BTW: regarding anti-rotation devices. Under normal conditions, the roller wheel self-aligns to the tappet lobe like wheel caster: the roller cannot clock more than a few thousands without rising in the bore. When it rises to height "X" (whatever, varies with base circle), the corner of the wheel catches the rising lobe and ploughs across it until they both die. There are 2 major reasons why this is not safe, viz. the tappet will rise and clock. The obvious one is excessive lash - a maintenance problem (and may be with hydraulics if the local oil pressure isn't always high enough to keep the plunger inflated to full height) The other one is not enough spring tension, where the tappet is momentarily separated from the lobe, and may not come back where it should. I'd be interested to see if a solid roller with tight lash but no anti-rotation devices could run a few races without self-destruction...
