Rebuilding Cylinder Heads - Why? 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. Guide to Rebuilding Cylinder Heads The biggest issue with rebuilding 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. Rebuilding Cylinder Heads: Valve Grinding After disassembling the heads, the first thing Peart did was degrease the castings. These are 2843675 castings with 1.78/1.50-inch valves that use rocker shafts instead of individual studs. The center two exhaust ports are siamesed and employ an odd dogleg design that can best be described as weak. The intake ports measure a small 119 cc, while the chamber measures 68 cc. 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 then used that spec to establish a guide for a dial bore gauge. The original valves were in surprisingly good shape, so we decided to reuse them. The clearance turned out to be 0.004 inch on the intake and 0.006 inch on the exhaust. This demanded new guides. 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 way for the replacement inserts. This is a critical step, since the guide should be maintained perpendicular to the seat as much as possible. 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 that are designed to compensate for guide wear and bypass the guide work. This is an option you might consider, especially if you have to replace the valves due to excessive stem wear. 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 head integrity yet produce an excellent foundation. These bronze K-Line guide liners also improve lubricity, allowing a tighter valve stem clearance that improves durability. 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 possible. Pressing them in from the bottom collapses the opening in the guide, creating a slightly undersized inside diameter. 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 bond the guide to the head, which improves heat transfer characteristics. 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 is somewhat time consuming since it requires quite a bit of honing and then measuring. This is one area where the money you pay for good machining pays off. 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 edge caused by the broach. Note the spiral oil path that improves lubrication, keeping the valves cooler. 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 (arrow) is the portion of the valve between the face and the outside diameter of the sealing face. A wide margin is essential especially for exhaust valves. Valve margins often become very thin if subjected to excessive valve face grinding. A thin exhaust valve margin not only decreases flow but can also overheat, which can contribute to detonation or pre-ignition problems. 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 quick stem honing to ensure a square surface from which to work. 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 experiment with 30-degree seats, which improves low-lift flow but at the cost of flow at higher valve lifts. The key to grinding the valves is to remove as little material as possible to establish a concentric sealing surface. 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 lot of bodywork). However, we did try a simple 30-degree back-cut (arrow) on the intake valve to see if that would help the flow. This additional angle generally helps low- and mid-lift flow. It's a simple trick that your machinist can perform during the valve-grinding step. 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 just in the stainless material. Most factory valves employ a sharp radius from the stem to the valve seat. Worse yet, many exhibit a ditch or undercut directly behind the seat that decreases flow. Depending on the application, new performance valves are only slightly more expensive than stock replacement pieces. Note the swirl polish and gentle radius from the stem to the seat in the aftermarket valve on the left. The advantage of a performance aftermarket valve like a Manley, for example (left), is not 1 | 2 | » | View Full Article By Jeff Smith Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!