Ask The Mechanic

By Any Other Name

Image by Getty Images

READER: I’ve always wondered why a flathead engine has so many names? I’ve seen it called a flathead, side valve and L-head engine. I know that the engine head is flat for flathead, and all the valves are on one side of the engine for side valve, but I can’t see where the name L-head comes from. Why is this engine called an L-head engine? What are the advantages and disadvantages of this old engine design?

STEVE: The name L-head for this engine comes from the way the engine is built. If you were to take the letter “L” and turn it upside down, and stick the long leg of the letter in the cylinder bore, the lower part of the upside down “L” would rest flat on the engine block, which contains the valves. Thus, the L-head name. As far as advantages, the engine is a simple design and has no pushrods, which allows the lifter to push directly on the valve stem. Because the valves are in the engine block and offset from the cylinder sphere, a stuck valve will not be hit by an upcoming piston. The incoming air mixture and exhaust enter and exit the same side of the engine. The biggest disadvantages of this engine design are that they are not highly efficient and not exactly environmentally friendly.

READER: I got mad at a stubborn stump and tried to get even with my 640 Ford tractor. I kept ramming it in reverse with my box blade. That was a mistake. I broke the ring gear on the rear end. So, I got a new ring gear and pinion (I’m not a tech), and was in the process of putting them in. My neighbor came over and told me that the clearance between my new ring gear and pinion should be the width of a piece of beer can. He just happened to have a few empties on his floor. Is he joking or what?

STEVE: I have heard the beer can story before, and I have no doubt that some use that for gear lash measurement on a ring gear and pinion. I don’t think it is really that far off. However, since the ring gear has the biggest load of anything on the tractor, it needs to be precise. So, I recommend a dial indicator with a magnetic base. You will need to check with the tech manual on this tractor for the specs and setup instructions. This job is not that easy to do correctly. But, you have finished this one, so grab a glass of tea and hire a dozer the next time.


Fan blades that are not surrounded by a shroud can cause a fire on top of a tractor’s engine, especially if the tractor is driven through tall crops … like making the first round in sudangrass or other cane-type crops. The fan blades can clip the tips of crop leaves and deposit them between the head and exhaust manifold, igniting the superdry material. My neighbor called me one night at 3 a.m. and told me my tractor beside the sudangrass field was on fire. Nope, not vandalism. Spontaneous combustionism.

Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email


Past Issues