How do you check piston-to-valve clearance?
The targets: at least 0.080 in on the intake valve and 0.100 in on the exhaust (the exhaust runs hotter and expands more). Don't go below these, especially for high rpm.
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The clay method, in outline:
- Make sure everything is in good shape first — no sloppy timing chain, worn guides, loose rockers or bent pushrods — or the reading is off.
- Install the cam at the timing you'll run, with solid checking lifters (or lifters that won't collapse) on the cylinder you're testing.
- Spread clay (Play-Doh works) on the piston top, install the head with its gasket and light checking springs, and torque it.
- Set the rockers, rotate the engine by hand through the valve events, then pull the head and measure the thinnest point in the clay over each valve pocket.
A degree wheel, piston stop and dial indicator give a more precise check.
The only way to know for sure is to check piston-to-valve clearance yourself on the actual engine rather than relying on a vendor's yes/no. What we've seen used is the modeling clay method: put clay on the piston, assemble the head with the cam installed, turn the engine through by hand, then pull the head and measure the thinnest spot of clay under each valve. This matters most when changing heads, for example 706 heads on a 6.0 to raise compression, or when running a bigger cam.
Check this first:
- Cam installed at its intended position before checking
- Turn the engine by hand only, never on the starter
Careful: Skipping this check on a head or cam change risks valve-to-piston contact and major engine damage.
Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.
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Check this first
- Cam installed at its intended position before checking
- Turn the engine by hand only, never on the starter
Try this, then this
- Clay check first
- If tight, degree the cam to confirm installed position
- Flycut or smaller cam if still under minimum
How you know it's fixed
- Measured clearance at or above .080" intake / .100" exhaust on the thinnest clay spot
Still not fixed?
- If clearance is under the minimum, flycut the pistons or choose a smaller cam
Time and cost: A tube of clay and an afternoon; flycutting is machine-shop money