Moparts Tech Archive

Big Block

Checking Piston-To-Valve Clearance

Checking Piston-To-Valve Clearance
With Heads Installed
If the heads are on the engine, you can check the clearance using a degree wheel, a dial
indicator with magnetic base, a hand style valve spring compressor, and two valve checking
springs.
Use the following method for measurement:
1. Remove Rocker Arm Covers
2. Remove Fan, Fan Shroud, Belt(s), and Crankshaft Pulley.
3. Remove Spark Plugs.
4. Determine Top Dead Center on Cylinder Number 1 using a degree wheel.
5. Remove Intake and Exhaust Rocker Arms From Cylinder Number 1
6. Remove Intake and Exhaust Valve Springs From Cylinder Number 1
7. Install Checking Springs.
8. Install Dial Indicator directly above intake valve, and set between .300 and .400.
9. Note reading on paper.
10. Rotate Engine to 15 degrees before top dead center (15 BTDC).
11. Press down on valve until valve meets piston. Note reading on paper.
12. Subtract value from Step 11 from value in Step 8. This measurement is TC in the formula below.
13. Repeat steps 10 thru 12 at 10 BTDC, 5 BTDC, 0 TDC, 5 ATDC, 10 ATDC, 15 ATDC
14. Repeat steps 5 through 8 for exhaust.
15. Install dial indicator tip against hole in intake pushrod ball.
16. Set dial indicator between .000 and .100. with lifter on heel of cam (base circle).
17. Note reading on paper.
18. Rotate Engine to 15 degrees before top dead center.
19. Note dial indicator reading. This measurement is LL in the formula below.
20. Repeat step 17 at 10 BTDC, 5 BTDC, 0 TDC, 5 ATDC, 10 ATDC, 15 ATDC
21. Repeat steps 13 through 18 for exhaust.
22. Apply these measurements to the formula below.

Where:
Total Clearance, Seated Valve To Piston = TC
Cam Lobe Lift Measured at Lifter = LL
Rocker Arm Ratio = RR
Piston to Valve Clearance = VC

Apply This Formula:
TC - (LL x RR) = VC

This will give you a valve-piston total clearance map, and a lobe lift map for the cam you have installed.
You would need to get the lobe lift figures at TDC for the cam you want to install to determine clearances
with the new cam.

Jason
Dartsport 73 340
Duster 70 340

I prefer checking the clearance with the rocker arm and pushrod installed, and either “0” lash or lash as will be run. Otherwise, there is too much variance from the theoretical rocker ratio and other geometry phenomena to get a completely accurate number, in my opinion. Also, if the clearance shows to be anywhere near the minimum I would also advise to go back and check at every degree in the area of least clearance as there can be substantial changes within 5 degrees rotation, especially with an aggressive cam.

FYI, for those who haven't done it yet. I used the following set up:

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