To be honest, its worth dropping the engine out and whipping the heads off...gives you the chance to do it properly, have a good rummage around, check everything, clean everything and pop it back...its a weekends work if you can spare it its a worthwhile exercise......
+1 I did my shims after an engine refurb this January, heads were rebuilt on the dining room table :-D Now knowing all the fiddly bits I would attempt the next check in the frame, but it is Tricky...
