Racer Brown Chapter Fourteen

The cutting edge should contact the piston on the far side of the piston and in a plane perpendicular to the crankshaft axes. If all's well, chuck the valve stem in a 3/8-inch or larger electric hand drill, flood the valve stem guide bore with lubricating oil, fire up the frill and take a light cut, no more than 0.030 to 0.040-inch. Release the chuck from the valve stem, verify this cutter position (the cutter must be in the newly formed valve relief), then verify the depth of cut with the depth micrometer, subtracting the last figure from the first one. When making the cut, don't force the drill down; this is an interrupted cut and the rate of feed must be very slow. Frequent stops are necessary to re-flood the valve guide bores with lubricant; you don't need to wear out the valve guide bores before the engine is fired up.
Assuming there was piston-to-valve contact initially, the first light cut won't be deep enough, so proceed slowly and cautiously until the relief depth is 0.085 to 0.090-inch. Of course, you won't have much choice because it's a very slow process. Don't move the piston while the cutting operation is in what we laughingly call progress because some of the chips you've made are bound to get caught between the piston and the cylinder bore and the piston will become scored. Remove the head once in a while to blow the chips out, admire your handiwork and to touch up the cutting tool with a sharpening stone. Observe the quality of the surface finish in the machined area. If it's shaggy, the tool needs sharpening and perhaps the rake and relief angles of the tool should be changed. Chatter marks are another story; they can be downright obstinate with a single-lip-cutting tool and an interrupted cut. If things look good, trade cutters and do the same thing for the other valve in the same cylinder, and to the same 0.085 to 0.090-inch depth.
Now for a piston-to-valve clearance check. Remove the head, blow out the chips, clean the gasket face of the head and block and both sides of the head gasket. Remove the piston to clean out the rest of the chips, reinstall the piston but make certain there are no chips on the crankpin or the rod bearing halves or on the mating faces of the rod and cap. Remove the cutting tool from the head; install the service valve assemblies, again using the fuel pump springs in place of the valve springs. Install the head gasket and head and torque the head bolts down fairly snugly. Install the timing chain and cam sprocket and make certain the camshaft dowel pin is engaged in the correct pin hole in the sprocket and that the sprocket pin hole number corresponds with the sprocket tooth number and finally, that the bright (or marked) timing chain links straddle the correct sprocket teeth at the crank and at the cam. Tighten the cam sprocket-retaining bolt fairly snugly. Install the valve lash pads and the rocker arms, set the valve lash to zero and mount the dial indicator on the spring retainer of number 1 intake valve as previously described. Remove the wooden chain wedge. Crank rotation gets difficult with this thing in place. Now, for experience, exercise and to be certain nothing has changed, run through a valve timing check on the intake valve. Rotate the crank and as the piston passes top centre of the induction stroke, go very slowly indeed, particularly if there was piston-to-intake-valve contact initially. If any but normal resistance to crank rotation is encountered, S-T-O-P!! Try to open the intake valve further by pushing down on the rocker arm and watch the dial indicator. If there is no indicator motion, rotate the crank backwards a few degrees and try again, then sneak up on it slowly by rotating the crank in the normal direction and pushing down gently but firmly on the intake rocker arm, all the while watching the dial indicator. When this instrument shows there is but 0.001 to 0.002-inch piston-to-valve clearance, stop and reconsider.

It gets iffy. If the valve timing is correct, if the intake valve relief is properly located and the correct size, if the relief is from 0.085 to 0.090-inch in depth, if actual piston-to-intake valve contact still exists and if stock pistons are being used, one factor announces its ugly self loudly and clearly: Either the stock pistons or the camshaft are (is) O-U-T for this particular application. You'll puncture the piston crown in two places before you can possibly gain adequate piston-to-valve clearance. It's your choice and your decision. If it's a budget operation and/or street or dual-purpose vehicle, or a strange rule in a strange rulebook, then there's not much of an alternative. Keep the stock pistons and exchange the camshaft for one shorter in effective duration, or less intense in valve action, or both. Most camshaft manufacturers will exchange the camshaft and related pieces at little or no cost except shipping charges, provided all pieces show that they have not been operative in a functioning engine. Or go with an after-market piston. These may come equipped with valve reliefs of sufficient depth to put an end to the problem right there. If not, you're up the same creek without the same paddle - but in a different canoe, where there is some piston-to-valve clearance but not enough to be safe. The advantages of most after-market pistons are that they have fairly deep valve reliefs in the first place and, due to additional crown thickness, the reliefs can be readily made deeper safely.
So let's back up a few notches to cover another possibility. Assume there was no initial piston-to-valve contact, but whatever clearance exists may not be enough. After the valve timing has been checked and corrected as required, start with number 1 piston at top centre of the compression stroke and the dial indicator mounted on number 1 intake spring retainer pre-loaded to something more than full valve lift, and with zero valve lash, and with the indicator dial zeroed with the cam lobe in the centre of the heel of the cam lobe. Rotate the crank about 350 degree or until the piston is about 10 degree closer to top centre and push down on the rocker again. Continue to do this until the piston-to-intake valve clearance is about 0.100-inch, then reduce the crank rotation increments to 5 degrees, depressing the rocker arm until it contacts the piston at each stop. Someplace along the line, the piston-to-intake valve clearance each time, as shown by the indicator. At some point, the piston will be as close to the valve as it will get. At this point, push the rocker arm down until the valve contacts the piston three or four times, watching the indicator closely to be certain there is no error. Release the rocker arm, zero the indicator dial, push the rocker arm down three or four more times and read the piston-to-valve clearance directly on the indicator from the zero point.
Let's say the minimum piston-to-intake valve clearance is 0.065-inch. To obtain the required minimum of 0.0900-inch, the intake valve relief must be made 0.025-inch deeper than it is. No sweat with most after-market pistons. There is usually no problem with a stock piston that has been previously modified with a valve relief from 0.085 to 0.090-inch deep. Much more and things get shaky, but this is a reasonable figure. It's best not to consider the valve lash when making these measurements; the lash may be very valuable sometime as a relatively slight but extra "cushion" against engine overspeed damage.
All ya gotta do now is - the same thing with the exhaust valve, but allow a minimum of 0.100-inch piston-to-exhaust valve clearance. The extra 0.010-inch clearance from the intake to the exhaust valve doesn't seem like much, and sometimes it may not be enough, but as previously explained, the camshaft retards itself as a function of engine speed and also as a function of normal attrition of the sprockets and chain. And as the camshaft retards itself, gradually, abruptly, or whatever, the piston-to-exhaust valve clearance decreases while the piston-to-intake valve clearance increases.

YOU'RE ALMOST THERE

With things lookin' good, the lower half of the engine can be assembled and buttoned up. Next, the cylinder head can be assembled, this time using the real valve springs and service valves, lash pads, the correctly numbered rocker arms for their respective valves and the "mousetrap" rocker arm springs.
Now consider the camshaft sprocket-retaining bolt. These things can behave very badly at times by working themselves loose, or falling out altogether, in either case allowing the camshaft sprocket to slip off the nose of the cam. At this point, your world will be filled with many instant problems. In the case of the L-16 and L-18 engines, the mystery of the self-loosening bolt is caused by secondary vibrations inherent in any four0cylinder engine configuration. With the L-24, the same effect is caused more by torsional oscillations of the longer six-cylinder crankshaft, transmitted from the crankshaft to the camshaft by the timing chain. A few precautions taken here may save an engine. First, scrub out the threaded hole in the nose of the camshaft with lacquer thinner and a bristle brush and blow it clean and dry with compressed air. Ditto for the bolt itself. Then make certain the bolt doesn't "bottom" in the threaded hole. Place a new split-type lock-washer against the head of the bolt, followed by the fuel pump eccentric-flat washer assembly. Put a few drops of Loctite along the threads of the bolt and make sure it is evenly distributed. Install the bolt and tighten it to 70 pounds-feet of torque. The book says 43-1/2 pounds-feet, but 70 is a better number. That should hold it, but recheck the torque occasionally just to be certain it isn't in the process of working loose.
If the stock fuel pump is to be discarded in favour of a high-volume electric fuel pump for a race engine, a more positive fix can be made. In this case, the fuel pump eccentric is not necessary and should be left off, replacing it with a steel flat washer of about the same thickness. Do everything outlined above, except drill the bolt head through to accept a strand of 0.060-inch diameter stainless steel safety wire. Don't forget the lock-washer and the: Loctite." Tighten the bolt to 70 pounds-feet of torque, then safety wire the bolt head tightly and in approved aircraft fashion to one of the spokes in the cam sprocket in such a way that the bolt cannot work loose. Then make a plate to plug up the fuel pump opening in the front engine cover and bolt it in place with a fuel pump gasket between the plate and front cover to prevent and oil leak at this point.
When the cylinder head assembly is completed, install the head with a new cylinder head gasket and (preferably) new cylinder head bolts that have been Magnaflux inspected. Needless to say, the crankshaft and camshaft must first be positioned correctly, and you'll have to fight the battle of the chain links and sprockets again. The boltholes in the block should be cleaned with the correct bottoming tap and the holes blown clean and dry with compressed air. Install new hardened flat washers between the cylinder head and the bolt heads.
Just to keep in practice, and to be sure the chain and sprockets were installed correctly, and to find out what has been lost by a full valve spring load, run through a valve timing check again, hopefully for the last time before the Great Day of the first engine fire-up. You will probably lose a couple of degrees in effective duration and the camshaft may show as being slightly retarded from the last valve timing check figures. If it's a slight amount, don't worry about it; you will no doubt have to advance the camshaft soon enough. If it's a large amount in either direction, something was installed incorrectly and the reason for the discrepancy must be tracked down and corrected before going any further.

The piston casting or forging permitting, the intake and exhaust valve reliefs should be joined in a straight line, eliminating any "scallops" between the reliefs to improve breathing during the overlap period at very high engine speeds. This is not advisable with stock pistons relieved to the maximum (about 0.125-inch) depth. Finally, a radius should be ground at the intersection of the top of the piston and the deep side of each valve relief.
When you do-it-yourselfers get sick and tired of doing it yourself, send the) after-market only) pistons back to the manufacturer with explicit instructions regarding the required corrections to the valve relief depth, diameter, bottom corner radius, top corner radius, etc. They may not like it, but they will probably do it for a nominal (?) charge. If the manufacturer gives you any romance, bundle the pistons up and send them to: Smith Brothers' Manufacturing Company (The guys who make pushrods. You remember pushrods?), c/o Hank Smith, 1201 North Azusa Canyon Road, West Covina, Calif. 91790 (213) 338-8026). Give Hank the same instructions and he'll do it quickly, accurately and usually for no more than about $5.00 per piston. The Smiths will also do it to stock pistons but you will have to supply tem with a sample marked piston, the valves used for marking the piston and their relationship in diameter to the service valves. Be certain to mark each piston correctly "front" so the offset piston pin bore won't be reversed. Include the required depth of the valve reliefs from the top of the piston, a plan and cross-sectional sketch of the desired reliefs, the valve-to-cylinder bore angle of 12 degree and last but highly important, a sketch of the valve sequence down the cylinder had from front to back there is a valve sequence reversal in the centre of all L-series cylinder heads. It's a good plan to number all pistons and refer to intake or exhaust reliefs with a felt-marking pen on the top of each piston.
After the pistons have been reworked, it is essential that they be rechecked for piston-to-both valve clearances, just to be certain no one goofed. By now you probably wish you had a zipper on the engine. It's a drag but a necessary one. This time, the only difference is that the piston-to-valve clearance check must be performed for all pistons and all valves, and in the manner described with the dial indicator to give a direct reading. A "plus" tolerance is fine, and a "minus" tolerance, as long as it doesn't exceed about 0.005-inch, is acceptable.