[Old Fart]

Well the expansion rate of steel is 8 millionths of an inch per degree F. Aluminum is,,, aww heck, why bother. The rockers obviously move to or from the cam location X distance as the head heats. The closers on Testastrettas are a 90 degree thing with a ratio of ?:1. The openers have a ratio of ??:1. If you're bored, for a 998 testastretta exhaust valve the stem is 4 1/16" from the center of the seat to the stem tip. I'm not going to calc the stem length increase with heat in it!

Aah, umm,, on a stretta, .004" works well. I agree that excessive clearance will hammer the valve train and likely be the cause of broken half rings as the valves are bouncing betwen the opener and closer rockers and cam at a high frequency during the excursion. Yet on the single cam engines the closer clearance can be almost nothing which seems most logical to me.
So why do the 4v engines have .004? Is it because the factory can't allow the time to shim them tighter so they shoot for larger values?

Running too tight may inhibit the time needed for an oil film to develop between cam and rocker. Yet again on the sohc engines this closer clearance can be almost nil. Yeah, why can't we run them tighter on both closers and openers.

I never checked clearance on my engine when it's hot. Will the clearances tighten or open up when hot? Dunno but I'll check next time I'm it as it's easy to pop the rear cover on a stretta. If the clearances diminish significantly I'll have my answer as to why .004" is the lower limit.

Perhaps a seasoned guru can weigh in on this and splain why we shouldn't run tighter on these engines.