Piston/Valve Clearance

[Robert Simpson]

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]

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]

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

[randy wilson]

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

[Adger Smith]

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]

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.

[Robert Simpson]

Thank You guys for your input. It all makes sence. In order to keep things as stable as possiable what spring pressure do you guys run over the nose of the cam? You know min/max? But, to go over what has been said, it seems that people agree that .025" Intake and .065" Exhaust would be safe? With proper spring pressure of course.

[Aubrey N Bruneau]

Robert... these fellows CERTAINLY know more than I do here, so I'm just put'n this out there, as an example of where it can be.... if you're an idiot.
I run a 108 lobe-sep cam, installed 4 degrees retarded. There was just too much gained from this confifuration, for me to turn my back on it.
However... the idiot part
I have about .065" clearance on the intake... and, uh.... .038" on the exhaust
! !
My exhaust lobe is much "calmer" than the intake lobe, so my control problem still emminates on the intake side, which has required some pretty outrageous spring pressures in order to rectify.
I don't use a "chip" in my burnout, and refuse to let the engine go past about 5500-6000 during it.
I have yet to connect my two step, to properly control my launch RPM, so I'm letting out the clutch as the engine accelerates throught the 5000-6000 range.
I shift at 7000-7100, go through traps at 7200... IMMEDIATELY drop into neatral and let engine idle.
Maybe it's a combination of my "old style" techniques, that's letting this ridiculous exhaust piston/valve clearance survive ?

[Adger Smith]

Aubrey,
I can understand the reasons for yours working that way. The ex valve is small, relatively in length and size to the hulk of an intake. Longer stem bigger thicker head on that intake. I would say weight has become your enemy. What about the pusrods and pushrod angles? They are way different in the '09, aren't they?

[Aubrey N Bruneau]

Basically the same angles as a small block, Adger. My pushrod is a custom made heavy-wall Manton. Yes, they helped too. Rockers are BBC 1.7.
Intake and exhaust valves are same length, but you're absolutely right... those intakes are heavy !
Then again, if you saw the intake lobe on my cam, you'd wonder how it can run !
doh !
It really IS that crazy.
Looks like I finally do have it under control though. Didn't get out racing this year ( just closing the deal on our house, so after what we went through the last couple years, this has still been a good one ), but I DID get the valve train under control. Went out, took a few gears. My new tattle tale-tach "blew the wistle" on me, with a couple 7300+ RPM shifts
HEE HEE !