We started this rebuild with a basic set of iron 318 castings, but the process is the same
Car crafting is all about power-enough to run 12s in the quarter-mile and spin the tires at will. But not everybody can afford a shiny new set of aftermarket aluminum cylinder heads. The next best thing if you can't afford a trick set of alloy castings is to lay a little TLC on the heads you have. Last month, we introduced you to Steve Strope and his stepson Seth's '66 Mopar Coronet. While the resurrection bodywork continues on that somewhat abused B-Body, the pair also opened up the 318 Mopar engine that will eventually find its way into the engine compartment. The Coronet's original powerplant was an ancient polyspherical 318 that only the most diehard Mopar fan could learn to love. Instead, the pair chose a '70 LA version that promised to be in better shape. At first, the Strope plan was to perform a quickie Krylon rebuild by just cleaning and painting the low-mileage donor motor while adding an intake and a cam. That's when the, "Well, as long as we've got it apart" song started. Before the final note had played out, the heads fell off in anticipation of a quick valve job. We took the heads to Jim Grubbs Motorsports where ace machinist Ryan Peart transformed the greasy leakers into like-new castings.
Most guys don't have machine shops in their garages, so the focus of this exercise is to point out areas where a couple of simple and inexpensive machining steps can improve airflow for cheap and to illustrate just what goes into a quality rebuild on even stock iron cylinder heads. Check it out.
The biggest issue with used cylinder heads is valve stem and guide wear. Generally, used heads will have loose guides that allow the valves to move laterally relative to the valve seat. This movement tends to beat up the seats, deteriorating their condition, which also diminishes the seal under cylinder pressure. If this isn't bad enough, loose guides allow the valves more room to move around, which trashes the valveguide seals. From this short description, it should be clear that a set of properly installed and clearanced valveguides is essential in the build process. So that's where we will start.
After disassembling the heads, the first thing Peart did was degrease the castings. These
To accurately establish valveguide clearance, Peart first measured the existing valves and
The first step in rebuilding the guides is to ream the original cast-iron guides to make w
As an option, it is possible to purchase stock replacement valves with oversized stems tha
While entire replacement guides can be used, bronze wall guide inserts retain cylinder hea
The guides are chosen in an attempt to create as close to the desired inside diameter as p
With all the guides in place, Peart pressed a broach through the guide from the top to bon
Peart then passed a hone through each guide to establish the ideal inside diameter. This i
The last step with the guides is to spot-face the top of the guide to eliminate a sharp ed
Besides guide wear, it's also a good idea to check the valve face margin. The margin (arro
Since valve stem wear on the OE valves was acceptable, Peart first subjected them to a qui
Virtually all performance valve jobs use a 45-degree angle on the valve. Some Pontiacs did
We elected to reuse the stock valves to save a little cash (that Coronet is gonna need a l
The advantage of a performance aftermarket valve like a Manley, for example (left), is not
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 th
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 mino
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 inst
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
|Valve stem to guide
||Intake, 0.001 Exhaust, 0.0015
|Valve seal to retainer
|Clean and Magnaflux
|Install new guides
|Back-cut valves (8)
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.
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.
2700 California St.
Jim Grubbs Motorsports
28130 Avenue Crocker