Aftermarket cams aren't something you can just bolt up and go. Dozens of painstaking measurements are used by engine builders to ensure a non-original camshaft’s compatibility with the engine. Such a camshaft absolutely must be installed intelligently, and this involves making several valve-related checks on the engine. Here's a summary of the most common ones.

Spring free length
The first check is valve spring free-length. This is the length of the valve spring uninstalled and is usually the only spring specification given in the Honda service manual. Honda’s purpose for considering it is merely to enable you to easily spot fatigued springs, which interestingly enough is not that common on Hondas. However, when building a high performance engine we go way beyond this simple goal, to using spring free-length as the jumping off point for calculating several other important spring specifications.

Spring installed height
One of these is installed spring height. This is the height of the valve spring installed, that is, slightly compressed already, even before the valve has moved. The purpose of knowing installed height is primarily to gauge valve seat pressure. Valve seat pressure is important in any engine, stock or modified, but the modified engine's increased cylinder pressure requires no-nonsense valve sealing. Also, at high rpm there is less time for valve cooling, so good seating becomes more crucial on a cooling basis as well.

Spring full open length
Then there’s valve spring full-open length. This is the spring's shortest working length, when the valve is fully open, the hardest the spring can be expected to work. Among other things, knowing spring full-open length helps the engine builder determine if there is sufficient spring pressure to control valve float. All performance camshafts increase how quickly the valve opens, adding to valve acceleration and making controlling valve "float" more of a concern. The spring's full-open pressure is your first line of defense against valve float, and it will often need to be more than stock in a higher-performing engine.

Spring coil bind
And then valve spring coilbind. This is when the spring's coils touch one another, metal to metal. The spring is compressed past the valve's full-open point. Coilbind must never occur during engine operation, and the spring that comes near to doing so is the wrong spring or misinstalled. In fact, the very reason for checking for coilbind clearance is to validate your choice of non-original valve springs, to point out when they are not the correct rate.

Valve free travel
And don’t forget valve free travel. Also called retainer-to-guide, this is the amount that the valve can travel from closed all the way to where the spring retainer hits the valve guide (or valve guide seal). One of the most important checks, many engine builders have failed to respect this one and have paid dearly.

Valve-to-piston clearance
And of course, valve-to-piston clearance. Valve-to-piston clearance necessarily changes in an engine modified with a different piston or camshaft, or when having remachined castings almost anywhere in the top end, or even different thickness gaskets. Even when the cam is stock. To do this check, carburetor slide springs are put in place of the valve springs. A degree wheel on the crankshaft and a dial indicator on the valve tell us how much the valve can move (when pushed with a finger) before touching the piston, at points from 0 to 15 degrees either side of TDC overlap, the danger zone, in 5 degree steps. This is unarguably the most important of all hot-rod engine building top end clearance checks. Don't believe what high performance parts retailers tell you. It's your engine, find out for yourself.

Valve-to-valve clearance
And we mustn't leave out valve-to-valve clearance. Valve-to-valve clearance is critical whenever the cam, cam timing, or valve sizes are changed. At TDC overlap where the valves come closest to each other, the valves must be confirmed to have enough space between them. Many engine builders use 0.060" as their target. If too close, one of the correctives is to "sink" the valves into their seats slightly. This is not ideal however as just a little sinking will alter port flow, and it is problemstic to do this on a shim type valve train as it reduces the shim sizes needed. Another tactic is to lightly reduce the diameter of the valves at their margins.

You can see from this that most of the work of putting in an aftermarket cam centers around the valve and valve spring. Honda designed both for the exact forces and geometry inherent in the stock engine, and changes to that engine can run afoul of this careful design. And there are still more "gotchas" to look out for, such as clearance of a larger cam's lobe with engine castings, rocker arm interference, valve protrusion, and more. Hopefully it is clear that a professional performance engine builder never "throws" an aftermarket cam at an engine.

Speaking of valve protrusion, in most vintage Hondas it is not a real concern. The adjusting screws on rocker arm valve trains provide considerable compensation when a valve sticks up farther in the head due to valve seat machining, certainly enough to accommodate the typical 0.005”-0.010” extra valve height. But those Hondas having limited adjustabiltiy—the eccentrically-adjusted 350 and DOHC 450, plus all of Honda’s later shim type engines—will present difficulty. In these cases valve protrusion becomes an issue to be dealt with. This is one more reason to prefer stainless steel replacement valves. Unlike the plated stock valves and even their aftermarket substitutes, stainless valves can safely be shortened, thereby reducing excess valve protrusion and eliminating problems associated with limited-range eccentric adjusters or with a fixed range of available valve adjustment shims.

I said you must do these checks on an engine has been changed in almost any way: different cam(s), different pistons, milling, cam degreeing, oversize valves, gasket thickness changes, etc. Even when the cam is stock if any of the other things have changed. However, one of the secret scandals in this industry is that cam makers today make cams that are "safe", almost foolproof. They do this in self-defense. Most aftermarket cams are "dumbed-down" to protect the cam manufacturer from inexperienced installers.