After 10 years stored in various sheds it wasn't looking too hot so while the barrels are being stripped and 'stove enameled' this time instead of that chipped powdercoat it seemed a good idea to freshen up the heads and polish the rocker covers.
The rear cover just didn't want to come off. So after an hour or so of 'give me a fucking break, this fuckers fighting me all the way', it did at last without me damaging it. And this is why, the valve spring collar is tight against the wall of the rocker box and a groove worn in the inside wall. Not something an engineer would notice I suppose.