I remember that wee OHV engine quite well. Done more than a few rockers too! It was indeed recommended to adjust the clearances with the engine running and wasn't difficult to do until you came across one with wear on the "toe" of the rocker - where it pushed against the end of the valve - The worn rocker then acted rather like a punch/hammer and munched up the feeler blade! I always did them static after I encountered that problem but it wasn't as simple as doing most OHV engines of that era - BMC "A" & "B"series, Ford Kent, etc, etc - because each rocker had it's own stud which incorporated a hemispherical bearing surface on which it swiveled and incorporated the adjusting nut ( a very "American" way of doing it but even then the "Yank" engines usually had hydraulic lifters so needed no adjusting) so you had to gently wiggle the rocker side to side as you adjusted it so you got the point of maximum clearance. I seem to remember that that engine also had problems with abnormally rapid distributor wear? Typically the car would come in with a very poorly running engine, very poor idle, possibly misfiring, and lacking power. When you put the Sun/Crypton machine on it you would immediately notice the points dwell was all over the place. When you removed the distributor (Delco I think?) the shaft would be found to be slopping about in it's bushing. I think the problem was that the bush was a very short length, compared to the Lucas for instance, and just not up to the job?Yes same here my 1.8 astraGTE had no problems.
Another bad vauxhall to suffer was the viva 1.3, my mates first car, he bought it to me because it "sounded tinny", took the rocker box off and found the pushrods had bored through the pressed steel rockers.
The parts bloke at the dealers just got them off the shelf and said " we sell thousands of them" think I had to set them with engine running or was that my ascona 1.9
Ah, all these "delicious" forgotten memories. Problems which were so simple to solve compared to today's vehicles.