This photo illustrates where the 30-degree back-cut is placed on the back side of the 45-degree seat on the valve. If you have access to a flow bench, you might also try experimenting with alternative angles.
A Question Of Back-Cutting
Rob Scheck, via CarCraft.com: I just finished reading the Dec. '06 issue. Your timing could not have been better. I was a bit of a gearhead during my misspent youth. I read all the engine hot-rodding publications that were available. I still have copies of Car Craft from the mid '70s. But I have been out of the game a long time. I have a '65 Mustang convertible with a 289 that is long overdue for a rebuild. I found Jeff Smith's article about getting more horsepower without spending a lot very interesting. I have one question. Is the 30-degree back-cut done in lieu of a triple-angle valve job?
Jeff Smith: The question, Rob, points out that I should have done a better job of explaining the process. The 30-degree back-cut is placed just above the 45-degree seat angle on the valve and is used in addition to a typical three-angle valve job. The idea is to make it easier for the air to make the transition from the throat area of the port past the intake valve. The exhaust-valve back-cut merely enhances flow in the opposite direction. Again, this trick back-cut does not work on all ports, but in your case, with a stock Ford head, we would definitely add the back-cut on both intake and exhaust valves. It will help.
A Lockable Question
Robert Adams, Fall River Mills, CA: Please settle an argument for us. How many times can you use a polylock in valve adjustments, and is the presumed protection offered by this setup significantly better than standard locking nuts? My son and I hear various arguments about the benefit of a nonscarring locking device, and we've noticed that the manufacturers of most roller rockers send some sort of locknut setup with the roller rockers when you purchase them.
To adjust a polylock so it's tight, we prefer to tighten the locking Allen screw against the stud and then gently tighten the polylock an additional 11/48 to 11/44 of a flat. This eliminates the need to put excess pressure on the Allen wrench.
Jeff Smith: You've landed right in the middle of one of my pet peeves, Robert, and the source of one of the more durable high-performance automotive myths. The polylock is probably one of the best inventions for adjustable valvetrains in the last 40 years. It is simply a deep, threaded hex nut featuring a small, locking Allen screw in the top of the fastener. This allows the polylock to adjust the lash on any engine and then be held in place when the Allen screw is tightened down against the top of the rocker stud.
The old pinch nuts as used on original small- and big-block Chevy engines worked well as long as they were not disturbed after their initial adjustment. The problem surfaces when the locking nut is removed and subsequently reused. Two things happen when the nut is reused. First, the pinched top portion of the locking nut tends to score the threads on the stud. Worse yet, after several adjustments, the nut is no longer capable of maintaining its lock on the position, requiring more frequent adjustment.
I believe this is where the wives' tale or urban myth was born that mechanical-lifter camshafts require almost constant attention to maintain their lash adjustments. If the locking mechanism, such as a polylock, is properly secured, the only way the lash can change is if something is either wearing or bending. I can tell you from personal experience with a mechanical roller cam I've had in a small-block Chevy for 15 years that over the course of perhaps 5,000 street miles and two, 90-mile, wide-open-throttle, 6,000-rpm runs at the Pony Express Open Road Race in Nevada the lash has not changed more than 0.001 inch or so for all 16 valves. This is probably the best way to illustrate how well a polylock works in maintaining the lash adjustment either for a hydraulic-lifter or a mechanical-lifter camshaft.