John Mc. I used to do the Jag heads in large engine recond machine shop. Dismantle head, cut seats, face valves, adjust clearances, deck, assemble would take two full days. all exhausts would be fitted and clearances adjusted by tipping valve stems one at a time, then on to inlets, this kept cam loaded in bearings by springs. Both cams would then be fitted timed to no1 which on a jag is rear cylinder[ why only the poms would know]. Owner or mechanic? would then turn camshafts attempting to time to no1Δ] you can guess the rest. Remove and replace heads and timing chains would go another 30 hours, who needs that sort of torture. I had to get that in.