Not meaning to hijack this thread, but if you have a stock engine or even slightly built one, what if any, might be the advantage to having an adjustable valve train?? Pontiac designed them to be torqued not adjusted, if I recall correctly. So, why do some, change things? Doesn't Chevy have the only factory adjustable valve train that GM produced?? Cadillac, Olds, Pontiac and Buick, all are non adjustable. Maybe that is the reason the Chevy V-8 design last so long?? I don't know.
Hydraulic lifters automatically adjust valve lash to zero and elimnate the play in the valve train and will self adjust by allowing for wear and thermal expansion. (Automatically adjusting the vlave lash to zero is not the same as what is termed "zero lash.") Hydraulic lifters are also quieter and don't require valve lash adjustments like a solid lifter. A hydraulic lifter is a piston within a piston. The outer piston is the lifter body; the inner piston is called the plunger. The plunger is retained inside the lifter body by a snap ring at the top of the lifter. There is a compressed spring under the plunger. The plunger is free to travel up and down in the lifter body so that the lifter can adjust itself to the size necessary to eliminate valve lash. When the lifter is on the base-circle of the cam, oil passages are aligned so that the chamber in the lifter under the plunger fills with oil under pressure. The oil pressure fills the oil chamber below the plunger and expands the plunger with enough pressure to eliminate all play in the valvetrain but with insufficient force to open the valves. The lifter, when filled with oil, then behaves like a solid lifter because oil resists compression and the passage by which the oil entered the lifter is soon closed after the lifter begins its upward travel.
The plunger needs to rest somewhere down in its bore and away from its travel limits within the lifter body. This is called "lifter preload" which is typically .020" - .060" from the snap ring which holds all the internal workings in the lifter body and maintained by the oil filling the lifter and pushing up on the plunger - done by the metering hole where the oil enters. But, the lifter also self adjusts by allowing oil to escape and this is called "bleed-down." Oil pressure is in essence removed to maintain a steady pressure under the plunger and eliminate an over-extension of the plunger, lifting the pushrod higher up and making it act as if it was longer in length, and hold the valves open and off their seats. All these variables, expansion of metals, oil viscosity, oil pressures, metering holes, plunger depth, pushrod length, valve spring pressures, rocker arm ratio, are factored into the engines specifications for operation AND RPM range.
Pontiac essentially set its RPM limit at 5200 RPM's, but the engine was certainly capable of higher RPM's. But, as you begin to go past the 5200 RPM range, this is where lifter pump-up can become a problem. AS the RPM's get higher, the lifter does not have enough time to "bleed down" BUT, it does fill up - causing an imbalance between the metered flow of oil in and the metered flow of oil out used in keeping the plunger (and valve lash) automatically adjusted at its zero lash properties. The oil/oil pressure continues to enter the metered hole and push the plunger further up the bore. This forced the pushrod higher up from its intended seating in the lifter's cup making it act like a longer pushrod, which is turn changes rocker arm geometry and creates more lift at the nose of the rocker arm/valve stem and holds the valve slightly open, rather than the valve being fully seated in the head. With the valve slightly open and no longer sealing, the engine may backfire through the exhaust or carb and simply run poorly. The engine will no longer rev nor produce additional power. Once the RPM's have been dropped, the plunger readjusts as the oil bleeds down and it goes back to running smooth again.
The term "zero lash" is a term used in a different sense from the original meaning in that it is an adjustment that allows for a higher RPM without the lifter going into lifter pump-up. What is being done is that the normal lifter pre-load of .020" - .060" is being minimized, pushing the plunger higher up in its travel and essentailly turning the hydraulic lifter into a solid lifter. You are also in effect lengthening the pushrod as it rides higher in the lifter's cup and need to adjust your rocker arm so that the valves seat closed as they should. With the oil filling the valve lifter and pushing the plunger closer to its limit, the problems with bleed-down is eliminated thus eliminating lifter pump-up - but you still have the benefits of a quieter hydraulic lifter that does not need constant adjustment.
In many of the magazines, this is one of the things that is a standard adjustment on any Pontiac engine seeking to maximize performance. This is also why the Bobcat kit came with the nylock rocker arm nuts so that the non-adjustable factory nuts which get torqued down, could be substituted to turn the non-adjustable valve train into an adjustable valve train and get the lifters to their "zero lash" setting and gain additional RPM's. Milt Shornack stated that this proceedure would allow the engine to spin 6,000 RPM's without problem, but should be kept below 5,800 RPM's for the 400CI and 5,700 RPM's for the 389CI. The factory cast iron rods in the later 400CI are a little better than the 389CI and it is really the rods that limit the RPM's of the stock Pontiac engine - among other things all being equal. Milt would "zero lash" the lifters hot & with engine running as described many times here on the forum and once the clicking sound stopped, turn the adjusting nut 1/8 of a turn. This may be extremely close to the limits of "zero lash" while turning the adjusting nut 1/4 of a turn is a safer bet and gives a little more preload on the plunger. If too tight, the plunger can hit the snap ring which holds all the guts inside the lifter, and pop out. Now you have a problem. Pontiac did use a different lifter that had a shorter plunger travel and faster bleed-down rate to prevent lifter pump-up and raise engine RPM's. It was used in their hi-performance engines beginning with the 1956 285HP dual quad engine right through to the Ram Air series of engines. All street engines used hydraulic lifters while those specific factory drag cars, intended for racing use only, used solid lifters.
So, if you are going stock and do not plan on spinning your engine past the engine's redline, then torquing the factory rocker arm nuts is the way to go. If you want more RPM's for performance applications, then an adjustable valve train is what you want.
BUT, as bigD points out, if you have done any work on your engine to the heads, block decking, using different thickness head gaskets, longer valve stems, etc., you want to make sure you have your rocker arm geometry correct and use pushrods that match your needs. An adjustable valve train may be
a way out for not having the correct length pushrods, but you want to ensure that the rocker arm moves evenly throughout its travel over the top of the valve stem otherwise side loading of the valve will wear out valve guides much quicker and you may have other issues as well.