Under the Bonnet: Fall 1995

VALVE TRAIN BASICS

Valve train condition is of vital importance at all times to the correctly running engine. Unfortunately, many of the factors involved are often neglected in “do it yourself” engine rebuilding and in normal maintenance.

The simplest and most common valve train operation performed is valve lash (clearance) adjustment. Yet even this common operation involves features which are not always known or well understood. For example, how does one deal with rocker arm tips which are worn where they scuff across the end of the valve stems? While valve lash usually loosens due to wear, what causes the opposite situation of reduced valve lash, which is sometimes encountered? Why is valve lash necessary in the first place?

Before developing answers to these questions, a quick review of valve train operation is in order. (For simplicity, we will discuss a simple single cam pushrod system.) On the front of the crankshaft is a small diameter sprocket which drives the timing chain. The larger driven sprocket on the camshaft has twice as many teeth as the crankshaft sprocket, causing the camshaft to rotate once for every two rotations of the crankshaft. The rotational movement of the camshaft is translated to linear movement by the individual cams pushing the lifters (“tappets”) upwards. These, in turn, move the pushrods, which operate in ball and socket fashion between the lifters and the rocker arms. This ball and socket arrangement is required by the rocker arms’ oscillation through small arcs (i.e., the pushrods do not only move up and down, but they also wiggle back and forth slightly). The rocker arms pivot on the rocker shaft, changing upward movement of the pushrods to downward movement at the valves. Return motion of the entire system is provided by the valve springs.

Every part of this system is subject to wear. When you consider that at an engine speed of 3,600 rpm, this operating cycle is repeated 30 times a second, it is amazing that these parts last as well as they do. There are many subtle design features present. For example, cam lobes may have a slight sideways taper, and the ends of lifters may be slightly crowned, or the centers of the lifters may be slightly offset on the cam lobes. Either combination causes the lifters to rotate in their bores, preventing wear from occurring in localized spots. This points to one reason why regrinding tappets is not always successful—they are often reground flat. Another is that most tappets have a relatively thin hardened “case,” and when reground, the softer metal beneath is exposed.

A frequently asked question is, “Is it really necessary to return components to their original positions?” The answer is, “Yes.” If assembled and lubricated correctly when new, there should be a short period of initial wear followed by a long period of well-mated parts working smoothly together. If these parts are later mixed, they are not worn to fit each other, and will usually wear excessively. It cannot be expected that such parts will bed into each other as satisfactorily as do new parts. This is especially true of cams and lifters. Although a good used cam may be successfully used with new lifters, a new cam with used lifters is sure to destroy the cam.

If the geometry between the rocker arms and valve stems is incorrect, the nose of the rocker arms tend to push the valve stems sideways instead of sliding on their ends, causing highly accelerated wear to the valve guides and valve stems. For correct operation, the radiused end of the rocker arm must contact the center of the valve stem end when the valve is 40 to 50 percent open.

Milling the cylinder head, regrinding worn rocker arms or valve stem ends, especially in combination, moves the contact point towards the rocker arm pivot, and requires compensation in the form of shorter pushrods to reestablish the proper relationship. Refacing of valves or valve seats has the opposite effect. Generally, stock “street” engines will tolerate a slightly less than ideal situation much better than engines with high lift cams and heavy valve springs. If in doubt, ask your automotive machinist for advice.

When valve faces and valve seats are ground, another important dimension, the valve spring height, is increased. This reduces valve spring pressure, which lowers the allowable rpm limit of the engine. Valve springs should be checked for “free height” and “fitted height,” figures for which will be found in your workshop manual. If springs are out of specification, it is best to replace them with a new set, although springs with a fitted height in excess of specification may be shimmed. With stock components and a good cylinder head, it is unlikely that the springs will have a too short fitted height. If, however, this is encountered, the spring seating areas on the head may be milled to suit. In all cases, ensure that the coils of the springs do not touch each other when the valve is fully open.

If the entire valve train were set up absolutely perfectly, and nothing in it expanded from heat, flexed under load, wore, or in any other way deviated from its theoretical path, we could use a tight, zero clearance setup. Real life being what it is, valve lash is required to allow for thermal expansion, wear, and other variables in the system. Allowance for a specified amount of valve lash is even factored into camshaft design, as changes in valve lash affect the cam timing. Too much lash allows the valve train components to slam violently against each other, while too little can lead to valves not closing properly (which causes them to burn), parts bending, and other nasty happenings.

Sometimes the valve lash will tighten in service, particularly on exhaust valves. This is usually due to valve seat recession, which is erosion of the valve seat in the cylinder head. This allows the valve to sit deeper in the head, reducing the available valve lash. The only real cure for this is to have hardened valve seat inserts fitted. Stretching of the valve stem can also reduce lash. Stretched valves must be replaced.

In checking or rebuilding a valve train, visual inspection of the components is usually sufficient to determine whether parts may be reused or not. If there is any question or doubt, replace the part. Cam lobes may show polished areas and slight discoloration, but must be smooth and flat. One lobe showing scoring, rounding, or “wiping” (smears of metal) is enough to cause rejection. Lifters must be good fits in their bores, without looseness, but should just fall into their bores under their own weight when lightly oiled, and rotate smoothly. Their bases must be smooth and show only very light even wear in a circular pattern, with no signs of chips, spalls, or cracks (a magnifying glass is useful for this, as they sometimes develop tiny surface cracks which are difficult to see otherwise). Pushrods must be straight. A good way to check this is to roll them on a flat hard surface; if bent, they will rise and fall as they roll. Pushrod ends will show polishing from wear, but should be smooth and without ridges. In some engines the ball end may have a slight projection in the center. This may be carefully ground off, but don’t grind any more than absolutely necessary.

The rocker shaft assembly should be disassembled for examination, being careful to note the correct order of the parts on the shaft for reassembly. If the shaft shows more than smooth polishing on wear areas, replace it. Check the rocker arm bushes (if fitted) by putting each rocker arm on a clean unworn section of the shaft. If there is any more than just the slightest detectable sideplay, replace the bush. New bushes must be reamed after being pressed into the rocker arms, and some must be drilled for oil passage. If the rocker arms have no bushes, they must be replaced if worn. The adjuster screw balls may show—even wear—but must be round. If any have a flat area on one side, replace them and check your rocker arm to valve geometry after reassembly.

Rocker arm “noses” should show a radiused bottom in profile, and be straight across the width. It is common to find a flattened area in the middle of this radius where the rocker arm slides across the top of the valve stem. Among other problems, this makes accurate adjustment difficult unless you have a very narrow feeler gauge which will fit inside this narrow worn area. It is generally considered permissible to regrind such worn rocker arms if they aren’t worn too badly, as long as the correct radius is retained.

Valves that show burning, cracking, or bent stems must be replaced. Valve faces may be reground if they are not in too bad condition, and if the valve stem is not worn or scored, and the end has survived being battered by the rocker arm. Valve head margins must not be thinner than specified in the appropriate workshop manual.

Valve guides are easily checked with a valve with an unworn stem, preferably a new one. Insert the valve in the valve guide so that the valve is approximately in its open position, and wiggle it back and forth and side to side. Triumph allowed a very generous 0.020″ side play for TR250-TR6 valves, but half of this figure is considered generous by many. If your guides show wear, replace them, ensuring that the new guides are properly reamed to size after installation.

—Eric Wilhelm

(The next Under the Bonnet article will deal with cam timing and how to install and set up your camshaft without relying on pre-marked sprockets, bright links on chains, or other factory “fiddles” which may not be available.)



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