Racer Brown Chapter Four

SOHC DESIGNS

The Datsun L-16, L-18 and L-24 camshaft and valve train system could be described as "contemporary single-over-head camshaft with rocker arms," but with some variations. The basic camshaft and valve train layout is not new. It's been used by such venerable makes as Mercedes-Benz, Porsche 911 series, the old OSCA 750's, various Ferraris, Maserati, BMW, Rover, etc.
Later-comers to the same scene include small Peugeots, some Mazda and Toyota models, Dodge Colt, Ford Pinto 2000 and others not as well known in this country.
Even a couple of domestic engines were involved: The short-lived Ford 427 SOHC V8 and Pontiac's in-line six-cylinder that ran from the 1966 through the 1970 model years. Oh yes, there was a Willys engine, too!
With only three known exceptions, all of these engines suffered (or still suffer) from the same disease - usually in the earliest production runs. ALL had (or have) severe camshaft lobe and/or rocker arm wear... Three exceptions were Fords SOHC V8 and some Ferraris because these used rollers that bore on the cam lobes instead of the more usual radiused pad machined on, or inserted into the rocker arms. Ferraris were kind of shaky for a couple of reasons but the Ford was virtually indestructible.
Datsun's L-24 is the third exception…not to imply that the L-24 has been totally free of camshaft and rocker arm failures. But with this engine, incidence of this type failure has been vastly reduced.
Basically, the sever cam-loge/rocker arm wear problem revolves around two significant and interrelated factors: (1) Incorrect metallurgy between the two rubbing surfaces of the cam lobe and the rocker arm; or, (2) Improper lubrication between the cam lobe and rocker arm interface; and (30 a combination of the first two.
Datsun has had their share of the same problem, first with the discontinued U-20 four-cylinder two-litre cammer and with earlier L-16's.
It isn't my intent to bad-mouth or be hypercritical of this or any other type of camshaft-valve train layout. Where inherent problems do exist, I believe they should be examined for cause, and hopefully, to show how such grief can be cured or at least avoided.
So an overhead camshaft system by itself does not necessarily solve all valve train problems - no matter how attractive it may seem initially.

DATSUN SOHC VALVE TRAIN DESIGN

Now specifically to the current Datsun single-overhead-camshaft types.

Drive

Over head camshafts in L-16/18/24 are driven directly from the front of the crankshaft by a long single-stage double roller chain and sprocket drive assembly, with no intermediate sprockets. An automatic chain tensioner on the "slack" side of the chain (left side of engine viewed from front) is just above the crankshaft sprocket. Engine oil pressure and a compression spring combine to apply the necessary load to a piston, at the end of which is a curved "shoe" bearing directly on the chain. A curved guide extends from just above the shoe to just below the camshaft sprocket to control chain whip and vibration. A similar but straight guide does the same job on the "tension" side of the chain (right side of engine viewed from front). These two guides and the shoe are faced with wear-resistant plastic where they contact the chain. Twenty teeth on the crankshaft sprocket and 40 on the camshaft sprocket give the necessary two-to-one reduction between crank and cam. In this type cam drive system, the crank and cam rotate in the same direction; clockwise when viewed from the front.
The cam sprocket is secured to the camshaft nose with a 16mm bolt which also holds the separate steel fuel pump eccentric in place. A 6mm-dowel pin pressed into the cam nose is offset from the camshaft centre. It matches up with any one of the three holes in the camshaft sprocket. The pin locates the sprocket relative to the cam; the three holes and three timing marks allow adjusting valve timing, which will be explained later.
A cam thrust plate bolts to the front side of the forward cam tower. Fore-and-aft movement of the camshaft is controlled by the thickness of this plate and the depth of the counterbore in the back of the camshaft sprocket. The thrust plate is available in three different thicknesses to allow minimising axial camshaft movement.

Cam Towers

The camshaft is supported in the aluminium alloy cylinder head by aluminium ally cam towers - four in the L-16/L-18, five in the L-24. There are no separate camshaft bearings as such; the camshaft journals ride directly in the cam tower bores.
Cam towers are bolted to the cylinder head located with large diameter hollow dowels, then align-bored after assembly at the factory. It is an ABSOLUTE NO-NO to remove the cam towers from the cylinder head because it is nearly impossible to restore correct cam bearing bore alignment after they have been removed.

Cam

The camshaft is a one-piece iron casting with induction-hardened cam lobe and bearing journal surfaces. There's no fancy metallurgy here: Analysis shows the material to be a close relative of plain grey cast iron. However, the casting technique is excellent and the castings show uniform density and are generally very good. Cam lobes are coated with a non-metallic phosphate compound to minimise scuffing with their mating rocker arm pads. Camshaft journals are left clean and bright because the phosphate coating material is not at all compatible with the aluminium tower bearings.

Combustion Chamber/Valve Layout

Combustion chamber configuration of the L-16/L-18 is more =-or-less "conventional" wedge; the L-24 chamber is more like an "open" wedge. Valve are in-line in all engines and are tilted the relatively small angle of 12 degrees from the cylinder bore centralise. Valve stems are tilted to the right when viewed from the front. All intake and exhaust ports are on the same right side. On the opposite side, the aluminium head casting is drilled and tapped to accept hexagonal head steel bushings, which are threaded internally and externally. Above the hex section, the bushings are grooved to accept "butterfly" type spring clips. Each of these bushings, one for each valve, is screwed tightly into the head casting.
Valve guide inserts are a dense cast iron alloy similar to Meehanite. These are an interference fit in the head with the head heated and the guides frozen. Nominal guide bore diameter is 8mm and nominal valve stem diameter is close to 5/16 inch, a point of occasional convenience, which will be discussed later. Valve stems are hard-chrome plated for wear resistance.
Aluminium-bronze valve seat inserts are used for all intake valves. Exhaust seats are cast iron alloy inserts. Valve seat inserts are fitted to the cylinder head in the same manner as the guides.

Rocker Arms

Rocker arm pivots thread into the steel bushings and are secured by locknuts bearing against the bushing tops. The pivot tops have spherical segments, the geometrical centres of which form the rocker pivot points. Just below the spherical segment and integral with the rocker pivot is a hex section that permits using an open-end wrench to raise or lower the pivot in the bushing, thereby adjusting valve lash. The rocker pivot locknut fits beneath the hex section in the rocker pivot and locks against the top of the threaded bushing.
Rocker arms are steel forgings spanning the distance between the rocker arm pivots and indirectly to the valve stem tips. Here, there are no questions or doubts about the use of hydraulic, mechanical or roll-type camshafts; all are mechanical with a positive and predetermined amount of lash or clearance between cam lobe and rocker arm pads.
A hemispherical socket machined into the under side of the rocker arm corresponds to the spherical segment of the rocker pivot. Each rocker arm fits over its pivot and is thus located by the pivot and oscillates about the pivot's geometrical centre. The rocker arm tip, which normally bears against the valve stem tip but doesn't in this case, is also on the underside of the rocker and has a single-plane radius machined into the rocker arm tip pad. An oil hole drilled in the top of the rocker socket permits splash lubrication of the socket and pivot, adequate because of the small linear distance the rocker arm socket travels in relation to the pivot.
Topside on the rocker arm between pivot socket and rocker arm tip, is a pad with a single-plane radius in the same plane as the valve stem tip radius, that is, coaxial with the camshaft. This pad bears against the cam lobe. At this point there is some controversy about early and late rocker arm types, though I can see no valid reason for argument at all.
Earlier rocker arms were made of one piece and the rocker pad that contacted the cam lobe had a slightly larger radius, which gave a slightly flatter arc to this pad. Some claim that these are the rocker arms to use in modified L-series engines because the larger pad radius results in a slightly higher effective rocker arm ratio and also slightly higher valve velocities and valve lifts.
As far as it goes, this is correct. However, what these proponents fail to recognise or realise is that the forged steel rocker pad contacting a relatively low-alloy iron cam lobe is metallurgically one of the worst possible combinations with commonly-used materials. This is true even with very light valve spring loadings and lubrication that, by sheer volume, makes up for what it lacks in direction and placement. Simply reversing the materials would have helped some but would not put a complete fix on it.
Datsun learned about the same lesson from its failures that nearly all previous designers of this general type of camshaft/valve train layout had learned: Ya gotta have metallurgically compatible materials at the interface of the cam lobe and rocker arm rubbing pad. In a conventional pushrod engine the cam lobes have a slight taper, are offset from the lifter bore centres, and the lifter faces are slightly crowned. All of these combine to force the lifters to rotate as they rise and fall in the bores in accordance with cam motion. This action produces a very smooth and even ear pattern on both lighter faces and cam lobes. In this day and age this is completely predictable, if there is metallurgical compatibility in the first place.
In the Datsun and similar overhead camshaft layouts neither the cam lobe nor the rocker arm rubbing pad are protected in this way from each other, so the material for each surface must not only get along with the other - each must also be scuff and abrasion-resistant, even if loads between the two are high enough to squeeze out or vaporise the interface oil film. This problem was very effective in finally killing production of the Pontiac OHC-6.
In any case, late L-16 engines and all L-18 and L-24 engines are equipped with two-piece rocker arms. The second piece is an insert furnace-brazed in place, and this insert contacts the cam lobe. When the two-piece rockers were introduced, the single-plane radius of the insert was, decreased slightly, slightly decreasing the effective rocker arm ratio and the valve velocity. This resulted in lower dynamic stresses at the cam lobe/rocker pad interface.
Analysis shows the current rocker arm insert pad material to be the approximate American equivalent of chilled cast iron. Radius of the current species of rocker arm pad is 50mm (approximately 2.00 inch).