by Mike Benthin
I see the question of why the hydraulic lifters are adjnusted by many on the short side (3/4 turn, i.e.)...so here's my spiel:
A hydraulic lifter is designed to allow ZERO clearance in the valve train (although I believe that the valve "cooling" on the seat is allowed for) even while engine dimensions are changing due to warming up and wearing of parts involved. You gotta know how these things work to understand the mumbo jumbo in suggested clearance adjustments. Zero clearance is achieved by oil pressure under the piston that contacts the pushrod. This means a quieter valve train and less wear due to "slapping" of the parts, as well as a longer engine run before checking the clearance. The piston in the lifter has a certain amount of total travel when no oil pressure is present, and I once checked it with a depth gauge as about 2.7 mm when the spring was removed.
Generally, you want the lifter adjusted so the piston is NOT bottoming out or slapping at the top against the retaining clip at the extremes of engine dimensions. If you adjust it loose, you are guaranteed to slap the retainer when no oil pressure is present. If you adjust it too tight, the valve may never fully seat and cool within its seat when the engine is hot. Usually one wants it in the center of its range to allow for the extremes of engine dimensions and future wear or valves seating deeper. Hence the 2 full turns in adjustment protocol. I believe this protocol also allows for optimum ANGLE of rockerarm contact to the valve stem end.
To allow for the typical valve clearance
diminishing over long engine use, the trend is to adjust the
lifters on the loose side of its range so it will run longer before
the adjustment should be checked/reset. The rationale is that more
harm is done if it's too tight than too loose, but that thinking
comes from the "adjust it every 1500 miles" mentality. Older
lifters that may drain out (even new ones can if the little hole in
the side faces down when you have hot oil) and require 8 miles of
driving to pump up may cause more wear in the valve train if
adjusted too loose? (Also a loss of power until clearance
diminishes). Personally I'd go with the factory setting and check
it every 20K (or when you change the plugs) to catch any unusual
changes beyond the others in any one valve setting. Remember, if
you have one tightening up due to a seat pounding in, you will not
realize it until a compression test or obvious loss of
power/imnbalance reflects it has gone past the adjustment
range.....which will take longer and be much closer to catastrophic
failure if you belong to the "3/4" turn crowd!!!.