Do You have a leakdown tester or compression tester w/a detachable hose that has a std. Milton or I/M quick disconnect? If so, take the light-springed schrader valve out, screw it in & plug the air hose on.It wont matter w/the rocker shafts off, if You put 100psi in there, there's 300+ pounds of force holding the intake closed..200# on the exhaust, piston up down, it won't matter. If You hook the stem hard enough to unseat one, You'll be dressing it for a good gouge/nick, that's for sure. Carefully stuff some paper towels to block keepers from escaping into the valley, then You can concentrate on the task, yeah it's not too late to get/build a better means of compressing the retainers...but it's frustrating to be there then stop to do so.....oh, a steady-handed helper is a plus for sure.