Adjusting valves on a flathead engine

July 1, 2019 | By Staff


I have acquired a 1937 Packard 120 coupe that is all original. A retired mechanic who lives down the street tells me that its valves need adjusting, based on the exhaust sound. I have adjusted valves on cars with overhead valves, but I have never done a flathead. How do you go about it?


There’s not much to it. Most older cars such as your Packard need their valves adjusted hot with the engine running, but other makes specify that the valves be adjusted cold. Since you need to adjust them hot on your car, work outdoors; preferably with a nice breeze blowing because of the danger of carbon monoxide poisoning from exhaust gasses.

Things You’ll Need:

• New valve cover gaskets

• Permatex sealer

Tappet wrenches (If you can find a set; otherwise open-end wrenches will do.)

• 100% cotton work gloves with fingertips cut off.

First you will need to jack up the car, remove the front passenger-side wheel, then the inner fender panel, and then climb under the fender to get to the side of the engine where the tappets are located. On some other cars, there is a door or panel in the fender apron that permits you to work on the valves.

To warm the engine, start the car, then pull out the throttle or adjust the carburetor idle speed screw so the engine runs at a fast idle (about 600 rpm) while you’re warming it up. Let it run for at least 20 minutes so all the metal parts can expand to running clearances. Don’t trust your car’s temperature gauge; it only measures how warm the head is.

Before beginning work on the engine, put on a pair of 100% cotton work gloves with the fingertips cut off. The gloves are to prevent burns caused by contact with the exhaust manifold. Never use gloves made of synthetic material because they can melt and cause burns.

When it is thoroughly warmed, adjust the engine back down to a slow idle (450-500 rpm) but not so slow that it bucks or rolls. If the idle is too slow the cylinder walls will be starved for oil—a very dangerous situation. Next, loosen the bolts holding the tappet chest covers on, and then pop them free with a sharp putty knife. Take care though, because with the covers off, a little hot oil may run out onto the side of the engine. Don’t worry about it now though; you can wipe it up later.

Adjust all of the exhaust valves first, and then do the intakes. Check each valve clearance with a feeler gauge of the correct dimension as specified in your shop manual. I always like to add an extra thousandth to the specifications for the exhaust valves over what is called for in the shop manual to help prevent them from burning later. The engine will be a tiny bit noisier, but you will be less likely to need a valve job in the near future.

Many side-valve engines made after about 1940 have self-locking adjusters. With them you only have to loosen the locknut with one wrench and make the adjustment with another wrench. But because your car was made in the ’30s, it will have the old-style lifters, and that means you will need to use three wrenches simultaneously to do the job.

The bottom wrench is required to hold the lifter still while you use the middle wrench to loosen the lock nut, then you use the top wrench to make the adjustment. Tappet wrenches, which are thin open-end wrenches, are the easiest to manipulate, but ordinary open-end wrenches filed thin enough to fit on the adjusters will work for the task.

Start by placing a wrench on the lifter, then using the second wrench, loosen the lock nut just enough to permit the adjuster to move. Hold these wrenches in your left hand with your index finger between them, then turn the adjuster with the third wrench using your right hand. (Reverse this if you’re a lefty).

Check for the correct gap using a feeler gauge, then verify it by attempting to slip the next larger size gauge in the gap. If you’ve got it right, the larger gauge won’t go. As an example, if the proper clearance for a valve is 10 thousandths, a gauge of 11 thousandths should not clear. When you have the gap where you want it, squeeze the two bottom wrenches toward each other to lock in your adjustment, then check the clearance one final time.

When you are satisfied that all the valves are adjusted properly, reinstall the valve covers using new cork gaskets. First scrape the mating surfaces clean with a putty knife, and then apply a little sealer to the tappet covers and gently press the gaskets into place on them. There is no need to apply sealer to both mating surfaces as there is little pressure on these seals, and then you will be able to remove the valve covers for future adjustments without destroying the gaskets if one side is not glued in place.

If you use silicone sealer, don’t overdo it because bits of cured sealer can break loose and be swept into tiny oil galleries and clog them. (I prefer good old Permatex for this job myself.) Don’t over-tighten the bolts that hold the valve covers in place because doing so will crush and ruin the cork seals, and can distort the tin valve covers as well.

That’s all there is to it for your Depression-era Packard, but some makes such as early Studebakers and the air-cooled Volkswagens require that their valves be set cold with the engine shut off. Cold means that the engine has not been running for several hours and is at room temperature (about 70 degrees). Never attempt to do a cold valve adjustment on an engine that has been running within the last couple of hours because most engines—especially old cast iron ones—will hold heat for a long time. Also, don’t try to adjust your valves cold on a very hot or cold day.

One by one, each cylinder must be brought to top dead center on its compression stroke so that both of its valves are completely closed before you can adjust them properly. There are a couple of ways to do this: One is to attach a timing light to each spark plug wire in turn, then turn the engine over until the light just comes on. Disconnect and ground the coil high-tension wire before attempting to crank the engine so it won’t start while you’re turning it over. Another way to determine when a piston is at the top on the compression stroke is to take its spark plug out and put your thumb over the hole. Then crank the engine until air is forced past your thumb.

Flathead engines that are normally set with the engine hot and running can be adjusted cold if you know what the correct setting is when it is at room temperature. One way to find out is to set the valves hot and running the first time, then let the engine cool completely to room temperature and measure the gaps. Keep sliding thicker gauges into the gap until you get to one that won’t go. The thickest gauge to go in is the correct cold setting.

The only reason manufacturers specified that engines be hot and running to set tappets is to ensure they are adjusted at a specific temperature. The engine’s thermostat helps guarantee this. So if you set your valves cold, they must be set at the specific temperature for that particular clearance. That’s all there is to it. Don’t forget to check for oil leaks after you reinstall the tappet covers.