The instance of piston striking valve is right up at the top of the list of things that will ruin your day. It's fundamental tothe four-stroke cycle that valves and pistons come close to one another, but even more so that they never make contact. Checking piston-to-valve clearance may seem like an esoteric procedure that only high-performance engine builders perform. In reality, this critical measurement isn't hard to check-and check it you should, since a host of common modifications can lessen the margin for error.
The impetus for this investigation came during our Dec. '06 story on building your own cylinder heads, "Some Assembly Required." When we asked Speed-O-Motive's Oscar Alvarez to share some tips with us, he immediately replied, "People have to check the piston-to-valve clearance." At first, this didn't really seem to be part of the head-building process, but swapping cylinder heads onto an existing short-block is just one of many actions that can change piston-to-valve clearance.
With this in mind, we returned to Speed-O-Motive for a session with engine assembler Bill Shoemaker. Rather than talking about this procedure in terms of professional engine building, Shoemaker and Alvarez emphasize its importance for the home builder.
"Many people slap a set of high-zoot heads and a 0.600-lift cam on an OEM bottom end that's not made for them," Shoemaker observes. While many aftermarket pistons are now manufactured to accommodate today's high-lift cams, some brands-as well as OEM slugs-aren't made to work with much more than 0.500 lift. Alvarez agrees: "When people buy parts and install them at home, checking piston-to-valve clearance is critical. But most people don't know that." So now you know. We hope to save you some grief-and provide an excuse to buy Play-Doh.
Regardless of what checking...
Regardless of what checking method you use to determine piston-to-valve clearance, several things remain constant. First of all, the cylinder head must be bolted to the block. (Snugged-up is fine; there's no need to torque the head down.) As a matter of course, the entire valvetrain must be installed on the cylinder being checked: lifters, pushrods, and rocker arms. The job can be done with regular valvesprings in place, but a set of lightweight checking springs makes it much easier to rotate the engine.
The first checking method...
The first checking method Speed-O-Motive's Bill Shoemaker showed us doesn't yield a precise piston-to-valve clearance measurement; rather, it's a way to quickly determine if you have at least the minimum amount of clearance needed. Using an old fuel-pump plate as a platform for its magnetic base, he began by setting up a dial indicator on the valve retainer. With the valve closed, the indicator was zeroed.