It's All About Length
Chris Warner via CarCraft.com: I am the proud owner of a '94 Mustang. When I got the car, it had the original 3.8L with 173K on it and bad rod bearings in it (big surprise). I traded a pool table and $200 for the car, so I wasn't even close to complaining. At the time, I owned a '67 Galaxie 500 four-door with 80K on the original C-code 289 and FMX transmission, so I yanked the drivetrain out of the Galaxie and swapped it into the Mustang. While the motor was out, I replaced the head gaskets, stuck it with a solid cam (0.477 inch lift and 310 degrees of duration-advertised) I ordered from Tony Branda Shelby along with solid lifters, and replaced all the other parts I removed to swap the cam. I topped it with a Performer 289 intake and 600-cfm carb, both from Edelbrock. Fumes are expelled through 151/48-inch headers to an H-pipe with 40-series Flowmasters hushing it just a bit. The car is fast, but I know the heads are holding it back. I purchased a set of 1965 C5AE iron heads that have been fitted with 1.94-inch intake and 1.60-inch exhaust valves and dual valvesprings. My friend tells me those are Chevy valves and will likely cause an interference with my pistons saying the valves are longer. My pistons are stock. With my camshaft and 1:6 roller rockers and poly locks, can I put on these heads without either replacing or fly-cutting my pistons? All but one of my friends hound me regularly to put a real motor in it (EFI 5.0), but I'm keeping the 289, so please help!
Jeff Smith: Let's start by addressing the differences in valves in your head. Your friend says the 1.94/1.50-inch valves are Chevy valves, and he's probably correct. This valve-size combination is a common upgrade for small-block Ford heads over the stock 1.78/1.45-inch valves. The Chevy valves use the same valve-stem diameter, but stock Ford valves came with a taller tip length, which is the dimension from the lock groove to the tip of the valve. Original Ford valves were designed to be used with rail-type rockers, using a pair of rails parallel to the rocker arm tip that straddle the valve tip and keep the rocker aligned over the valve tip.
According to our handy Manley catalog, small-block Ford valves used with rail-type rockers have an overall length of 5.08 inches that includes a tip length of 0.395 inch. Valves designed for the small-block Chevy, like the 1.94/1.50-inch valves in your head, have an overall length of 4.911 inches with a tip length of only 0.250 inch. The difference in the overall length is the difference in the tip length. This does not affect valve-to-piston clearance. This clearance is determined by a combination of camshaft timing, piston top design, piston deck height, head gasket thickness, whether the heads have been milled, and a few other minor variables. The best way to know if you have sufficient valve-to-piston clearance with this big cam is to place a wad of modeling clay on the piston top in the valve-relief areas, install a head with the head gasket you intend to use, and then set up a solid lifter, pushrod, and rocker arm for both the intake and exhaust valves for that cylinder. Turn the engine through several valve-lift cycles and then pull the head and measure the thickness of the clay impressed by the valves. Minimum valve-to-piston clearance is 0.060 inch on the intake and 0.100 inch on the exhaust side.
You also mentioned you will be running roller rockers, which is good because you cannot run stock Ford rail rockers with the shorter valve-tip-length Chevy valves or the rail rockers will contact the retainer and cause more grief that you'd be better off avoiding. However, the shorter Chevy valves will affect rocker-arm geometry. This means you should check your rocker geometry, which is also very easy to perform.
Use a Sharpie or machinist's dye to mark the end of an intake-valve tip. Next, mock up a solid lifter with a pushrod and rocker arm on that same intake valve. Make sure the intake is on base circle of the cam lobe and then wiggle the rocker arm to make a witness mark on the valve-stem tip. After removing the rocker arm, determine where the witness mark places the rocker arm roller tip relative to the valve tip. If the pushrod length is correct, the mark should be on the inboard third of the valve tip. If the pushrod is too long, the mark will be somewhere near the middle of the valve tip or closer to the outboard side of the engine. If the pushrod is too short, the mark will be too close to the inboard side of the valve tip.
Generally, with a shorter Chevy valve, your pushrods may be a bit too long for ideal rocker-arm geometry, but this is why you should check. Using a shorter pushrod to establish a more ideal geometry will create a happier valvetrain that will be more durable. It would also be a good idea to check valvespring coil bind and retainer-to-seal clearance to be sure the valvesprings can handle the valve lift you intend to run.