Trying to figure out how much I can shave the heads with 0.030" Cometic head gaskets, and a cam with 590 lift on the intake and 610 lift on the exhaust. With stock pistons.
What would stock P:V clearance matter if you're changing the cam? Things like cam duration, valve lift, and how the cam is indexed all play a part in how close the valves come to the pistons.
You know they make a putty that you stick on the top of the piston, put the head/gasket on, turn the engine over, and measure the clearances. It's the only right way to measure it and be safe.
What would stock P:V clearance matter if you're changing the cam? Things like cam duration, valve lift, and how the cam is indexed all play a part in how close the valves come to the pistons.
You know they make a putty that you stick on the top of the piston, put the head/gasket on, turn the engine over, and measure the clearances. It's the only right way to measure it and be safe.
DB's right on with this one. It's a PITA, but it's the only right way to do it.
Most engines have so much clearance that it never should matter really. I am running a .653 lift even 2 or 4 degrees advanced in my Pontiac motor which is like .800 for a BB Chevy and I have zero issues.
Steve, you *should* be able to mill .030" off a stock head and be safe with a .030" gasket. I've milled .050" off stock heads that were used with stock .051" head gaskets and had cams with your duration. It's not the peak valve lift that gets you, it's the valve lift at TDC that affects the clearance. A larger duration cam will have more lift at TDC and that's where the piston and valve get close (specifically 8-10 degrees BTDC for the exhaust, 8-10 ATDC intake).
What will get you in trouble on the exhaust side even if the clearance is ok is valve float. You can have good clearance but if the spring pressure is low you'll bend a valve.
Ultimately, you do need to check it. The way I do it is I replace the valve springs with light checking springs on cylinder number 1 and assemble the valvetrain on that cylinder with *ZERO* preload on the lifter. Then I bolt a small steel plate to the valve cover rail and set a magnetic base dial indicator on it with the tip of the indicator on the retainer. You need to make sure that the plunger on the lifter does not collapse into the lifter after you set the preload for the check or you'll get a false high figure. A stock lifter that has been modified to a solid lifter is great, but you can also take an oil can and pump oil into the hole into the side of the lifter to get it completely full. When you can't push the plunger into the lifter with finger pressure you're ok.
On the intake side, start your checking with the piston a few degrees before TDC. Zero the indicator and push the valve down until it lightly touches the piston. Write down that number. Rotate the engine a couple of degrees, zero the indicator, and check again. You're going to see about a .007" change per degree of crank rotation. Do it over and over. As you do this you will see that the clearance gets tighter, then starts to grow again. The tightest figure you obtain is the P to V number. On the exhaust you'll start well ahead of TDC checking it.
A degree wheel is nice to have, but it's not absolutely necessary.
Steve, you *should* be able to mill .030" off a stock head and be safe with a .030" gasket. I've milled .050" off stock heads that were used with stock .051" head gaskets and had cams with your duration. It's not the peak valve lift that gets you, it's the valve lift at TDC that affects the clearance. A larger duration cam will have more lift at TDC and that's where the piston and valve get close (specifically 8-10 degrees BTDC for the exhaust, 8-10 ATDC intake).
What will get you in trouble on the exhaust side even if the clearance is ok is valve float. You can have good clearance but if the spring pressure is low you'll bend a valve.
Ultimately, you do need to check it. The way I do it is I replace the valve springs with light checking springs on cylinder number 1 and assemble the valvetrain on that cylinder with *ZERO* preload on the lifter. Then I bolt a small steel plate to the valve cover rail and set a magnetic base dial indicator on it with the tip of the indicator on the retainer. You need to make sure that the plunger on the lifter does not collapse into the lifter after you set the preload for the check or you'll get a false high figure. A stock lifter that has been modified to a solid lifter is great, but you can also take an oil can and pump oil into the hole into the side of the lifter to get it completely full. When you can't push the plunger into the lifter with finger pressure you're ok.
On the intake side, start your checking with the piston a few degrees before TDC. Zero the indicator and push the valve down until it lightly touches the piston. Write down that number. Rotate the engine a couple of degrees, zero the indicator, and check again. You're going to see about a .007" change per degree of crank rotation. Do it over and over. As you do this you will see that the clearance gets tighter, then starts to grow again. The tightest figure you obtain is the P to V number. On the exhaust you'll start well ahead of TDC checking it.
A degree wheel is nice to have, but it's not absolutely necessary.
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