearbox with the square section to drive the cable. This plastic shaft was turned down to length so that it was the same as the Alfa 75 dizzy base plate to driven gear.

So that's what was done.

Guy

[Guy Mayers]

Finally the AHM cover plate needed to be drilled through for the shaft and a new threaded boss for the cable attached.

Job done.

Guy

[Guy Mayers]

And with all the components sorted out this is what the assembly looks like on the bench :o.

[Guy Mayers]

Time to install the assembly then. Well, you can see it went in okay but it took a while to get in.... The retaining nut and washer came off very easily but the driven gear wouldn't pull out with a magnet. Or a pair of pliers. Or welding rod slid under the gear and pulled up.

So, time for a bit of thought and a coffee. Key in the ignition and cranked it over a couple of times and the gear rises up the shaft easily and could be pulled out.

Dropped the new assembly in but the top plate wouldn't settle down fully. Slight panic - was the shaft too long? A dry assembly without the "O" ring showed that it was okay and that's how it has to stay for the time being. Unfortunately the engineering company removed the AHM one when machine the the top cover and misplaced it so I need to get a new one. I attempted to use one from the old top plate but it was too wide and had to be cut down. Even then it was too thick or the groove in the cover plate too shallow as it just wouldn't fit into the engine with the seal in place.

So now I'm waiting for a new one from AHM now... Hope Adie stocks them!

Guy

[Guy Mayers]

Okay, so I didn't want to run the motor without the missing "O" ring just in case....

But I changed my mind and fired it up and the rev counter worked!!

RESULT!!!

[john]

A nice report Guy. Well done with the dedication!
A few years back I bought an "o" ring selection box from Machine Mart. Don't know if they still do them?
Anyway, if you can get hold of one there is a fantastic selection of diameters and thicknesses in it and I've never been short of a ring since.

[Rotor1]

Brilliant work there Guy! Just one part of the puzzle that I didn't quite understand; how did you solve the issue with the shaft turning the wrong way and thus unscrewing the nut, or was that solved by eliminating that arrangement all together and pressing the machined down square drive into the top hat?

Henry

[Guy Mayers]

The nut has gone, the function of the nut and washer was to stop the driven gear coming off the shaft when the engine stops, that's now done by the entire assembly pressing down on it. Should anything fail in service it ought to show up quickly by the rev counter stopping and it's just a case of tracking the driveline back to where it's stopped. The weak point ought to be the plastic cable drive.
Guy

[Rotor1]

Understood now Guy.

Henry

[Guy Mayers]

Great Henry! My only (slight) concern is a failure of the plastic part that could free a wear washer that sits between it and the bearing in the cover plate that might let it drop into the gears below. I think I'll be monitoring that for a while.

I do have one issue left to resolve - the original cover plate has a tube connected to a rubber pipe that goes to the bottom of a black canister on the back of the plenum and, assuming this is a scavenge system that vented onto the oil pump drive I need to reinstate that somewhere.
However, given that the oil pump has it's own supply I'm sure I can just feed that into the cam cover.
Unless the pipe is a vent for this area of the engine then I may need to drill through the block for it?

Can anyone enlighten me on the function of this pipe?

Guy