Some tips for doing a valve job
Question:
After checking the compression on my 1951 Willys with the 473 F head Hurricane four-cylinder engine I determined that it needed a valve job. I used a Magnaflux Spotcheck Junior kit as suggested in a previous Mechanic on Duty tip to check for cracks and we found none, so I sent the head out for a valve job, and told the machinist to take 20 thousandths off the head in the process in order to increase the compression and eliminate any warping. We did not take the engine out of the car, and the machinist has agreed to come to my garage to do the exhaust valves. He will grind them in, and I will do the lapping. Do you see any problems with that?
Answer:
That should work out fine provided the block is still flat. You can check for flatness with a steel straightedge placed on edge the length of the block, using a feeler gauge to make sure that it is still flat all the way across. Check in several places to make sure it is uniform. Otherwise you may have to get the block decked too. A good thick Fel-Pro head gasket can make up for slight unevenness, but if the block is out by four thousandths, you will need to have it ground flat for it to seal properly, especially with the increased compression.
I have never examined one of those engines carefully enough to know how much clearance the valves have in the combustion chambers when the engine came from the factory. And the head may have been shaved before during a previous overhaul. If you are going to knock off what may be an additional 20 thousandths, you may want to verify the clearances.
The easiest way to do that is to “clay” both the intake and exhaust valves to make sure they will not bump the piston on the intake side, and will not bump the head on the exhaust side. To do that, the valves must be installed and adjusted cold. You then put a lump of children’s modeling clay on the top of the piston under the intake valves, and another lump in the head above the exhaust valves. Shoot a little WD40 on the clay so it won’t stick to the valves. Install your old head gasket and torque the head into place per the diagram in your shop manual. And then—using a big wrench on the vibration damper nut—turn the engine over very slowly with the spark plugs removed. If it starts to bind at all, stop immediately. You will damage the valves if you don’t. If you encounter no resistance, rotate it through for two revolutions. When the valves are fully opened they will make impressions in the clay, and that way you can check the clearance.
Now remove the head. Use a knife to cut straight down through the clay and then measure the clearance. Valves need at least 40-thousandths clearance fully opened. That’s because as the engine heats up all of its components will expand, including rods and pistons. If you do not have at least the above clearance, you may need to recess the valves, and possibly install new valve seats.
Any parts, gaskets etc. that you might require are available from Egge Machine at modest prices.
