The decision not to use cam followers with hydraulic compensation, was taken early on during the design phase. Main reason is that hydraulic lifters in fact only belongs to engines with an overhead camshaft. Having an underlying camshaft you need push rods to command the rocker arms. While the engine heats up, cylinder and cylinder head “grow” and become longer. Also camfollower , push rod and rocker arm “grow” and become longer. If they do not grow all the same length, the valve clearance change and the hydraulic lifter adjust the clearance. There will be no problem as long as the extension of valve train is smaller than the extension of cylinder and head. But when vice versa, you create a situation where the valve is not complete closed when she is supposed to be.
There are several interfaces in the valve train where wear occurs. The normal wear characteristics of the valve train components should be well known by the manufacturer, so their maintenance and TBO schedules will require inspection and or replacement of components subject to wear at safe intervals. However if abnormal wear is happening in an engine with hydraulic compensators, this problem will probably go completely unnoticed until such time that the compensation needed to make up for the wear becomes greater than the compensator can accommodate. Serious damage to valve train and cylinder head components could easily be the result because the problem has not been noticed in the early stages.
Another problem with hydraulic compensators is that they can be the cause of valves not seating properly. If for some reason a valve has not been able to seat correctly once because of some small hard foreign matter sticking to the valve seat or valve lip, the hydraulic compensator will adjust to the valve's new (not completely closed) seating position. So even if the foreign matter causing the improper valve seating is eventually dislodged through vibration, higher gas flow, or other normal factors, the valve will continue to stay partially open! This situation will quickly lead to a burnt or misformed valve. If the hydraulic compensator's internal check valve is partially worn, it may leak some of it's internal pressure when the engine is stopped for a longer period of time. This could allow the valve to reseat itself properly again, but chances are that the valve and seat have suffered some damage and proper seating is no longer possible....
Our basic philosophy is this. Checking valve clearance is a simple and easy task which can be performed by almost anyone with just a couple of simple tools. If tappet clearance is changing significantly and you need to adjust the tappets relatively often, then this is a sure sign of trouble. This is a warning sign you simply may not ignore. Take action before any serious damage occurs and you will probably save yourself a lot of money for a serious repair job that could have been avoided. Don't let the perceived "luxury" of hydraulic compensators hide valve train problems which should not go unnoticed.