Racer Brown Chapter Twelve

Selecting the Rocker Arms

If you haven't already done so, now is the time to become friendly with your local Datsun parts source. Buy a couple of sets or more of new Datsun L-series rocker arms (Datsun part number 13257-21000) with the understanding that you can return the ones you don't need. NEW rocker arms are an absolute necessity with a new camshaft. The purpose here is to select a set of rocker, plus some spares that are as equal in all areas of measurement as possible.
Place on valve in the cylinder head with a light mechanical fuel pump spring to hold the valve closed, the correct spring retainer, valve locks and lash pad. Place a dial indicator spindle on the spring retainer with the spindle parallel to the valve stem in both planes, and in such a way that rocker arms can be removed and replaced without disturbing the indicator. Pre-load the indicator to something more than maximum valve lift. Install on rocker arm pivot and locknut in the head for the valve to be used, and run the pivot down in the bushing then rotate the camshaft until the nose of the cam lobe for the valve to be used is pointed away from the head. Install a clean, dry and new rocker arm by engaging the valve end of the rocker in the lash pad slot first, then move the rocker under the cam lobe and engage the socket end of the rocker with the rocker adjusting pivot. Adjust the pivot height so the valve is off the seat about 0.002 to 0.003-inch and tighten the pivot locknut a bit tighter than finger-tight. Rotate the camshaft to be certain the cam lobe does not run off wither end of the rocker arm pad. This is easily done by daubing a very thin coat of Prussian blue paste on both flanks of the cam lobe, leaving the rocker pad clean and dry. Rotate the camshaft one full turn by hand and observe the traces of Prussian blue paste that have been transferred to the rocker pad. If the cam lobe runs off the valve stem end of the rocker pad, a thinner lash pad is required, and conversely, a thicker lash pad is required if the cam lobe runs off the pivot end of the rocker pad. It may take a couple of tries with lash pads of different thickness, but the cam lobe contact patch should be pretty well centred on the rocker arm pad, and at the moment, "eye-ball close" is OK. For each attempt, the rocker must be removed and the rocker pad wiped clean of all traces of the Prussian blue paste, and the cam lobe re-blued. Just use a touch of the stuff; a little goes far.
When the cam lobe contact patch is centred on the rocker arm pad and the valve is off the seat 0.002 to 0.003-inch with the centre of the heel of the cam lobe contacting the rocker pad, rotate the indicator dial to zero and lock it in place lightly. Rotate the camshaft by had until maximum valve lift shows on the indicator, log the number and rotate the cam until the indicator shows zero again, remove the rocker arm and number it to coincide with the maximum valve lift figure. Do not move the rocker pivot adjustment of the indicator dial. Now install another rocker arm and repeat the process. I f the indicator needle moves more than about 0.002-inch either direction from zero, reject that rocker and got to another. A wide variation here means that the relationship between the socket, pad and tip of the rocker is inconsistent, but a tolerance of plus-or-minus 0.002-inch is acceptable. Don't forget to re-zero the indicator dial, if necessary, for each rocker arm. Also, rig some kind of a pointer aimed at one specific sprocket tooth when the rocker pad contacts the cam lobe in the centre of the heel of the lobe. Mark the sprocket tooth so the same starting point is used for each rocker arm. At this stage, the actual rocker arm ratio is not important. What is important is to find a full set of rocker, plus a few spares, that are as close as possible to being identical. It is also important to use the same valve, the same retainer, the same valve locks and the same cam lobe when checking rocker arms for consistency. Number all rocker arms in their correct sequence from front to back. If one or more rockers are removed, make certain they are replaced on the same cam lobes. DO NOT mix them up.

Getting the Geometry Right

Observation of the rocker arm geometry is next, but this cannot be done without a cutaway lash pad, like that shown in the photos. Rocker arm geometry can be considered correct when the centre of the rocker arm tip radius coincides with the centreline of the valve stem at exactly half valve lift as previously explained. It may take a small variation in lash pad thickness to make this condition come about, but when it is done. There will be some "free" power (the best kind!) due to the reduction of friction in the valve train, as well as lowering the wear rates of the valve guide bores and valve stems. Unfortunately, the rocker arm geometry must be an "eyeball" check, a condition that not only calls for good light pointed in the right direction, but also for critical examination of the surfaces of the rocker arm tip and the lash pad to be certain their relationship is correct and a low-power magnifying glass will help. For this check, it is safe to assume that the centrelines of the lash pad and the valve stem are coaxial. Because the lash has "ears" on two sides, you may want to do as we did for the accompanying photos. The ears can be ground off and a cutaway retainer helps to visualise what is happening.
A point should be made here related to valve lash pads. We make our lash pads from SAE 52100 steel, the exact same material from which are made the best ball and roller bearing assemblies. Heat treat specification on the lash pads is from 52 to 55 Rockwell "C" scale; hard enough to be very wear-resistant, yet extremely tough without being excessively brittle. After finish machining the pads are 100% Magnaflux inspected. The lash pads furnished with our camshaft assemblies are selected for thickness by (1) having the cam lobe contact patch as close to being centred as possible on the rocker arm pad with zero valve lash for a given cam lobe profile and (2) based on a new cylinder head and new valves.
This means that when the valve seats and valve faces are ground, and the valve lash set properly, the lash pads should be slightly thinner than as-supplied. However, these pads are machineable to some extent. The heat treat process is a "through" heat treat and not a "case". This means that they are within 1 Rockwell "C" scale point at their centres as they are on the outer surface. This gives you something to play with because it is always much easier to remover metal than it is to add it, once too much has been removed. And the use of loose shims in the L-series valve train is a no-no. These lash pads can be machined in a lathe with a tungsten-carbide cutting tool; ain't easy, but it's possible. The best method is to surface grind the flat surface that contacts the valve stem tip. Any decent machine shop will have a surface grinder, but make certain that the grinding operation is done with lost of coolant. If it is done "dry" there is a very good possibility that grinding cracks will be encountered or that the temperature of the pad during grinding will be high enough to anneal (soften) the pad in just the areas where it should be hardest. Of course, we do exchange lash pads if the ones supplied are too thick or too thin, but we must know the correct lash pad thickness requirement.
If a large variation in lash pad thickness is required to make the rocker arm geometry right, it is absolutely essential a recheck be made of the cam lobe contact patch on the rocker arm pad to be certain the cam lobe doesn't run off one end of the rocker pad. It if is not possible to satisfy both conditions at the same time, it becomes necessary to concentrate on making the cam lobe contact patch fit correctly on the rocker pad as a primary action but secondarily, with the least amount of compromise to the rocker arm geometry.
Just in case you think this lash pad setup procedure is going to be super simple or fast, consider that the contact area may move completely off the pad with a change of only 0.015-inch in lash pad thickness. If the rocker arms have been selected as described and if the valve stem lengths have been equalised, then the rocker arm geometry and the cam lobe contact patch exercises need only be performed on one valve. If there has been no selectivity in picking out rocker arms and/or the valve tem lengths have not been equalised, then you will have on hell of a time optimising each valve to each rocker arm, and you will very likely end up with a lash pad of different thickness for each valve, a condition that should be studiously avoided.

Checking Valve Timing and Valve-to-pistons Clearance

Now for a valve timing check and if we're clever and lucky, a piston-to-valve clearance check at the same time. Using number 1 cylinder for convenience, install the intake and exhaust valve assemblies and all related pieces in the cylinder head using a light mechanical fuel pump spring in place if the valve springs for each valve. Engage the camshaft dowel pin in the number 1 hole in the camshaft sprocket to start and tighten the sprocket retaining bolt finger-tight. Set the valve lash to zero. Rotate the camshaft so the dowel pin is vertical, above the cam bolt. Timing mark 1 toward the outer edge of the sprocket will be on the right side when viewed from the front, three teeth above a horizontal plane. In this position, the valves correspond to the TC piston position of the compression stroke for number 1 cylinder with both valves seated.
Exact TC for number 1 piston must now be located as closely as possible if it hasn't been done before. This is most accurately determined with a dial indicator reading in "tenths" (1/10,000-inch graduations). The so-called "positive-stop" method of locating top centre is not recommended. The stop usually isn't positive enough, and the piston motion curve is not the same on either side of top centre if the piston pins are offset in the pistons, like stock pistons, so exact top centre will not be exact, and can vary by more than one crankshaft degree. We want it exact. Bolt the graduated crankshaft damper or degree wheel to the nose of the crankshaft or a flywheel with a graduated rim to the back of the crankshaft and run through it enough times with the dial indicator to be certain that it will repeat and that it is dead accurate, then leave the piston at exact top centre and adjust the pointer until it, too, shows exact top centre. With number 1 piston at TC of the compression stroke, the crankshaft key will be vertical on the topside of the crank nose, and the crank sprocket timing mark will be on the right side when viewed from the front, about a half a tooth below a horizontal plane.
Without moving the crankshaft or camshaft, install the cylinder head with a used but usable head gasket of the exact type you'll be using on the final assembly. The gasket is necessary to take up its own space; otherwise the chain tensioner will retard the camshaft when it very likely shouldn't be. Torque the cylinder head bolts in steps to no more than 50 pounds-feet. Remove the camshaft sprocket; install the timing chain and lotsa luck! This long thing has more ways of getting caught, hung up, entangled, etc., than you'd believe. Now there are two bright (or marked) links (count 'em) on the short side of the chain. Holding the chain vertically, the bright (or marked) links must be on the right side and must face forward when viewing the chain and engine from the front. The lower bright (or marked) links must be on the right side and must face forward when viewing the chain and engine from the front. The lower bright (or marked) link must be engaged with the crank sprocket tooth that has the timing, mark while the top bright (or marked) link must engage the tooth for the number 1 timing mark on the camshaft sprocket. When this mess is sorted out, reinstall the camshaft sprocket and make certain the camshaft dowel pin is engaged in the pinhole 1 in the sprocket, then tighten the sprocket bolt fairly snugly. Without moving anything double-check the chain installation to be certain the bright (or marked) links are engaged with the proper sprocket teeth, at both the crank and at the cam. This is no time to be off a tooth or more in either direction. One tooth is 18 crankshaft degrees and the broadest possible tolerance isn't broad enough to cover an error of that magnitude.
If everything is OK, or seems that way, mount the dial indicator on the spring retainer of number 1 intake valve, make certain the indicator spindle is parallel to the valve stem in both planes, and pre-load the indicator to something more than maximum valve lift. Rotate the crankshaft slowly and gently in the normal direction of rotation (clockwise from the front). If there is any above-normal resistance to rotation no matter how slight, S-T-O-P!! And investigate the cause. Aside from the piston striking the cylinder head and/or gasket, about the only other possibility is the piston tagging a valve, so move both valves by pushing down on the spring retainers. Now you see one reason for using light springs instead of valve springs.
Assuming there is no conflict (yet), rotate the crankshaft until the dial indicator shows a maximum valve lift and make a mark on the degree wheel, damper or flywheel at which the pointer is aimed. Rotate the crankshaft exactly one full revolution, right back to the same mark, zero the indicator dial. Then rotate the crankshaft slowly in the same direction until the indicator shows a valve lift of 0.025-inch, or whatever the check height for valve timing may be. This will be the intake valve opening point and, unless the camshaft is of extremely short effective duration, this will occur at some point before TC. Read the degree wheel and log the number. Again assuming there have been no collisions between piston and valves, continue to rotate the crankshaft, observing the cam lobe and rocker arm, past maximum lift and down the closing flank of the cam until the dial indicator shows that the valve is 0.025-inch from zero or the correct check height. This indicates intake valve closing point and it will occur at some point past BC. Read the degree wheel, log the number then add the intake valve opening point to the intake valve closing point plus 180 degrees to arrive at the intake valve duration. Compare the valve opening and closing points to those shown on the cam's timing card. The duration should be right there; just what the card says, within plus-or-minus one crankshaft degree. There may be some disagreement in the valve opening and closing figures, and the comparison will give you a pretty good idea how much and in which direction to go to correct the valve timing. Run through the procedure enough times to be certain the figures repeat then transfer the dial indicator to the exhaust valve and go through the operation again. When the intake opening and closing points and the exhaust opening and closing points have been determined, then you have a pretty good fix for amount and direction for any corrective action that must be taken to make the valve timing fall in place.
If, during the initial checkout, there proves to be piston-to-valve contact, simply remove the piston and connecting rod assembly from the engine, after exact TC has been located. The piston isn't really necessary for a valve timing check, or for subsequent valve timing corrections. The normal course of events indicates that the valve timing should be checked first; next correct the valve timing, if a correction is required; then the pistons can be notched after the valve timing has been established.