For some reason, the engine just doesn’t run quite right. Or maybe it fired right up but now doesn’t want to run at all. After ruling out all the easy stuff, you finally begin pulling components for a more complete inspection and discover one or more bent valves.
That’s the kind of thing that an engine builder’s nightmares are made of. Practically every engine builder will crash a valve into a piston sooner or later, and the damage can range from a single bent valve to a completely grenaded engine. So you definitely want to avoid pistons kissing valves whenever possible. Sometimes it will happen during a wide-open-throttle, high rpm blast when the springs can no longer maintain control and a valve lofts into a piston.
Other times a valve can get bent because there’s simply a miscalculation and the valves want to be in the same spot as the pistons at the same time. These can be avoided with just a few easy checks during assembly.
Most people are aware that you should check piston-to-valve clearance to make sure the valve doesn’t contact the piston on its travel up and down the cylinder bore. But there’s also a second check that should be done any time you are using a piston with valve pockets, also known as “eyebrows.” Valve pockets create a little extra room for valve movement while also keeping the engine’s compression up. Checking the piston-to-valve clearance will make sure the valve pocket is deep enough, but there should also be enough room between the edge of the valve and the wall of the pocket. This is called the “radial clearance.” Any time you are running larger than stock valves, custom pistons, or are mixing and matching components, radial clearance is a must check.
Generally speaking, you want at least 0.050in clearance between the edge of the valve at the lip and the wall of the valve pocket. Building a race engine where a flat-top piston is required, aggressive engine builders may try to get away with less clearance and will have their valve pockets cut as small as possible to help improve compression. On the other hand, more clearance between the lip of the valve and the pocket wall won’t hurt anything and likely will only bring your compression down slightly.
For this story we stopped by Prestige Motorsports in Concord, NC. Prestige builds all types of engines, but this particular bad boy is a Ford Windsor stroker punched out to 427 cubic inches. It uses a World Products Man-O-War iron block fitted with a forged crank with 4.000 inches of stroke and 6.250-inch connecting rods. At the end of the rods are a set of forgings from JE’s SRP line that have been custom designed for this engine package.
The pistons fit a 4.125-inch bore, have a 1.245-inch compression distance to work with the stroker package, and a 23cc dish to help keep the big-inch engine happy on pump gas. There’s a Comp Cams solid roller camshaft with BAM tie-bar lifters, Scorpion rocker arms and a pair of AFR’s CNC-cut aluminum heads with 220 cc intake runners. The mad scientists at Prestige Motorsports say this package makes for a dependable motor with tons of torque and is very popular with classic Mustang owners looking to maximize fun behind the wheel. On the dyno it made 625 horsepower and 540 lb-ft of torque. For more information on Prestige Motorsports and the variety of engines they build, check out www.PrestigeMoto.com.