318 Cylinder Head Clearances & Machine Cost

Seat Work
In ancient times (about 20 years ago), seat preparation was performed using a series of grinding stones to establish the seat angles. Generally, this involved three stones that were dressed at the specific angle required. The accepted standard valve seat for a typical performance intake and exhaust valve seat uses 30-, 45-, and 60-degree angles. The 45-degree seat matches the angle ground on the valve. The 60-degree angle is also often referred to the throat angle that directs the air (on the intake side) from the port toward the seat. Then the 30-degree top angle helps radius the flow into the chamber. Different cylinder heads can and do respond to different combinations of angles, but the classic 30-45-60 is tough to beat.

With the advent of the dedicated valve-cutting tool machines such as the Serdi, Sunnen, and others, the machinist can specify a custom-made cutter that machines all three seat angles and widths simultaneously. Not only is this faster and far more consistent through all 16 valve seats, but it also ensures far more seat concentricity, which also helps flow.

This is the stock chamber after cleaning. You can see that if there are three angles on the intake seat, they are very thin. Oddly, before and after flow testing revealed that we did not improve airflow even with a performance-oriented valve job.
This is the stock chamber after cleaning. You can see that if there are three angles on th
Peart set up the heads in JGM's Sunnen SGM-1500 machine and efficiently cut all the intake and exhaust seat angles using a specific cutter for each of the two different seats. In this case, JGM chose a slightly different 38-degree top cut along with the standard 45-degree seat, followed by a radius in the throat as opposed to the typical 60-degree angle.
Peart set up the heads in JGM's Sunnen SGM-1500 machine and efficiently cut all the intake
This photo illustrates the three angles on the intake and exhaust valve seats. A very minor amount of grinding just beneath the throat cut would help the flow. The stock throat diameter on this Mopar exhaust port measures almost 85 percent of the valve diameter (1.270 versus 1.500). The rule of thumb is the throat diameter should not exceed 88 percent for the exhaust, which in this case means there is very little room for modification. On the intake side, the recommended percentage is a maximum of 90 percent. This Mopar intake measured 1.48 inches, which calcs out to 83 percent on a 1.78-inch valve.
This photo illustrates the three angles on the intake and exhaust valve seats. A very mino

Head Assembly
With the machining complete, all that's left to do is assemble the heads. We went with an Edelbrock valvespring to match the camshaft, since the springs were really inexpensive. There are a couple more steps that include choosing the right valveguide seals and valvesprings and ensuring that we have plenty of clearance for our new cam. The cam is a mild Edelbrock Performer-Plus with only 0.420-inch valve lift, so coil bind and retainer-to-seal clearance aren't issues. We also upgraded the intake seals to Viton-style positive seals that mount on the outside of the guide. Once the spring clearances were verified, Peart assembled the heads with a little lube on the stems, and we carried them back to Strope's shop to bolt them on the engine.

While the cam we decided to use in the 318 is very mild, it doesn't hurt to check for installed height, which is the distance from the spring seat to the bottom side of the retainer. As you can see, with a 0.030-inch-thick spacer, the height measures 1.630 inches. The spec is 1.650, so we're slightly tighter, which should bump up the seat pressure to around 95 pounds.
While the cam we decided to use in the 318 is very mild, it doesn't hurt to check for inst
There are several types of valve seals. We decided to use a positive-style seal (left) instead of an umbrella seal (middle) that was on the heads. Another style of umbrella is the hard-plastic style (right). On the exhaust side, we used new O-rings that are placed on the valve in a groove just under the retainer. This directs a little more oil to the exhaust side.
There are several types of valve seals. We decided to use a positive-style seal (left) ins
Here is the finished head with the springs installed and ready to be painted Mopar orange and bolted back on the engine.
Here is the finished head with the springs installed and ready to be painted Mopar orange

CLEARANCES
COMPONENT	CLEARANCE (INCHES)
Valve stem to guide	Intake, 0.001 Exhaust, 0.0015
Valve seal to retainer	0.050
Coil bind	0.050
Installed height	1.650

MACHINE COSTS
OPERATION	COST
Clean and Magnaflux	$88.00
Install new guides	170.00
Back-cut valves (8)	32.00
Surface heads	56.00

These are JGM prices that may not be typical of other machine shop prices, but they will give you an idea of what the operations cost.

FLOW TESTING
VALVE	STOCK	VALVE JOB	BACK-CUT	CHANGE	INTAKE	CFM	(PERCENT)
LIFT	INTAKE	EXHAUST	INTAKE	EXHAUST	INTAKE	CFM	(PERCENT)
0.100	52	54	54	47	55	1	2%
0.200	108	97	107	90	118	11	10%
0.300	157	119	156	116	163	7	4%
0.400	172	128	177	126	164	-7	- 4%
0.500	166	135	167	132	168	1	0.5%

Note that our new valve job really didn't improve the flow, which is not unusual. The change that really helped was the 30-degree back-cut to the intake valve. Note how this improved the flow at 0.200-inch valve lift the most and then fell off, actually hurting flow slightly at 0.400 inch. Since our cam's max lift is only 0.420 inch, this should be worth a little torque if nothing else.

SOURCES
Edelbrock 2700 California St. Torrance CA 90503 310-781-2222 www.edelbrock.com	Jim Grubbs Motorsports 28130 Avenue Crocker # 331 Valencia CA 91355 661-257-0101