Racer Brown Chapter Ten

INSTALLING THE CAMSHAFT

The best method for installing a camshaft in a Datsun cammer engine is to have the cylinder head on the bench. This allows the valve seats and valve faces to be ground, as well as any other machine work required for the installation. Therefore, it is best to be armed with the Datsun shop manual for the engine concerned. If the rest of the engine is in pieces, that is one thing, but if the engine is to stay in the chassis, it is another matter. In the latter case, first loosen the camshaft sprocket-retaining bolt. The bolt is (should be) usually very tight, so just barely loosen it - DO NOT remove it. Then the engine must be rotated until the first (bottom) bright (or marked) link on the camshaft drive chain is engaged with the tooth that has the timing mark on the crankshaft sprocket (which you can't see) and the second (top) bright (or marked) link on the chain is engaged on the tooth with the timing mark on the camshafts sprocket.
If the engine is not taken apart, you can count 41 links (not link plates) from the second bright link engaged with the timing mark on the cam sprocket until the first bright link engages with the timing mark on the sprocket, then reverse the procedure to get the bright link back down on the crank. That way you can be sure that you have the cam timing correct, assuming that the chain was correctly engaged with the correct tooth on the crank sprocket to start with. At this point, both sprocket timing marks will be nearly horizontal on the right side of the engine when viewed from the front, with the camshaft sprocket mark above horizontal and the crankshaft sprocket mark below horizontal, and the crankshaft key and the camshaft dowel pin will be pointing upward nearly vertically. This will (or should be) top centre of the compression stroke for number 1 cylinder (both valves closed), and the TC mark on the crankshaft damper should be in line with the point. If there is any disagreement in the reference marks, poke a long screwdriver through the spark plug hole in number 1 cylinder so the end of the screwdriver contacts the top of the piston. With one finger lightly pushing the screwdriver upward at the end, rotate the engine in both directions just enough so that the screwdriver tells you that the piston is at top centre as close as you can feel it. The longer the screwdriver, the more sensitive an indicator it will be, and this will be close enough for the moment.
Before removing the camshaft sprocket, us Datsun tool number ST-17420000, a hardwood wedge. Slip it gently but firmly down between the timing chain links to keep the timing chain correctly oriented with the timing marks and better yet, to keep the chain tensioner piston from falling out of the chain tensioner housing and into the oil pan. If this happens, the oil pan must be removed to retrieve the piston, and it will only be a stroke of sheerest luck if the piston can be replaced in the housing without removing the entire front cover assembly, including the crankshaft damper, water pump, oil pump, ignition, etc. With the wedge firmly in place, it is now safe to remove the camshaft sprocket and continue with removing the cylinder head assembly.
Pistons - Any engine to be used strictly as a race engine should obviously be completely disassembled and rebuilt with the race application in mind. Similarly, a street engine, or a dual-purpose engine with lots of mileage on it should also be completely rebuilt. In the latter case, a suggestion is in order. Stock Datsun pistons are permanent-mould aluminium castings with integral steel struts to control piston skirt expansion available in oversizes up to 0.060-inch (1-1/2 mm). These will give much better oil control than an aftermarket forged piston because you can use tighter piston skit-to-cylinder bore clearances. However, they are only available in a flat-top configuration without valve reliefs. If the engine is to be compromised more toward street operation, use 'em. The price is right, too, and you don't have to fight the battle of full floating piston pins, which can be much more bother than they are worth.

Cylinder Head

Any cylinder head, new or old, should have the valve seats and valve faces ground and lapped, as well as checking and correcting the valve stem-to-valve guide bore clearances. If any port and/or combustion chamber work is to be done, now is the time. Before the cylinder head is completely disassembled, it's a good plan to measure and record and dimension from the valve stem tip to the valve spring pocket in the head with the valve closed for all valves. This is a pretty critical dimension, so a "yardstick" measurement isn't good enough; it must be made with a depth micrometer. The best way to do this is after the camshaft, rocker arms, valve springs, lash pads, retainers, etc., have been removed from the cylinder head assembly. Next, with all valves held closed, lay a straightedge along the line of valves in the head to see if there are any major discrepancies in valve stem length from one end of the head to the other. Ideally, the straightedge should be parallel with the cylinder head gasket face with the straightedge contacting all valve stem tips. There will usually be differences in valve spring pocket depths in the head, so the valve stem tip-to-valve spring pocket dimension will apply to only one valve. The valves must be numbered in the proper sequence so they can be reinstalled in the same guide bores.
These days, nearly every cylinder head artist has his own "trick" method for doing valve jobs, and some of these are OK, but the purpose for which the engine is intended must be considered. Thinning the valve heads, fully-radiused valve seats and other exotica are not for a street or dual-purpose engine where longevity is an important factor. A multiple-angle cut in the valve pocket, beneath the valve seat, is fine as long as the cuts are consistent and conservative, with a "topping" cut of no more than 15 degrees and a flatter angle may be better. Intake valve seat width should be from 0.070 to 0.075-inch and exhaust valve seat width should be from 0.085 to 0.090-inch. Valve seat and valve face angle should be 45 degree. Shallower seat angles may be OK in race-only engines but they don't seal as well or last as long as 45 degree seats.
In any case, it is important to keep the valve seats as close as possible to the cylinder head face. In other words, DON'T sink the valve seats in the head. This practice generally ruins airflow characteristics, particularly at the intake valve and port. The depth of the intake valve seats from the cylinder head gasket face must all be equal as close as it is possible to make them. Ditto for the exhaust seats, except they won't necessarily be the same as the intakes due to the difference in valve sizes.
Once the valve seats and valve faces have been ground - and preferably lapped - and any other new pieces installed such as new valves, new valve guides, new valve seats, and all related work completed, make another measurement from the valve stem tip to the valve spring pocket with the valve seated for all valves and also make another straightedge check along the line of valves in the head. There is no doubt that the valve stem tips will extend further than they did originally. It is now necessary to find the valve stem that projects the least amount from the topside of the cylinder head. This will be visible with the straightedge check assuming the straightedge is held exactly parallel to the cylinder head gasket face. Next, shorten all remaining valve stems to match the shortest one. This must be done with a valve-refacing grinder with a fixture for grinding valve stem tips. The grinding wheel must be clean, sharp and dressed so the finished valve stem tips are square to the valve stem axis, flat and with a smooth surface finish.
The purpose of this exercise is to equalise all the valve stem tip lengths, but in addition, it permits the same thickness of valve lash pad to be used on all valves. If this procedure is not followed, very likely each valve will require a valve lash pad of different thickness, which is a pain in the neck and elsewhere. This is itchy work because any variation in valve stem tip length is reflected as a valve lash pad thickness requirement as a function of rocker arm ratio. If one valve stem tip extends 0.010-inch more than its neighbours, then the valve lash pad thickness requirement is changed by 0.015-inch, assuming a rocker ratio of 1.5 to 1. Therefore, the closer a zero-tolerance condition is approached, the better.

Porting

Before we leave cylinder head modifications, some additional points must be made. If, due to the application, modifications to the intake ports and pockets, exhaust ports and pockets, valves, combustion chambers, etc., are indicated as they usually are even in engines with milder states of tune, it is highly recommended that the cylinder head and valves be sent to a cylinder head expert equipped with an air flow bench and the intelligence to use it correctly. Port, valve and combustion chamber design, to say nothing of induction and exhaust systems, have become extremely sophisticated within recent years, and nothing can kill a cylinder head quicker and deader than gouging it out in the wrong place by someone who may have the best intentions in the world, but who lacks the know-how, experience and air flow measuring equipment, all of which are required these days to do the job correctly. So go to an expert in the first place and save yourself the cost and frustration of having to do the job again after it has been bungled. Be very explicit in telling him your exact requirements for the engine and let him decide what is appropriate in the area of valve and port sizes, etc. The same expert will usually be equipped to equalise the volumes of the combustion chamber cavities in the head. All this may not be cheap but it will be cheaper than having to do the job twice in order to get it done once, and right. If the guy is sharp and knows his Datsuns, it probably won't be necessary to tell him that all L-series engines need more help than the intake ports, and also that total exhaust air flow should fall within the range of 75 to 80% of total intake air glow.
There are two separate and distinct approaches to the proper reworking of L-series Datsun cylinder heads and the correct route depends upon the application. For street or dual-purpose engines, shoot for as much airflow as possible at relatively low valve lifts and let air flow at maximum valve lift fall where it falls. The air flow curves should be good and fat at low valve lifts, on both intake and exhaust ports, without dips or "holes" in the curves, and if air flow doesn't increase much beyond valve lifts of 0.450 to 0.475-inch, who cares? When this is done correctly, gas velocity will usually be quite high for both intake and exhaust and the engine will show it by being very throttle-responsive throughout the normal engine speed range, but will be at tis best in the mid- range and upper mid-range. If air flow through the ports more-or-less "signs off" at say, 0.460-inch valve lift, and the actual valve lift is in the 0.470 to 0.480-inch range, so much the better. This means the valves will be at maximum air flow rate for a longer period of crankshaft rotation and this will keep engine performance alive and well in the higher engine speed ranges. In such cases, camshafts with longer effective durations will not be as strong at lower engine speeds as those with shorter effective durations in conjunction with fairly healthy valve lift numbers. But don't get carried away; those 0.600-plus inch lift numbers are not for street or dual-purpose engines.
Strictly race engines are of another planet. However, airflow at lower valve lifts cannot be abandoned in a search for the highest possible airflow at some ridiculously high valve lift figure. L-series Datsun cylinder heads can be modified to produce very good air flow figures at valve lifts in the 0.650-inch range, but this happens at the expense of air flow at lower lifts and has a detrimental effect on performance in the mid-range and even upper mid-range engine speeds. This occurs because the valves may reach the point of maximum air flow for only the shortest period of time, if at all, so the ports and air flow numbers, as magnificent as they may be, cannot be utilised effectively and the whole episode could easily dissolve into an exercise in futility.
By far the better plan, and one that really works, is to make every attempt to retain as much air flow at lower valve lifts as possible without giving anything away at higher lifts; not necessarily at the most extreme lifts, but at some reasonable and acceptable number say, in the range of 0.575 to 0.600-inch. Then the cam lobe profile can be made with enough additional lift so the valves are at or above the point of maximum port flow for a considerably longer period of time and/or crankshaft rotation. In this way, the engine does not give nearly as much away in the mid-range speeds, yet maximum power will very likely be at least as good, but more likely better than if extremes are attempted. I have seen some L-series Datsun ports that flow some extremely impressive numbers up to and including 0.750-inch valve lift but attempts to make use of these ports have been the most total, dismal and dreary failures imaginable, and for two very good but separate significant reasons. First, airflow at valve lifts below about 0.480 to 0.500-inch has never been enough to blow the dust off your desktop. Second, the L-series Datsun rocker arm pads are simply too short to accommodate the type of cam lobe profiles necessary to generate such enormous valve lifts without running off both ends of the rocker pad, but there are secondary, nevertheless important, mechanical considerations as well. And the Datsuns are of relatively modest piston displacement, and they don't really need, nor can they use, all the valve lift in the world. Besides, you'd have to drain the oil to get the valves open.