Piston/Valve Clearance

Robert Simpson · 09-08-2011, 11:27 PM

What min piston to valve clearance do you guys recomend on a sbc stocker? I was discussing this with some others with mixed results. A SBC stocker, flat tops, shubeck lifters etc, automatic trans vs a manual? What do you guys run?

Alan Roehrich · 09-08-2011, 11:34 PM

Robert, in all honesty, if the clearance between the intake valve and the valve relief in the piston is GREATER than the clearance between the piston and the head, you'll really have to have something stupid happen to hit an intake valve. For example, if the piston to valve clearance is 0.055", and the piston to head clearance is 0.38" total, they'll almost never hit, you'll have to spin it 9 grand in the water to make them hit, 999 times out of 1000.

Now the exhaust is another story, you really need at least 0.075" to be safe and comfortable.

All of this does of course assume you have really good valvetrain, 7/16" screw in studs, light valves, light retainers, excellent springs, and 3/8" x 0.120" wall pushrods.

Paul Precht · 09-08-2011, 11:46 PM

.015 on intake and .060 on exhaust can work if you're careful.

randy wilson · 09-09-2011, 09:43 AM

I agree with Paul, But to be safe, .025 and .070.

Adger Smith · 09-09-2011, 10:40 AM

Robert,
The thing to remember when you have real tight (close) PTV clearances is they usually never hit when the engine is under load. The valve dynamics change when the engine is in a free wind (no load) situation. That is when the valve gear gets unstable and close clearances can come in contact with one another. Overwinding in the burnout and streching the RPM out at the top of a gear are your enemy. You might want to keep a constant watch on valve spring pressures and conditions. Keep then good and strong, esp the open pressure. Seat pressure can drop a few lbs, but the open pressure falling can put an ex valve in contact with a piston. I like to take a look at the contact of the rocker on the valve stem tip, esp when I'm adjusting valves. Signs of Multiple contacts of the rocker tip and the valve is a sure sign of valve float or loss of spring control. The spring seat or shim being eaten or beat up is also a bad sign of valve control. You can get away with close clearances if you are careful, esp with smaller head dameter, stem valves. Valve gear weight is a big consideration in just how close you can go.
Good Luck...

Alan Roehrich · 09-09-2011, 11:06 AM

One other thing to remember, according to David Reher, you'll hit the valves faster by being "on the chip" during a burnout than just about any other time. Another reason I use a 5500 chip in the burnout, and try very hard to never hit it. I just won't do a high RPM burnout, and will never do a burnout at WOT.

09-08-2011, 11:27 PM	#1
Robert Simpson Senior Member Join Date: Aug 2004 Location: Jacksonville, Florida Posts: 808 Likes: 7 Liked 20 Times in 12 Posts	Piston/Valve Clearance What min piston to valve clearance do you guys recomend on a sbc stocker? I was discussing this with some others with mixed results. A SBC stocker, flat tops, shubeck lifters etc, automatic trans vs a manual? What do you guys run?

09-08-2011, 11:34 PM	#2
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,123 Likes: 1,581 Liked 1,874 Times in 422 Posts	Re: Piston/Valve Clearance Robert, in all honesty, if the clearance between the intake valve and the valve relief in the piston is GREATER than the clearance between the piston and the head, you'll really have to have something stupid happen to hit an intake valve. For example, if the piston to valve clearance is 0.055", and the piston to head clearance is 0.38" total, they'll almost never hit, you'll have to spin it 9 grand in the water to make them hit, 999 times out of 1000. Now the exhaust is another story, you really need at least 0.075" to be safe and comfortable. All of this does of course assume you have really good valvetrain, 7/16" screw in studs, light valves, light retainers, excellent springs, and 3/8" x 0.120" wall pushrods. __________________ Alan Roehrich 212A G/S

09-08-2011, 11:46 PM	#3
Paul Precht Senior Member Join Date: Dec 2001 Location: Elysburg, Pa Posts: 733 Likes: 361 Liked 327 Times in 121 Posts	Re: Piston/Valve Clearance .015 on intake and .060 on exhaust can work if you're careful.

09-09-2011, 09:43 AM	#4
randy wilson Senior Member Join Date: Jan 2007 Location: N/A Posts: 882 Likes: 0 Liked 22 Times in 22 Posts	Re: Piston/Valve Clearance I agree with Paul, But to be safe, .025 and .070. __________________ don,t have one

09-09-2011, 10:40 AM	#5
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,446 Likes: 575 Liked 880 Times in 311 Posts	Re: Piston/Valve Clearance Robert, The thing to remember when you have real tight (close) PTV clearances is they usually never hit when the engine is under load. The valve dynamics change when the engine is in a free wind (no load) situation. That is when the valve gear gets unstable and close clearances can come in contact with one another. Overwinding in the burnout and streching the RPM out at the top of a gear are your enemy. You might want to keep a constant watch on valve spring pressures and conditions. Keep then good and strong, esp the open pressure. Seat pressure can drop a few lbs, but the open pressure falling can put an ex valve in contact with a piston. I like to take a look at the contact of the rocker on the valve stem tip, esp when I'm adjusting valves. Signs of Multiple contacts of the rocker tip and the valve is a sure sign of valve float or loss of spring control. The spring seat or shim being eaten or beat up is also a bad sign of valve control. You can get away with close clearances if you are careful, esp with smaller head dameter, stem valves. Valve gear weight is a big consideration in just how close you can go. Good Luck... __________________ Adger Smith (Former SS)

09-09-2011, 11:06 AM	#6
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,123 Likes: 1,581 Liked 1,874 Times in 422 Posts	Re: Piston/Valve Clearance One other thing to remember, according to David Reher, you'll hit the valves faster by being "on the chip" during a burnout than just about any other time. Another reason I use a 5500 chip in the burnout, and try very hard to never hit it. I just won't do a high RPM burnout, and will never do a burnout at WOT. __________________ Alan Roehrich 212A G/S