Ask any engine builder where he would concentrate his efforts if he could only modify one component in an engine, and most would say the cylinder heads. Even the sleepiest street engine can see power increases and improved driveability, emissions and mileage with a professionally rebuilt set of cylinder heads.
While there are thousands of “rebuilt” heads sold every year, many are run through high-volume production lines with less than optimal clearances. While those heads will work, they will never perform as well as if they had been treated to quality parts, and more importantly, quality machine work. While few Hot Rodders have access to cylinder-head rebuilding machinery, it’s important to know the difference between a dime-store rebuild and competent machine work. To get the lowdown on the proper technique for rebuilding a cylinder head, we went to Jim Grubbs Motorsports (JGM) in Valencia, California, and followed along as Grubbs and machinists Pete Christensen and Todd Roselund mended a set of small-block Chevy heads. All the procedures outlined here apply to cylinder heads of any make.
Foundations
While trick valvetrain parts are cool, they’re a waste of money without quality valve guides. Valve guides are essential to building any cylinder head. A straight, nontapered valve guide properly positions the valve relative to the valve job and keeps it there. If you’ve ever seen valve seats pounded out until the angles are completely indistinguishable, that occurred because of loose, tapered valve guides and/or worn valve stems.
There are a number of different types of valve guides on the market, but JGM prefers to use thin-wall bronze guide liners as opposed to rigid-replacement guides because they remove the least amount of the original cylinder head, maintaining cylinder-head integrity. In addition, bronze requires minimal lubrication, resulting in less oil leakage into the cylinder and allowing tighter valve-to-guide clearances.
Some shops prefer to knurl worn guides as opposed to replacing the guides. All the performance machine shops we spoke to agreed that knurling is not recommended since it deteriorates almost immediately and increases oil consumption. When rebuilding your heads, make sure the machine shop uses quality guides. If they insist on knurling your guides, find a new machine shop.
Next are the valves. While old valves can be reused, determining stem wear is critical. JGM prefers no stem wear, but a taper of no more than .0005 inch is acceptable. Valve-stem tip wear and the thickness of the valve margin (see “Clearances” chart) are also critical. JGM bead-blasts the valves, hones the stems and then butt-grinds and chamfers the tips. All valves, new and used, are subject to this process to ensure consistency. New, high-quality valves are a wise choice since straight stems allow optimal guide clearance.
With the guides and valves qualified, you can next address the valve seats. JGM prefers to replace stock exhaust valve seats on any head with recessed seats, a noninduction hardened seat or a head that has been treated to multiple valve jobs. Chevy began induction-hardening exhaust seats in 1974, but this process only surface-hardens the seat about .010 to .020 inch deep. If the seat has been ground more than once, the hardening has probably been removed. JGM machines out the old seat and installs a new hardened seat insert usually made of Stellite or other hardened alloy steel. This stronger material prevents the seat erosion that occurs from the combination of high exhaust-gas temperatures and unleaded gasoline.
The deck surface should be straight. If the deck surface of the head is warped, it should be machined to ensure the head gasket will seal properly. Deck-surface machining will also decrease the size of the combustion chamber slightly, depending on how much the head is milled.
Valve-guide seals are another important consideration. While O.E.M. seals do an adequate job, JGM prefers to use Viton rubber seals available through a number of sources including Fel-Pro. These seals are positive seals, meaning that the valve guide must be machined to allow the seals to fit properly over the guide. For more information on Viton seals, refer to “Oil Control” in the October ’93 issue of Hot Rod.
Proper installation of valvesprings includes checking the valvesprings for seat and open pressure and setting the installed height. For a small-block Chevy street head, this is usually 1.700 inches. Shims can be used underneath the springs to decrease the installed height if necessary. Excessive shimming to increase spring pressure is not recommended since it reduces the installed height. It also reduces both coil-bind and seal-to-retainer clearances. This is especially critical if you are using a performance cam. Most stock small-block Chevy springs, for example, limit total valve lift to no more than about .500 inch. This is why checking these clearances is so important.
The Extras
Up to this point, all of these operations are necessary for a proper rebuild of your heads. Besides trick parts, such as stainless-steel valves and porting work, there are also some optional modifications you can do to your heads if you have the money. In the case of the small-block Chevy, screw-in studs and guideplates are a common add-on. While they produce a performance look, don’t opt for these goodies until you’ve performed all the other machine work first. If you are using stock rocker arms, flat guideplates work best, while the stepped guideplates can be used for roller rockers.
New, hardened pushrods will be necessary if you’re using guideplates, since stock pushrods will gall and send metal chips all through your engine. New rocker arms are always a wise choice. A set of poly locks for adjustable valvetrain engines like the small-block Chevy is also a wise move. Stock rocker-arm nuts tear up threads on the studs and make adjustment of the valves a hassle.
If you follow these examples for your next set of heads, they will be far superior to a set of dime-store heads and well worth the money you’ve invested. But we’re not finished. Next month, we’ll take these rebuilt heads and baseline them on a dyno. Then we’ll show you how to make power with larger valves and also some easy home porting work. Stay tuned.
