The circlip doesn't hold the whole assembly. What happens is that the shaft with the circlip and the shiny part in the photo gets pushed back into the housing. Originally the shaft was knurled and pressed together, over time and with heat and vibration the grip relaxes and the entire generator mechanism can move back in the housing, disengaging from the camshaft.

 

Glyn,
operation dog drive succeeded and de rev. counter works well.

old siutuation

worn out dog drive and antic circlip

new situation with plastic dog drive and circlip

 

Excellent news! Problem solved!

 

WOULD ANYONE PLEASE...consider spinning that rev counter with a cordless drill and putting an oscilloscope on it..i would like to know what kind of signal comes out of it? is it a generator? or simply a rotary switch...in which case it would ground to tach circuit at different frequencies of square waves on off on off rate determines tach needle position on dashboard.
the reason i am asking, is that i would like to make an adapter that runs the tach guage off of the ign. coil. as i am putting a modern 4.2 engine that does not have the tach generator device on the cylinder head... into an older 64 3.8s

 



 

From what we have already discussed I believe the item you are talking about is the signal generator on the back of the inlet cam. The generator spins and creates an electrical current. The faster it goes the greater the current. The current is then passed along the wires to the Rev counter which is basically an amp meter but with a different scale relating to the amount of revolutions the engine turns rather than the amount of current produced. Not sure how the 4.2 produced its readings for the Rev counter. If you still have the older 3.8 you would be a fool to change it over. 3.8 was a better engine, more reliable and goes back in without any modifications. Better to rebuild the 3.8 but if that is not a choice why not use the 3.8 head on the 4.2 block. I think this is possible and then you can retain the generator on the inlet cam. Possibly even look at using the cam covers and the inlet cam from the 3.8. Someone will be able to tell you if this is possible or not but I am only guessing.

 

See other thread. You can't fit a 3.8 head to a 4.2. The 4.2 has siamesed bores & a narrower centre bearing.

The Rev counter generator from a 3.8 puts out a perfect AC wave at 10 volts per 1000RPM. The tach on a 3.8 is a volt meter calibrated in RPM.

You need a newer 3 wire tach (2 white, 1 green ~ excluding lighting) for the 4.2 that takes a pulse from the coil. Most 4.2's can't accommodate the cam operated generator. Your 87 certainly can't. You will lose your clock. Also fitted to 420 & Sovereign.

The instrument is earthed to make the full circuit








Some of these 4.2 rev counters were redlined at 5000RPM & some at 5500RPM.