Question about P-V Clearance

No - if you do it the way you describe, the valve spring force will depress the lifter plunger during rotation throwing the measurements off. You must either remove the valve springs and use light enough test springs so that the lifter plunger won't be depressed, or you must disassemble a lifter, and temporarily fix it so the plunger can't be depressed - in effect, make it a solid lifter for the test. Also, you need to be really careful when you remove the clay, and when you section it for measurement that you don't alter the thicknesses you're going to be measuring. You mention peak lift - it has nothing to do with p to v clearance. At peak lift, the pistons are WAY down the cylinder - there's plenty of clearance then.

Here's another way to measure clearance. http://www.diyporting.com/PVC.html

And below I've copied a bit on the 'drop valve' method for measuring clearance. BE SURE TO DEGREE THE CAM before measuring clearance - changing cam timing changes the clearances.

Positive stop – obtain an old spark plug for the Ford engine – most of us have at least one laying around. Put it in your vice, and with a tool of destruction you deem appropriate – cut off the ground tip and bust up the ceramic around the center electrode. Keep pounding to get the ceramic and center electrode removed so you’ve got a good ¾” or so of hollow space inside what used to be the ‘business’ end of the spark plug. If you happen to break the ceramic on the other end of the spark plug, don’t worry about it. Once you’ve got a nice hollow space in the tip of the plug, take a 5/16” tap, and cut threads inside that end of the plug. I found that diameter to go right in without any drilling required. Once you’ve cut threads, screw in a 5/16" bolt about 1.5” to 2” long. Then take your hack saw and cut the head off the bolt. Wouldn’t hurt to take your file/grinder and dress up the freshly cut surface so no sharp edges or shrapnel enter the combustion chamber. Also clean up the threads on the plug so that it easily screws into and out of the plug hole.

Determining TDC – remove the passenger side valve cover. Remove rockers from the intake and exhaust valves on the #1 cylinder. Rotate the engine (socket on crank bolt) until the #1 piston is down the cylinder on the intake stroke. Unscrew the spark plug in that cylinder, and replace it with the positive stop you just made. Only screw the positive stop in hand tight. Install your cam degree wheel and pointer (no easy task in some cases as harmonic balancer and other accessories will have to be removed). You may want to remove all spark plugs at this point because it will make it easier to smoothly turn the engine over in small increments. GENTLY rotate the engine clockwise until the piston comes to rest against the stop. Note the degree wheel reading. GENTLY rotate the engine in the opposite direction until the piston comes to rest against the stop. Note the degree wheel reading. TDC is exactly half way between these readings. Calculate where that position is on the degree wheel, remove the piston stop from the cylinder, and rotate the engine to TDC as you calculated it.

Degreeing the cam – install the camshaft in the straight up or dot to dot position to start with. Set your dial indicator up over the #1 intake valve lifter or pushrod. Rotate the engine clockwise until the intake lifter/pushrod just begins to rise. Follow the dial indicator and stop the rotation when the lifter is at .050” lift. Take a reading on the degree wheel. It should match your cam card information for the number of degrees at .050” lift on intake valve opening. For example, the cam card on an E303 cam says that the intake should be open .050” right at top dead zero – 0 degrees TDC. So if you rotate the engine until your dial indicator shows .050” lift on the intake opening, the your degree wheel should be pointing right at tdc. If you’re within a degree or two, then you can go ahead and check the .050” closing timing, as well as the opening and closing events for the exhaust valve. They should all match up with the cam card info within a degree or two. If, for example, at .050” you’re supposed to be at 0 degrees TDC (from the cam card), and after your dot to dot install you’re reading 4 degrees after TDC at .050”, then the intake valve is opening too late (4 degrees later than it should). To adjust this, you need to make the intake open 4 degrees earlier (right at tdc), so you’ll have to advance the cam by 4 degrees. To do this you slide your indexed timing set off, you select the “4A” slot on the crank gear (instead of the straight up position), align it with the dot on the cam gear and reinstall. Remeasure as described above – you should now read .050” lift right at TDC. Many contemporary cams have assymetrical lobes – so I wouldn’t install the cam using the lobe centerline method – the centerline can be difficult to find. I recommend using the .050” lift timing events for the install – check all 4 of them to be sure the cam is doing what the cam card says it should be doing.

Drop valve measurement – now that the piston is at TDC (be sure you're at TDC during overlap - between exhaust and intake strokes) you can safely remove the valve springs on the intake and exhaust valves. I used an inexpensive KD tool for a small block Chevrolet (Blasphemy!). It bolts into the pedestal and has a crow’s foot that sits over the spring retainer. It’s nothing more than a pry bar. Be sure to screw the rocker bolt far enough into the pedestal to hold the tool. You (or your big friend Bubba) are going to apply significant force on the lever to compress the spring. A small magnet is invaluable for grabbing the keepers once the spring is compressed. Also, a couple of taps (lightly – careful here) with a hammer on the retainer can help free up the keepers. The valves may or may not drop down and touch the piston – be sure it’s at TDC to limit the valve movement. Set up your dial indicator over each valve. Rotate the engine to about 20 degrees BTDC. Now, in 5 degree increments, until you reach 20 degrees ATDC measure the distance between valve closed and valve touching the piston. You can move the valve up and down with your fingers, or if the seals are loose, up with your fingers, gravity will take care of down. You’ll have maps of closed valve to piston contact distances every 5 degrees from 20 BTDC to 20 ATDC for intake and exhaust. Rotate the engine back to 20 BTDC (back the way you just came – if you rotate the other way, the piston will drop all the way to the bottom and you’ve got valves dropping down into the cylinders – DON’T DO THAT!!) and set the dial indicator up over the intake and exhaust lifters (or pushrods if you don't have a dial indicator extension). Make the same measurements on both lobes – a map of lift on intake and exhaust lift at 5 degree increments from 20 BTDC to 20 ATDC. Multiply the lift times the rocker ratio. That is valve lift. At each 5 degree point, subtract valve lift from the valve closed to piston contact measurement. That is your piston to valve clearance. It should be more than .080” on intake and .100” on exhaust for engines revving less than 6500 rpm. As your rpm increases, your clearance should increase. If you’re revving to 8500 rpm, you probably want on the order of .125” to 150” on intake/exhaust to be safe.

You'll need to remove the valve springs and REPLACE them with light test springs if you're gonna use the clay method. And even then, while rotating, you'll need to watch the lifter to be sure that the plunger (spring loaded) isn't depressing. I've found that even the resistance provided by new valve seals and light test springs can be enough to depress the plunger.

What I prefer about the drop valve method is that without the spring in place at all, there's no need to worry about what the lifter is doing. However, if you need to measure the clearance from the edge of your valve to the edge of the relief - clay is the only option. You're looking for .06-.08" edge clearance.

Are you absolutely certain the cam was degree'd when it was installed? I'd want to check the cam timing even if it was pre-assembled. If someone slapped a cam in 'dot to dot' it can still be as much as 4-6 degrees off from the cam card.

It's a heckuva lot easier to check the cam timing now than it will be to try and measure it after it's installed if you have problems with start up/drivability. And you'll learn a bunch about how the motor works. Besides, if you end up later having to go through the problem of changing the cam timing, then that will throw your p to v clearances off, so you'll have to remeasure that as well.

Question: If the whole thing is already assembled, cam installed and degreed, etc. etc. etc. etc... by some shop that is supposed to know what's what.... ummmm... wouldn't they have already checked P-V clearance?