The "modern" crank-dampers supplied by The Usual Suspects are of a smaller diameter than the original 3.8 one I replaced a few months ago, and the beautifully engraved timing marks are not in the same angular alignment either, so I decided to make a new pointer based on a couple of bolts down below the dynamo mounting, which were close to the TDC markings on the damper.
I am told there is an XJ6 one that fits (anyone got a reference ?) but suspect it would still need adjusting in which case this method is perfect for tweaking it towards accuracy.
The dial-guage method of finding TDC is not very repeatable, I find, probably because the piston crown and valve pockets form a slightly irregular shape under the front cylinder's spark-plug axis, and since you?re obliged to use something like an 8" extension on a Mitutoyo to get down there. Even with a decent support (coil mounting studs and a bit of 6mm plate ?) you're never sure what you've found is bang-on, and it's never quite the same twice, when I do it at least.
(hot tip : 2mm bicycle spokes make instant probes, a bit cross-threaded etc but nobody will know and they lock in firmly)
This imprecision, of course, is because the piston is stationary at TDC and the crankpin is moving at 90° to the axis of our dial guage, hence you get the worst possible sensitivity for flywheel degree measurement.
If you break out the porcelain of a standard NGK (at least) plug, you can tap it to take M10 studding ; with an approximate combined length of 100mm measured from the sealing washer, the studding locked with a lock-nut above the plug, and the ensemble tightened down on the front cylinder, you can then reliably and repeatedly stop the engine at roughly 90° BTDC and ATDC - the precision here is irrelevant, since what we're after is symmetry.
(I am assuming a std engine - better check that your big valves, wild overlap, or off-to-cock valve timing dont put your valves anywhere near the space occupied by the studding ! No experience of mofified Jag engines so this rider may either be very wise or wholly inappropriate)
This is the point at which piston speed is the highest, so the precision of angular measurement to linear displacement is at its most advantageous.
If you mark the flywheel as you come up gently clock-wise to the BTDC stop, then rotate back gently and mark it again at what is in fact the ATDC stop, you can then measure the circumference between the two marks and divide the distance. I use a flat steel tape strip that is usually used for making measuring jigs and cut-to-length tables for carpenters, but I bet a dress-makers tape-measure is good enough.
Half-way between the two marks - hey-presto, you have TDC.
I you make your pointer too wide deliberately, you can then file it to suit once you know where TDC is. If it's snug on its mounting bolts, it then goes back in exactly the same place if you've taken it off.
PS No, I don't use a crank tab-washer .... the socket fits so much better, and tab-washers are (IMHO, but based on long experience with conical fittings on i.c. cranks) quite hopeless at containing a loosening bolt or nut anyway ; blue Loctite works just fine as long as all is perfectly dry/clean/degreased beforehand.
