Rocker Adjustment 101 - I have a question regarding my Pontiac 400. It has a '72 block with '70 GTO heads. I have been adjusting the valve rockers 11/44 turn past zero clearance. But I read that all Pontiacs should be adjusted so the rocker nuts seat on the stud shoulder, and then they are torqued to 20 ft-lb.
But this engine has roller rockers, and a couple of them loosen up. The previous owner installed small-block Chevy rocker nuts, which I think can be tightened only once. According to the previous owner, there is an aftermarket cam in it, but I have no specs. Can I adjust the rockers all the way down? When I do, the valve looks like it may be open. Or should I get locking nuts and keep adjusting them? Do I turn past zero clearance? Any advice would be appreciated, as I am new to Pontiacs and still learning.Steve TruverVia Internet
Jim Taylor Responds: Your questions about camshaft and valvetrain issues are probably the number-one requested topics to Tech Q&A, just ahead of compression ratio ciphering. Along with the other tech Q&A contributors, I have written about your concerns before, but with "new to Pontiac" guys moving up to the great Pontiac V-8 everyday, I'll go through some basics.
You're correct that Pontiac rocker nuts seat on the stud shoulder and are torqued to 20 ft-lb. Technically, the 51/416-inch stud has a conical (cone-shaped) seat above the threads to which the conical shape inside the locking nuts seat. This type of locking device has a large area of interference (friction) fit. I've never seen a factory-installed rocker nut come loose.
Pontiac's stud and nut set will work if during a rebuild you do not mill the block deck, mill the heads, or use a thinner head gasket. Also, you must use the same length replacement valves, pushrods, and rocker arms because factory-assembled engines had predetermined (blueprinted) heights, lengths, and distance measurements calculated to allow proper preload on the lifter when factory jamb nuts were used. The camshaft must have the same base circle height as the original Pontiac cam. The base circle of the cam is where the lifter is not in motion (or valve closed position). This position is where valves are set. Your 11/44 turn past zero should be OK unless you have 0-inch (zero) lash lifters. If you have 0.020-inch preload on a zero lash lifter, the valve will stay open when it should be closed.
The 51/416-inch jamb nut you're using never lasts. You need to get "posi locks," most often called "Poly locks." They are barrel nuts with a setscrew on top. When the set screw is threaded down, it locks by bottoming on the top of the stud. Oil the threads of the barrel nut and moly lube the threads of the set screw for easier locking. Install them in place of the original jamb nuts using a 11/44-11/42 turn past zero preload adjustment.
Never adjust rockers all the way down, especially since you have no specs on your aftermarket cam. The valve staying open could be the least of your problems. Depending on the lift of the cam, the valve could hit the piston.
Try to identify your roller rockers' brand. Then call the company and ask for posi locksfor your Pontiac. The 11/44-turn adjustment suggestion from me is just that-it's probably OK since it has been working, but I recommend following manufacturer's instructions for hydraulic lifter preload.
Performance And Mileage From A Carb Swap? - I'm an HPP subscriber who seeks your vast knowledge. My '76 Trans Am has a freshly rebuilt (500 miles) 455 H.O. with an Edelbrock intake. The drivetrain features a T-10 trans and a 3.73:1 rear. My 750-cfm Holley 3310 vacuum secondary carb is leaking below the diaphragm, and it's old-not to mention it's a gas hog. If I buy a new carb, what would be your recommendation? Can I buy a smaller four-barrel carb and not lose performance? Or should I just get this Holley rebuilt? If a rebuild is the answer, do you know any reasonable shops? -Duane Walker Warminster, PA