Basic Valve Jobas published in BritishV8 Magazine, Volume XVII Issue 1, July 2009
by: Greg Myer
The head is one of the most important areas of an engine in terms of performance. A good valve job is critical for whatever you want out of your motor, whether it's better fuel economy, power, or just plain reliability. After years of use, for the valves to work at their designed capacity or better, a good bit of work must be done. There are a number of steps that must be taken, and several ways to do each of them. Some ways are better than others and some heads require special treatment.
After removing the heads, they need to be disassembled. You can do this in your garage if you have a valve spring compressor. You'll need a compressor designed for the type of head you're working on. An inexpensive lever style or a C-clamp style will work on many standard V8's: Ford, Chevy, Buick, or Rover. Late model overhead cam heads require something different. If you like, the machine shop of your choice can remove the valves and also reassemble your heads after machine work is complete; all you'll need to do is torque them back on.
After dissassembly, the next order of business is cleaning the heads. Cast iron
heads can be put in a hot tank of strong chemicals. Tanks for aluminum are available
in some shops. They use less heat and different chemicals, so as not to damage
anything. Other types of cleaning can be done too. Cleaning machines that throw
shot on the head seems to work very well.
Next comes checking the heads for cracks. Again there are various ways of proceeding. Cast iron heads can be magnafluxed: a strong magnetic field is created and will cause the powder to collect in any crack, making it quite visible. Neat! Aluminum can be checked with one of several methods. There are usually three or four steps involved. First a special cleaner is used, then a penetrating dye is applied, and next a developer. Some of these will produce results in plain room light, while most require a black light, or ultra-violet.
If a crack is found, find out your options. Most cracks extend from one valve seat to the next. Will a new set of hardened seats fix it? If not, and you are working on a set of standard, easy to obtain heads, it may be prudent to start over. Ford and Chevy heads fall into this category, unless they are specialty, aftermarket heads. Aluminum heads can be welded and remachined. This could, however, get expensive depending on the proper way to proceed. Many cast iron heads will need to heated in an oven first and welded while hot. This can be tricky, and while I've had it done, I would only let someone with plenty of experience attempt it. In my case it was for a heavily ported 4 cylinder head that was from a factory turbocharged application. Not easily replaced. We had excellent results. If you are unsure of any area of your heads, have them pressure tested. Better to find out now than after they are installed and the motor is in the car.
Now on to the machining process... Before the seats are touched, the guides must be established as correct. They must be both the proper inside dimension, centered and not out of round at either end. The latter is sometimes the case as the rockers have a tendency to push the valve stem away when acting on it. Guides with a little wear can be knurled and reamed to size. Guides may also be pressed out or drilled out, depending on type, and new ones pressed in and reamed to size. The reaming is critical as quite often the guide is made out of round by the press. Several materials are available for the new guides; however bronze is far and away the favorite. If you have decided to use smaller stemmed racing valves, the machinist needs to know as the guides he puts in must have the smaller diameter. These valves weigh less and are under-cut and swirl polished allowing better breathing through the port. Less weight here means valve springs will not be stressed as much and can contribute to higher RPM's as well. Very well indeed, but be prepared to foot a higher bill for the parts.
The reason the guides are established before other machine work is done is because
most valve seat work is centered on the guides. If they're not right the seats may
be off center or otherwise inaccurate.
Cutting the seats is critical to the whole job! There are various types of cutters and valve grinders. The key is the man who does the job. He must make sure of each step and check that everything fits and matches.
I have an older Black & Decker unit that uses cutting stones. I need a number of stones to cover all the diameters of valves as larger stones won't fit in the combustion chamber of a smaller head. Also, there are several types of stones manufactured to cope with the materials that need to be ground. Cast iron use the standard, relatively less expensive stones whereas the stones to cut stellite seat inserts require a tougher composition and are therefore more expensive. I also need one each of three cutting angles for all diameters of valves. OK, I need a bunch of stones!
Newer head machines can do a multitude of tasks within a short period of time,
so turn around on jobs is quicker. These machines vary in design, some even being
controlled by computers, making repeatable results easy to obtain. How fast are
This Rottler machine (above), as demonstrated by Harry Clark at the Salisbury, Maryland NAPA machine shop, can grind 3 angles on all the seats in a set of V8 heads and pressure test them in 10 minutes! Why isn't it finishing these Cleveland heads right now? They are waiting on a few guides to be delivered. Even great machines depend on many other factors. This machine can also drill the guides so new ones can be pressed in and then cut the tops of the guides for whatever type of seal you prefer. It can tap the rocker stud bore for screw-in studs as well as cut it shorter. To do this requires speed control of the drill head. This can be lowered to 40 RPM for tapping. It cuts seats for standard inserts and then cuts the angles. All of this is controlled by the operator after the machine's electronic leveler establishes positioning. This particular machine has a "Bowl Hog" attachment for opening up the throat of the port when larger valves are being installed.
Want one for your garage? I sure do, but at about $ 30,000, depending on attachments, it's going to have to wait.
When you take your heads in for work, if you bought special parts, take them along as well as any printed instructions that came with them. The machinist needs to know any special requirements to make these pieces fit and function properly. This example (at right) sitting on the bench at NAPA is from Ford Racing. It provides specs for cutting and tapping the rocker stud bores on a Cleveland style head.
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This head (at left) on Bill Cannon's head table shows Dykem Blue dye used to keep close
track of the various machining operations. Dye is available in various colors
and in spray or brush on depending on what is being done and which material is
being machined. Note also the gasket surface. These may corrode or gall. A
cleanup pass on the surfacer is called for. To raise compression, at times,
more material is removed the decrease the size of the combustion chamber. This
is an effective way of gaining compression if your engine doesn't have a wide
selection of pistons available or you are a half point lower then your targeted
ratio. If the heads are surfaced more than a small amount the intake manifold
gasket surfaces will need to be surfaced as well because the you have lowered
the overall height and the manifold won't match up. Quite possibly there will
be a serious vacuum leak. The amount will not be the same as taken off the
deck. It can be determined by a graph that the machine shop has. When
assembling your engine it's always good to check the manifold to head match.
At this time the tops of the valve guides may be machined for Teflon seals if they are to be used. The rocker arm stud boss may also be machined and tapped for screw-in studs, if applicable. In the picture, in the rear, there is a guide cutting tool. It has the diameter and depth established so the machinist can't mess this up. The tool is on the right, a freshly machined guide top is to its left and a Teflon seal is sitting between them.
Closer to us is the rocker stud cutting tool. Again, the depth is set. The studs need to be pulled first, as the stock one laying there has been. After machining the boss down, the hole must be tapped and then the new heavy duty stud screwed in. If the hole goes into the water jacket a sealant MUST be used. The pushrod slot may need to be lengthened if a high lift cam and/or rocker arms of great than stock ratio are part of your game plan. In fact, it's wise to spend the time and a few extra dollars now so it's easy to upgrade later. Playing with various ratio rockers can be eye-opening, both for power and mileage, although not at the same time.
Next are the valves themselves. The diameter of the stem must be within specs as well as the length. Some valves will wear on top due the lack of oil between them and the rocker arm. This affects the geometry of the valve train. Some valves use lash caps which can be easily replaced if worn. Once these dimensions are established the surface that mates to the seats needs to be ground smooth at exactly the right angle. This usually is the same as the seat. Most are 45°; however some manufacturers specify a one degree interference cut. Others indicate different angles depending on their research with flow benches or for longevity concerns. For example, for years Pontiac specified a 45° face on their exhaust valves but a 30° on all intakes. Starting in 1964, that changed to 44° and 29° respectively. Don't ask me why; just be sure to check the book. Aluminum Buicks all called for 45° on both intakes and exhausts.
Used valves that are "burned" are not usable. This can show up to the naked eye as pie-slice splits in the valve. If you have any like this, toss them. Some burned valves can be more difficult to detect. The face area may be badly burned and it will take turning it in the valve face machine before it shows up. The valve will cut on part of the face, but not all. The machinist may try to continue cutting, but that will reduce the margin too much. The margin is a major concern. This is the area on the outside circumference of the valve head, parallel to the stem. Cutting the face will reduce this measurement. If the margin is too small, the valve will burn in short order. If your machinist is concerned with this area you would be well advised to replace the valve or valves in question.
The 45° cut on the valve face affects the overall installed height of the valve too. This is usually minor, and with hydraulic lifters and / or adjustable rocker arms you need not be concerned. When everything is machined, and the valves assembled in the head a straight edge can be laid across the tops of the valves to check that they are level. As mentioned, slight differences are common and easily tolerated. If your engine has solid lifters and shaft mounted rockers the valve height needs to be right. Even then an adjustable valve train will be to your advantage.
With the valve seat in the head machined and the valve face done, the two can be mated. I like to use lapping compound for this. Years ago it was used to match the surfaces, much like the lid on a glass apothecary jar. It worked very well too. Today's equipment however leaves the two surfaces in such a nice condition that it's no longer needed. I use it to identify the contact area between the two. For street motors that will see 100,000 miles before the next rebuild, a wide seat is the ticket. Race motors however like narrow seats as this facilitates more flow at all lift heights.
With the face cut made and measured, the valve can be back cut. Perhaps a 30° cut just above the 45° cut. This narrows the face to the needed width and helps flow on both intake and exhaust valves. The illustrations above show a warped, burned valve that would not cut and one with a 45° face, a red dyed 30° back cut and a 15° back cut above that for comparison. Notice how this cut removes the inner lip on the back of the valve. That lip has been shown to cause turbulence, disturbing the air flow both into and out of the chamber.
Another machine operation to consider is a front cut. This process cuts a slight angle on the chamber side of the valve and also reduces the margin. Again it's been demonstrated to help flow on both intake and exhaust valves. I took an old valve and put red dye on it, cut the face 45° and put a 15° front cut on it to demonstrate. The margin is still clearly visible, but you can see how it is reduced.
The installed spring height needs to be checked next. This can be measured
with a special tool designed for the task. If there are differences shims
can be use to even out the gaps. Note: aluminum heads have spring cups.
Never install springs directly on the aluminum as the soft aluminum will
take a beating and you can even destroy the head.
The spring pressure at the installed height can be measured too.
Seals are next. There are a wide variety of seals available. The O-ring type was the Chevy standard for decades. High performance engine builders like Teflon seals. These require cutting the top of the valve guide to press the seals onto. These are the standard of performance engines today. Use them on both the intake and exhaust. It's a good idea to use new keepers and retainers too.
This assembled BB Chevy aluminum head at Bill Cannon's Awesome Engines
shows how it all comes together.
When the engine is assembled proper rocker arm geometry should be verified. If things are not where they are designed to work, the life expectancy of your motor will be greatly reduced.
Please keep in mind that these are basic operations and we have not touched on port matching, porting, and matching head cavity volumes ("CC-ing"), or other high performance and racing modifications. We'll save that for another time.
At least with this information you will know what the machinist is talking about, and you should be able to answer his questions. You'll know what you need to speed your heads on their way.
Disclaimer: This page was researched and written by Greg Myer. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.
Photographs by Brandon Myer at Brightside Photography and Greg Myer for BritishV8. All rights reserved.