How To: Check 302/351 Mustang Piston to Valve Clearance

Piston to Valve Clearance Kit

What is piston to valve clearance? In a nut shell, it is the clearance between the valve and piston; which is something any one should check for when building an engine. The magic number for a street application is one hundred and twenty thousandths clearance between the valve and piston. Things you’ll need before checking piston to valve clearance is obviously your short block, cylinder head, head gasket, roller rockers, push rods, and a 'dog bone' or lifter retainer. Throw in our special piston to valve clearance kit, digital or dial caliper, and some good old Play-Doh.

Required Tools

• Socket Set
• Socket Wrench
• Engine Oil
• Play-Doh
• WD-40
• Assembly Lubricant
• Digital or Dial Caliper
• Razor Blade

Installation

1. Apply a small amount of oil to the lifters.
2. Install the lifters into the engine block with the holes facing up and toward the center of the engine.
3. Position one of the lifter retainers between the lifters in their marked orientation.
4. These are also referred to as ‘dog bones’.
5. Position your head gasket and the Play-Doh across the piston closest to the center of the block.
6. Spray WD-40 on the bottom of the valves to prevent any Play-Doh from sticking.
7. Position the head onto the engine block.
8. Apply assembly lube to the ends of the push rods.
9. Slide the push rods through the guide plates.
10. Position two roller rockers onto the guide plate and push rods.
11. Tighten your roller rocker set screws to zero lash.
12. Zero lash is achieved when you can just barely turn the pushrod with your hands.
13. Whatever cylinder you’re working on, snug down the four cylinder head bolts or studs surrounding that particular cylinder. (Or whichever method you prefer to hold down the head while you turn the engine over.)
14. Make two full rotations and return the key-way to the twelve o’clock position.
15. Disassemble your rocker arms, pushrods, and remove the head.
16. If you notice, with a Trick Flow Stage 1 cam, 1.6 ratio roller rockers, and stock length push rods, the valves touched the Play-Doh.
17. Cut the Play-Doh in half and measure the center point width with a caliper where the valve made contact with the Play-Doh.
18. One hundred and twenty thousandths is the absolute minimum when building an engine for a street application.
19. If your valves made more contact and the Play-Doh was less than one hundred and twenty thousandths, contact your local machine shop.
20. If everything checks out okay and your piston to valve clearance is one hundred and twenty thousandths or greater, you’re all finished!

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