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When I first started out in the automotive world, most things mechanical were still a big joyous puzzle… Until I stumbled across a thing called “valve lash.” I could understand most terms I was encountering, but valve lash really got me. When you look up lash in the dictionary you will find, as the last entry, “the clearance or play between moveable, mechanical parts.” Now we are getting somewhere, but what does that have to do with the valves? “Valve lash” is the clearance between the rocker arm tip and the tip of the valve stem. For the beginner would-be tuner, no other operation is less understood, or more fear invoking, than a good lashing of the valves. Let’s break the chain of fear and misunderstanding once and for all…
My good friend Nate Tynan came over to the house in his ’65 Slant Six Dodge surf wagon, full of minor troubles. Among the most offensive of the maladies on the list was an engine lacking on low-end power that shook like Jell-O at idle. This was unacceptable behavior from an engine with less than 20K miles on it.
I had a hunch it was improper valve lash. It could have been a burned valve, but that was highly unlikely given the newness of the engine.
The Slant Six engine has what is known as a solid lifter camshaft and adjustable rockers. To allow the valves to fully close there must be some clearance between the valve stem tip and the tip of the rocker. If there is too little or no clearance, the valve will not shut, the idle will be nasty, and eventually the valves will be destroyed, especially the exhaust valves. By resting for a moment on their seats in the cylinder head, the valves transfer heat to the head. The rest of the time they are open and the exhaust valves are surrounded by hot gases.
If your Slant Six is rocking and you can’t hear the beautiful sewing machine sound of 12 little hammers while idling, then it’s time to get to work.