QUICK NOTES
The Task:
Find out what goes into building a set of top-quality, reliable cylinder heads.
Bottom Line:
Careful measurement and attention to detail are key.
Cost:
If something goes wrong, too much!
In one important respect, cylinder heads are like many other parts of our beloved Chevy powerplants. Done right, they provide miles of high-performance enjoyment; done wrong, they can bring the ride to a screeching halt. In this case, we're not talking about cylinder head performance per se, at least not when it comes to flow numbers and compression ratios. What head makes the most power has been covered elsewhere. What we're talking about here is the basic but complex act of cylinder head assembly. Basic because they sit atop our engines and do the will of the camshaft in regulating the internal combustion cycle; complex in that there's a lot of moving parts in a typical cylinder head, meaning there's room for error. And who wants that kind of misery?
Looking to understand what it takes to properly assemble a set of cylinder heads from bare castings to ready-to-install lungs, we went in search of professional help. Our friends at Speed-O-Motive take cylinder head assembly seriously-so seriously, in fact, that they build up all the heads they use from bare castings. The man who builds these heads for S-O-M is one Oscar Alvarez, who's been doing this kind of thing for neigh on a decade. We spent a good half-day with Alvarez, trying to glean all we could about the proper way to assemble a set of cylinder heads.
There's no way we can boil down our sensei's teachings into a few words, but we did ask Alvarez to start us off with a few tips:
Oscar's Top Three Tips
1. Check for correct spring pressure.
2. You must check the head surface.
3. Use top-quality parts.
After our time with Alvarez, we'd have to add this: there may be no substitute for experience, but making careful measurements, paying attention to detail, and learning from the experience of others can get you a long way toward a reliable set of cylinder heads.
Alvarez actually gave us a fourth tip-in fact, he feels so strongly about it that he gave it first. "People have to check the valve/piston clearance," he declared. "It's the most common mistake I see." Unfortunately, that process is a bit outside the scope of this article. On the other hand, what's the point in building a solid set of heads, only to have valves banging against pistons the first time you fire her up? With this in mind, we'll get to work on an article on checking valve/piston clearance. For now, here's a look at what it takes to build a bulletproof set of cylinder heads.
SPRINGBOARD
Whether you're assembling an all-new cylinder head combo or refreshing a tired set of lungs, much of the info we've given you here applies. Reusing a set of valvesprings, however, may require a few extra steps. First and foremost, the springs should be cleaned and checked for visual damage. When it comes to checking spring pressure and installed height, Alvarez left no doubt that the best way to make these measurements is to follow the manufacturer's specs for the cam you're using. Don't know what 'stick is in there? Get ready for some shimming. "If a spring pressure is very low," Alvarez told us, "you can shim it and bring the pressure up." As for installed height, you'll need to use your height mic on each valve, find the tallest installed height, then shim the rest to match.
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If you're working with used heads, it's critical that they be cleaned and crack-tested before rebuilding-even if you're the one who used them. It's also critical to check the head surface, and Alvarez suggests doing so even with new heads. If you have a 0.003-inch gap between a straightedge and the head surface, you should have the head surfaced. If the gap is 0.004 inch, the head is bad, and you must surface it, according to Alvarez. |
The first part of checking valve-guide clearance is to mic the valve stems. If the stem measures less than 0.340 inch, it's junk, according to Alvarez. This new Manley valve comes in at 0.341, which is right on the money. This is the figure Alvarez used to set up his dial-bore gague. |
Step two is checking valve-guide clearance. Alvarez began by checking the guides themselves with the dial-bore gauge. According to Alvarez, guides should come in at 0.001 inch clearance on the intake side and 0.0015 inch on the exhaust. If the guides are bigger than this (i.e., loose) they must be replaced. |
On the other hand, if the guides are on the tight side (i.e., the measure is smaller than spec), they can be honed to the proper size. It's a job best done by a pro, but if you choose to tackle it, be sure to use the proper tool and to keep the guide clean and lubricated as you hone. Work slowly and evenly, and measure frequently. |
On the other hand, there's also the seat-of-the-pants method for checking valve-guide clearance. With the valve head raised about 11/44 inch off the seat, you should feel virtually no movement. |
With all our valves and valve guides good to go, it was time to check out the valve seats in our subject heads. Alvarez began the lapping process by giving each valve seat a coat of blue layout dye, then each valve a layer of lapping compound. You don't want to overdo it here, since this substance must be totally removed before final assembly. |