OLDGMCTRUCKS.COM - GMC ENGINE VALVE ADJUSTMENT TRICK
Copyright 2003 oldGMCtrucks.com  
Do not reproduce this document for publication without prior written permission.

 Try this simple and effective method: 

Gather up the tools you will need; a 5/8” box wrench for the cover and valve “lock”0 nuts, and a large flat screw driver for the adjusters themselves, a trouble lamp (to see flywheel marks), a few clean rags.  Now fire up your motor.

1) Get the engine to operating temperature – I mean really good and hot!  Let it run for a while because you need the motor as warm as possible to do this right.   Shut it down.
2) Remove the valve cover and find Top Dead Center on the #1 cylinder.
To do this, crank the engine slowly by hand by turning the fanblade and pressing on the V-Belt to hold tension so it will crank the motor. Keep your eye on the #1 intake valve (second from the front). When the intake valve starts to close (rise up), continue to crank slowly while watching the flywheel timing peephole.  When the correct timing mark aligns with the pointer, (between the U and the C for the 228, 248 or the ball for the 270) you are at #1 TDC. It takes a little practice to hold the adjuster in the same position while you snug down on the lock nut.  

3) Use a feeler gauge set at .012 for the intakes and .020 for the exhaust valves. I usually use two different feeler gauges to make it easier. Slide the gauge in and adjust the clearance between the rocker and the top of the valve. Use a 5/8 wrench and a flathead screwdriver on the side of the rocker above the pushrod.  Turn them so the gauges are just slipping by with a little bit of nip/drag, but not tight! 
Make the adjustments in the following order: 

# 1 Intake & Exhaust
# 2 Intake
# 3 Exhaust
# 4 Intake
# 5 Exhaust

4) Spin the motor over by hand one revolution, the engine now should have #6 cylinder at top dead center (TDC).  Now you can adjust the remaining valves in the following order: 
# 6 Intake & Exhaust
# 5 Intake
# 4 Exhaust
# 3 Intake
# 2 Exhaust

**Another way to remember the order is with #1 at top dead center, start from the front and do the first three, skip one, do one, skip one, do one, skip one, do one. Then spin to # 6 TDC and start from the back and do the same thing, first three, skip one, etc.  Your GMC engine should sound like a nice quiet sewing machine. 

If it doesn't purr like a kitten, go back through the instructions above and re-check everything.