Valve Stem Protrusion; Knucks, Pans, and Shovels

Harleys are very rebuildable, and I would go so far as to venture that they may be among the most commonly rebuilt (using the term "rebuilt"somewhat loosely) of any engine family in existence. Such a supposition is somewhat bold, given the minuscule number of Harleys compared to the vast oceans of, say, small block Chevys. But face it, which engine is more likely to wind up in a scrap yard when it is in need of major repair?

Given that, along with the often less than spectacular life span of a top end rebuild on Knuckles, Pans, Shovels and Sportsters, many if not most have seen multiple valve jobs over the decades. Naturally with each valve job performed, the valves will seat a little deeper in the head. The method of gauging how much deeper is via the valve stem protrusion specification. Valve stem protrusion is one of those specs that is sometimes overlooked and to some extent misunderstood when dealing with Harley heads.

At issue are a several things. In no particular order; valve spring installed height, shrouding of the valve in the chamber, compression ratio, and finally rocker arm geometry. Having less than the minimum can lead to the devastating result of your valve springs reaching coil bind while your cam is still trying to lift the valves higher. Not a good situation and can usually be summed up as 'broken parts."  On the opposite end of the spectrum is the Panhead that looks as though the pan covers have been bashed out with a ball peen hammer (because they have indeed been bashed out with a ball peen hammer) so that the valve spring collars would not hit them.

Shrouding of the valve in the chamber from the valve being too deep is fairly easily remedied by a judicious modification of the chamber during the process of a valve job, though this too can overdone resulting in issues down the road when new seats are installed.  Along with deep valve seats comes a reduction in compression ratio (aggravated via de-shrouding) by making the chamber larger.  That may or may not be an issue depending on a number of factors.

Valve train geometry is also at issue, but I will attempt to address that later in the post.

To examine this subject I would like to start in the middle and work our way forward in time before going back to the beginning - that beginning being the Knucklehead.

On page 75 of the Harley Davidson Panhead Service Manual - 1948-1957 Rigid, we find what seems to be first official mention of the specification (at least that I can find).

The spec, which the drawing refers to as "Valve Seat Tolerance" is pretty self explanatory. It is the distance from the tip of the valve stem to top surface of the collar of the valve guide. The illustration also shows a gauge which was available for those lacking precise measuring tools or for quick checks. The gauge is simply a cylinder that straddles the guide. The "step" at the top of the gauge indicates minimum and maximum height; if the tip of the stem falls between the top and bottom of the notch, the stem protrusion is within spec.

The 1978-1/2 to 1984 FL/FX 1200/1340 4 Speed Service Manual (note the title may not be growing in length but it certainly is in use of numbers) shows the same illustration (page 3-18) for 1979 and earlier, but it might be worth noting that it offers a different illustration and spec for 1980 and later.

The difference, at least in part, is due to the changeover to valve guide seals. Earlier heads, both Pan and Shovel, only required a machined pad that was at least the diameter of the valve guide collar to locate the guide since the lower spring collar rested on the collar of the guide. The addition of seals made it necessary to rest the lower spring collar directly on the head to provide room for the seal, so the machined portion of the spring pocket was increased to the diameter of the lower spring collar.

Late vs Early

At first glance one might assume that the different spec is due to taking the measurement to a different surface, since it is now from the tip of the valve to the surface that the bottom of the guide collar seats against. And maybe that's the case, however, things don't seem to quite add up. If the collar on the guide is nominally .100" thick, then all is well. Add .100" to the early 1.500" to 1.545" spec and you come up with the '80 and later spec of 1.600 to 1.645". Ignoring the '80-'81 guides that used a .075" snap ring instead of having a guide with an integral collar, there is still the question of the gaskets that were under the guide collar on earlier motors. I had to look pretty close to even find the part number (18196-51) for this gasket in a Harley parts book since it does not appear in any later copies, though I have a small collection of them left over from top end kits. Measuring a random sample of these showed that they ranged in thickness from about .030" to .040". The James Gaskets catalog lists them as .031" thick with the application being 1951 to 1978.

Hmmm,... so with a window of only .045" in minimum and maximum stem protrusion, we find a variance of at least .030" just in whether or not a gasket was installed under the guide when rebuilding. And what about '48 to '50 Pans and '79 Shovels? Won't they show up as nearly at maximum protrusion right from the factory? And what does that mean when considering '80 and up which certainly never used the gasket? Now the .100" difference in stem protrusion spec doesn't add up so neatly because you have an "effective" guide collar thickness of .130" (collar + gasket) for many years.

Add all of this together and I think its safe to conclude that stem protrusion specification is probably not something will "make or break" your valve job unless you wander too far afield. My guess is that the spec was added after the fact as a guideline for mechanics rather than a part of the original design parameters of the Motor Company.

And if all doesn't throw enough margin of error into the equation, then consider this. If the Motor Company's stem protrusion specs theoretically provide correct valve train geometry (and that is a gigantic stretch given shops such as Baisley High Performance have presumably made a fair chunk of money over the years from their service of correcting Harley rocker arm geometry), then that still means that when you increase valve lift via a performance cam, you have also changed the stem protrusion numbers which should theoretically retain correct geometry.

Here is basically how it works. If you were to draw one imaginary line through your pushrod and another through the rocker arm's ball socket to the center of the rocker shaft, when your cam is at one half of its lift, the line should form a 90 degree angle. Likewise, an imaginary line from the center of the rocker shaft to the pad of the arm should also form a 90 degree angle with the centerline of the valve stem at that same half lift point. That way at zero lift the line through your rocker arm should be the same amount below 90 degrees as it is above 90 degrees at full lift. But that means that if you increase the lift of the valve with no other changes, then the angle with the valve closed will remain the same , but the 90 degree relationship between pushrod and rocker will no longer be at 1/2 lift. To get back to the theoretically correct valve train geometry you would need to lengthen the valve by an amount equal to 1/2 the increase in lift. Or, you could get the same effect by sinking the valve that amount. And guess which is easier and more cost effective, sinking the valve or having a custom valve manufactured?

All of that is to say that with a performance cam, the theoretically correct stem protrusion increases at a rate of half the increase in valve lift. In practice this also has the added benefit on a Harley of providing the increased valve to valve clearance during overlap (commonly referred to as Top Dead Center lift) which is needed for those performance cams.

Now, with all that to digest, I'll pause briefly before continuing with the question of valve stem protrusion on a Knucklehead.  Stay tuned.