As Bill said, a very likely cause of a sticking or jerking needle is dirt or wear in the movement. Apart from cleaning and lubricating (treat it as an expensive clock), there should be just enough clearance for free movement. As the movement is not the easiest to access or work on, I’d check the wire from the generator at the end of the camshaft to the instrument first and test for 10 Volts/1000rpm.

If it’s the generator, I’d do anything to repair it rather than cough up the price of a replacement. Mechanical repairs are pretty well always possible, though they may require significant ingenuity. It should also be possible to recover tired magnets or re-calibrate the meter to allow for them.

What to do if you swap engine or cylinder head is an interesting one, especially for me as I have a series 3 XJ head sitting in my workshop. I’ve not yet looked at the head to investigate modifications to fit a generator on the end of the camshaft. Advice on the internet seems to vary as to difficulty involved. For sure electric modifs are cheaper and more easily corrected if they go wrong. I’m inclined to try and adapt a Smith’s RCV ignition voltage sensing device to mate with face and needle of the standard Mk2 instrument. I think it should be possible to re-calibrate the instrument and keep the traditional clock in place. If necessary, it’s not difficult to replace the electronic circuit.

There’s a lot of excellent info on the Smith’s tachometers in
https://www.triumphclub.co.nz/wp-con...hometers-1.pdf

http://www.rastley.com/tach-1410.pdf

and

https://www.tccv.net/hints/SmithsTac...Conversion.pdf

It’s worth noting, for cost and info, that Smith’s instruments went into many makes other than Jaguar.

 

Thank you Peter. I had only previously read the New Zealand doc.

Out of interest. My car had 90,000 miles on the clock when restoration started thus knowing the car's history to some extent the tacho had done that mileage & is fine & smooth in operation after that mileage & nothing but semi stripping, cleaning & lubricating, respraying the bezel, fitting silicone glass anti rattle pipe seal that I had extruded and reassembly. Rest on my thread below. Lubricated with an ISO 46 Turbine Oil, but this is not a critical application. Used because typical life in service is up to 30 or 40 years in power stations & I had to hand.

Peter ~ just remember you can't fit a 4.2 head to a 3.4 or 3.8 block as previously covered. (wrong ~ see below)

 

Glyn, I'm aware (I think) of the difficulties of the putting the later head on the older block. I discussed it all with Ken Jenkins and the problems are all fairly surmountable. In fact, it may well be less complicated than attaching a tacho generator to the end of the camshaft. Certainly, it has been done more often.
Peter

 

The bore centres are different on a 4.2 block with siamesed bores/sleeves & a cut down in width of the centre main bearing. How do they achieve a fit with the respaced and increased size bores & new crank? I'm interested. I know the block is the same length & stroke unchanged. Ken Jenkins has a fine reputation on all things Jaguar.

Always willing to learn but don't understand how the 4.2 combustion chambers would line up with bores/pistons of the 3.4/3.8 and head gasket requirement.

 

Glyn, If I've understood your question correctly, the answer is that the 4.2 combustion chambers line up exactly with 3.4 and 3.8 cylinder bores. Jaguar never made them line up with the 4.2 block. With the wider 4.2 bore, there's a bit of squish area and the misalignment of the inner and outer cylinders doesn't seem to have significant consequence for combustion. I've not looked into the gasket, but expect a 3.4 to be OK for the 3.4 block. Unfortunately, I've a lot of bodywork to deal with before these problems become more than academic.

 

Ah! Yes you have understood me. So they left the head/combustion chambers misaligned with the 4.2 bores. Well that explains that. How strange.

Yes they moved the centre cylinders closer together & 1 & 6 moved further out nearer to the block ends on the 4.2.

Thank you!