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Ask Anything - February 2014


Jerry Phelps; MGYSGT USMC Ret., Newport, NC: I really liked your article on the 305 buildup using the Vortec heads. I'm in the process of doing the same thing to my 1985 Monte Carlo SS L69. Did you mill the Vortec heads, and what thickness head gasket did you use?

Jeff Smith: First of all, the alphabet soup after Jerry's name stands for Master Gunnery Sergeant, United States Marine Corps, Retired. My father was a career Marine aviator (and a Mustang, an enlisted man who became an officer). My dad always had the greatest respect for the Master Sergeants—he said they were the ones who really ran the Marine Corps. Hopefully, this answer will warrant an acceptable "fit rep," Gunny.

Over the years, Car Craft has done several 305ci small-block Chevy builds, so I'm not sure which specific engine build you referenced, but it doesn't really matter since this information will be mostly the same. Unlike the early '67–'69 302 engines with their 4.00-inch bore and 3.00-inch stroke, the issue with the 305 is a really small bore. The standard configuration of a 305 is a 3.736 bore and a 3.48-inch stroke, and the L69 version used in Monte Carlos and early third-generation Camaros had 9.5:1 compression, which was aggressive for the day. The engine was rated at 190 hp, while the lower performance version LG4 was between 150 and 170 hp. The L69 also used a slightly longer-duration camshaft than the lower compression versions. Both engines used the same iron head with a 58cc combustion chamber size. You can use the Vortec iron small-block head on the 305 because they came with 1.94/1.50-inch valves, which will clear the small bore. However, the Vortec chamber is larger at 64 cc than the 58cc 305 engine chamber. This can be rectified by milling the head to reduce chamber volume. The general rule of thumb is to remove 0.006-inch deck surface to reduce the combustion chamber volume by 1 cc. So this would require milling roughly 0.036 inch off the heads to trim the chamber down to 58 cc. That's assuming the heads have not been previously milled. I plugged a 58cc chamber into a stock-bore-and-stroke 305 with a flat-top piston equipped with four valve reliefs (roughly 8 cc) with a 0.015-thick shim head gasket and the piston below deck height 0.020 inch will produce a static compression ratio of 9.5:1.

The 0.015-inch head gasket thickness is equal to a stock-style steel shim gasket, which is the thinnest gasket you can run on these engines. The above situation is based on the piston being below the deck by 0.020 inch. It's possible that the pistons might be slightly closer, which will raise the compression. If the engine has been rebuilt and the machine shop has milled the deck and brought the pistons up to, let's say, 0.005 below the deck, then a thicker composition head gasket will be required both to keep the compression ratio around 9.5:1 but also to maintain a minimum of 0.038 to 0.040 inch of piston-to-head clearance. With cast pistons that run a tighter piston-to-wall clearance, this will reduce piston rocking, which should be minimal anyway, considering the small diameter. It's best to keep the piston close to the head to tighten the quench volume, which enhances combustion efficiency. Piston-to-head clearances beyond 0.050 inch tend to minimize the effect of quench, which can cause detonation problems. And this is something you'd like to avoid, since the 305 engines tend to be very detonation sensitive. This should get you in the ballpark for compression and help wake up that little 305.

Removing this much material from the deck surface of the head may cause problems when mounting the intake manifold. This tends to push the bolt holes in the head down toward the bottom of the mounting holes in the intake manifold. The best solution is to mill the intake, as well. The suggestion there is to remove 0.12 from the intake flange for every 0.010 removed from the cylinder heads. This means if you removed 0.040 from the heads, you'd need to mill 0.048 inch from the intake manifold flange and about 0.060 from the intake endrails. Or I'd suggest test-fitting the intake. As long as the endrails don't touch the china wall on either end of the block, you might be able to just elongate the intake manifold bolt holes with a file to make the intake fit. This might also create a slight mismatch at the port opening, but we've found through testing that this rarely affects horsepower, even when the intake opening in the head is smaller than the intake manifold's outlet port.

More Info
Federal-Mogul (Fel-Pro); 248/354-7700;

CC Quickies

We had so much fun with our first 496 ("707 HP for $6,720," Mar. 2007) that we are building another complete with a Ohio Crankshaft stroker kit, good H-beam rods, and a set of JE pistons. There's also rumor of a set of excellent oval-port AFR heads that promise big-time power. Stay tuned.

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