The picture shows a lash cap like were used on Ducati motorcycles in the early to mid 70s, and may still be in use today. Volkswagen also used lash caps.  You could buy lash caps for a Ducati in many thicknesses. I used the lash caps to set the gap between the lifter and the valve stem. They fit right over the push rod and cannot fall out because of the interference with the valve stem. This makes an easy way to change the valve lash, which changes the valve opening and closing degrees. It is much easier than grinding off the valve stem for more clearance or replacing the valve with a longer stem one for less clearance. With the lash caps I can also cut some small pieces of shim stock and place them up under the cap. This way I can advance or retard the valve timing with the thickness of the shim stock. No, they will not fall out.  Many times while I was racing I would change these pieces of shim stock between races to get more lower end or top end power out of the motor by advancing or retarding the valve timing.

Rear View of  Bob's Water Cooled Engine

There are many things that go into making a motor run well and the opening and closing of the valves at the right time is the most important. With a water-cooled Cushman motor, there is a great deal of valve lash to work with because the water for cooling ran only about 180 degrees. At this low temperature the valve stems did not heat up and expand nearly as much as they would have a much hotter air-cooled engine² so the valve lash gap did change as much. I have run a .008 lash gap on the exhaust valves and when checking the gap with the engine at maximum temperature after a long race I still had .004 clearance. The important thing is that this gives a wide usable range in valve lash to degree your valves opening and closing. You are not married to Cushman’s .016 & .014 minimum valve lash gap that is required for a hotter running air-cooled engine. The important thing is to set the lash to open and close the valves at the correct time. I have run as much as .025 inch valve lash to get the valve open when I wanted it to. I gave up a little valve lift in favor of the valve opening or closing exactly at the right time to produce the maximum power.

The .014 & .016 valve lash was set by Cushman to give a good all around running motor that was easy to start, whether on a scooter, or some piece of farm machinery. We also used Wisconsin valve lifters that had a larger head area, which also affected the open and closing of the valves. Before I go much further I should mention that I cut the length of the valve lifters to just long enough to stick up in the valve chamber and set a lash cap on. The new large valves had longer stems, which allowed them to reach all the way down through the valve chamber to the top of the lash cap. In other words, the valve lash gap is at the very bottom of the valve chamber rather than nearer the center of the chamber as in a stock Cushman engine.

The timing gear is the main thing that sets the open and closing timing of the valves. Cushman made timing marks on the cam gear and on the crankshaft gear to properly align them. The stock setting is great if you want to putt putt down the street on your scooter, but if you want to turn 7000 RPM, and run flat out for 8 miles on a 1/8 mile track, the stock valve timing will not get the job done. I spent many hours filing little offset keys to advance, or retard the valve timing to get it just right.

Photo of the New Cam and Gear. Note the Three Keyways and the Lash Caps

The first racing cams that were made by Bob Sawyer were ground to be in the 30 degrees intake open, and 70 degrees intake closes, 70 degrees exhaust opens, and 30 degrees exhaust closes, and a 109 degree lobe separation configuration. This was a big jump from Cushman’s 20 degree - 60 degree grind. All of Sawyers cams were ground with symmetrical cam lobes, that is, the opening and closing sides were the same profile. Cushman cams are a good example of this design. Many of the modern cam grinds are asymmetrical, with the opening and closing sides having different profiles. The opening side of the lobe opens the valve very quickly, and the closing side of the lobe to shut the valve quickly, but as easy as possible. I have seen some closing ramps on lobes that I wondered how the valve kept the head on the stem. Even though Bob's cams were ground to close tolerances, changing the exact opening and closing timing was still done with the cam gear and valve lash.

This is where my friend Clyd Sturdy, the owner of Associated Gear in Los Angles came thru for me. Clyd ran a Modified Midget as well as running racecars at Bonneville. Clyd machined a set of gears for my Cushman with the crank gear made from 8620 steel and the cam gear made from 4140 steel. These new gears had the same number of teeth as the stock gears, but these were straight cut gears. If you take a Cushman cam gear in one hand, and a crankshaft gear in your other hand and roll the crank gear around the cam gear you will feel the drag as the teeth mesh. We call it " hooking the gear ".  The drag in the teeth takes horsepower and that is why we used the straight cut gear. It makes a little more noise in the motor, but noise is not a factor with a racing motor. When Clyd cut the cam gears he broached three keyways in the gear. The first keyway was the same as Cushman’s placement. The second keyway was 1/2 tooth advanced from the first one, and the third keyway was 1/2 tooth retarded from the first one. Since the new cam was larger in size, we used a woodruff key to hold the cam gear that was the same size as the Ford and Chevy four degree offset key that you could buy at most speed shops. If a straight key or the four-degree offset key did not time the valves close enough, the larger woodruff key made it less of a job of making your own key for an offset anywhere between 2 and 4 degrees.  I really feel that this new gear set help me to win the #1 Championship in the National Modified Midget Association in 1973.

Parts Used in the Water Cooled Cushman Motor

 I will not get into exact valve degrees, as each person has his own idea on what is the best, as well as whether you should advance or retard the valve timing. I will say that since the Cushman had a good torque curve and produced a great deal of low end torque, I found that a little retard of the valve timing was the best for my application.