By Fergus Ogilvy:
Even a brief assessment of the valve lock will quickly convince us it’s at the heart of the competition valve train.
If engines are exposed to over-revving—provoking valve springs and sometimes valve retainers to float—we are dependent upon the valve lock to remain locked or else…
Pioneered by the original equipment manufacturers and virtually fail-safe, the ubiquitous 7-degree valve lock has been the standard for decades. Its 7-degree outer tapered shape fits into a similar taper on the valve spring retainer. Some engineers refer to its 7-degree angle as a locking taper.
But, the shallower the taper, the harder it is to separate and remove, and drag racers constrained by time, understandably, grew impatient. It was infernally tedious, frustratingly tight and thus inconvenient. In their desire for efficiency, the 10-degree taper emerged.
Though the 10-degree valve lock offers faster dismantling, the wider angle, unfortunately, cannot function reliably for sustained periods of valve float as the retainer can become separated from the valve lock, causing the locks to fall out.
Thus an 8-degree configuration is often the better compromise for the racer. It offers better retention during valve float than its 10-degree counterpart and remains in unity with the retainer under severe conditions. In addition, the 10-degree design will reveal wear on parts during valve float that don’t usually sustain wear.
Identifying the telling signs of over revving
Engine builder Jon Kaase says, “Sometimes if the valve train gets out of control and it’s bouncing around, the valve locks get chafed on their bead locks or scored on their half-round portion beneath the bead lock—that is, the part that sits on the stem of the valve.”
A common sign of over-revving is often observed in the contrasting appearance of the valve locks: the locks on the exhaust valves look like new while the intakes have a shiny appearance. This is probably explained by the fact that the intake valve is slightly heavier and also the intake cam lobe is more aggressive.
Kaase continues: “Sometimes when you remove the valve covers you can see what appears to be really dark oil on top of the retainer, which is a sign that something is awry. Things are moving around and the retainer is accumulating microscopic particles of metal, causing the oil on top of the retainer to be dark looking.”
Ideally the valve locks should have full engagement with the valve stem, the keeper groove, and the angled interface of the retainer—the fuller the engagement, the stronger the joint. Further, it is preferable that no part of the valve locks should protrude beneath the retainer.
Moreover, valve lock joints are strengthened by the presence of a deeper skirt—a greater clamping area beneath the keeper groove. This directs stresses to the valve stem below the keeper groove. Thus the groove and more importantly the area above it are less threatened.
Finally, the three critical dimensions of the valve lock are as follows: the outer angle, the outer diameter, and the inner diameter.