Lots of displacement, more compression, and a ton of valve lift used to be the recipe for a typical professional-level drag-race motor. Today, this same description could just as easily be a serious street engine. If your next motor has a combination including lots of compression and cam, you will need to know how to check valve-to-piston clearance. Today's aggressive cams combined with high-flow cylinder heads thrive on large valve-lift numbers, so even a flat-top piston motor may need some help in the valve-clearance department. The best way to prevent smacking a valve or two up against a piston is to know exactly what the clearance is before you torque that last head bolt. Let's go through the simple clay method of checking this clearance. Your valves will thank you. So will your wallet. Cam Timing versus Valve-to-Piston ClearanceThe exhaust valve is generally closest to the piston at 10 degrees BTDC, while the intake valve is generally closest to the piston at 10 degrees after top dead center (ATDC). Given these clearances, it should become obvious that either advancing or retarding the camshaft will result in tighter or looser valve-to-piston clearances. If you anticipate advancing or retarding the cam, you should check valve-to-piston clearance in both scenarios before assembling the engine. CHANGE RESULT Advance cam timing Decreases intake valve-to-piston clearance, increases exhaust valve-to-piston clearance Retard cam timing Decreases exhaust valve-to-piston clearance, increases intake valve-to-piston clearance VALVE-TO-PISTON SPECS Intake and exhaust: Minimum 0.100-inch depth Minimum 0.050-inch radially Piston dome to head: 0.050-inch minimum* Piston to spark plug: 0.050-inch minimum* *Steel-connecting-rod engines only Variables That Affect VALVE-TO-Piston ClearanceIntake centerline (cam position)DurationValve liftRocker ratioValve diameterPiston top (domed, dish, or flat)Shape and angle of piston valve reliefsValve angle Piston deck heightHead-gasket thicknessValve float at high rpm (loss of spring control)Pushrod deflection at high rpmAngle milling heads This is what happens when valve meets piston. Consider yourself lucky if this is all that happens.We were thinking of running aluminum rods in a daily driver. Who's with us?This is what happens when valve meets piston. Consider yourself lucky if this is all that We started with our dismembered 489ci Rat (4.250-inch stroke, 0.030-over 454), which will debut in a couple of months. We mocked up the crank, camshaft, and one piston and rod along with a cylinder head. The key to accurate valve-to-piston clearance is to first degree the cam to ensure it is accurately phased. Any change to cam timing, either advancing or retarding, will affect valve-to-piston clearance.We started with our dismembered 489ci Rat (4.250-inch stroke, 0.030-over 454), which will Use three head bolts to tighten the head. Next, mock up both the intake and exhaust valvetrains for cylinder No. 1. Rotate the valves through at least one complete cycle, preferably two. Move carefully here, and if the crank stops, don't force it. Back the crank up and pull the head. We used checking springs, but using the actual springs should duplicate any deflection in the valvetrain.Use three head bolts to tighten the head. Next, mock up both the intake and exhaust valvet We cut the intake valve clay impression in half with a razor blade to easily check the amount of intake valve-to-piston clearance. We had in excess of 0.200 inch on the intake side, so we have no problem here.We cut the intake valve clay impression in half with a razor blade to easily check the amo The first step is to position some modeling clay (Play-Doh also works-your kid won't mind) over the intake- and exhaust-valve pockets. Remember to use the same thickness head gasket as the one you will ultimately use. Spreading a tiny film of oil on the valves will prevent the clay from sticking.The first step is to position some modeling clay (Play-Doh also works-your kid won't mind) Yank the head. This will reveal the indentations left by the valves. You can use the long end of a Vernier caliper to measure the thickness if you like, but at some point, take a minute to carefully study the indentations and the relationship between the valves and piston.Yank the head. This will reveal the indentations left by the valves. You can use the long Next, we tried a much larger aftermarket aluminum big-block cylinder head with 2.300/1.88-inch intake and exhaust valves. The first thing we discovered was that the large intake valve hit the piston just before the piston reached top dead center (TDC). We hand-filed the area, but this moved the interference to the crown of the valve relief in the piston. This would require milling larger valve reliefs.Next, we tried a much larger aftermarket aluminum big-block cylinder head with 2.300/1.88- To mark the intake- and exhaust-valve centerlines in the piston, JGM uses a tool made from an old valve that acts as a transfer punch. We set the piston at 10 degrees before top dead center (BTDC) and punched both valve centerlines in the piston. Any good machine shop can then use these centerlines (along with the stock valve angle) to fly-cut all eight pistons for larger valve reliefs.To mark the intake- and exhaust-valve centerlines in the piston, JGM uses a tool made from Here we used a Vernier caliper to simulate the radius of a 2.300-inch intake valve. While in this photo it appears the larger valve will clear, it in fact hits all along the top of the piston relief crown.Here we used a Vernier caliper to simulate the radius of a 2.300-inch intake valve. While SOURCES COMP Cams Jim Grubbs Motorsports 28130 Crocker Ave. Unit 331 Valencia CA 91355 661-257-0101 Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!
