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Heads Up: Now It’s Time To Put The Camshaft In The Head And Bolt The Caps Down To Check The Clearance
By Dick Schaffner
In our last article (see Automotive Rebuilder’s April 1999 issue, page 76) we had just completed welding the head to straighten it. Now it’s time to put the camshaft in the head and bolt the caps down to check the clearance.
If the cam caps have rocker shafts that run through holes in the cap, we are careful not to distort the rockershaft hole. Rotate the camshaft and check for tight spots. Always make sure the cam is straight before installation. If the cam spins freely then we set up a dial indicator and check the clearance. If you don’t have an indicator, use a feeler or plasti-gauge.
Next, check the factory specifications. If the cam is too loose out of specification, we can correct it a couple of ways. If it’s .002" to .004" loose, it can be tightened up by carefully removing material from the cap with a good file or emery paper.
Stroke the cap across the file or paper slowly and easily, making sure not to tip the cap. One or two strokes should be enough, then replace the cap and check the clearance of that journal. If the clearance is in excess of .004" above the standard specification, we recommend cutting all the caps on a cutter or surfacing the top of the head and then line boring or line honing with a ridged hone and oil.
If it’s too tight, line honing or line boring will repair the problem. Be sure to check end play because sometimes the thrust of the cam is worn or broken. In these cases, replace as necessary.
Next we check and repair the valve guides. In most cases, our technician will check a valve stem for roundness and taper. If it’s good, then it is used to check guide wear. The guides are checked two ways; side-to-side and back to front.
If the wear is even and minimal, no repair is needed. If there is a large amount of wear, replacement is required. If there is a question of how much wear is present, use a dial bore gauge to check the taper and out-of-roundness. If you don’t have a bore gauge you can select a grinder pilot that measures the same as a valve stem, plus the recommended guide clearance and use it as a gauge.
Now let’s talk about replacing guides because sometimes they can be stubborn. Aluminum heads have replaceable guides that are made of different materials. Some have bronze guides, but most have steel or cast iron guides.
There are different ways to remove and replace guides, depending on the head and style of the guide. Most iron guides can be driven out with a guide driver, pounded with an air hammer, or by a ball peen hammer and standard guide driver. We have found that the air hammer will usually do the faster job, but if the driver isn’t in good condition, or it’s the wrong size guide, breakage may occur.
Another problem we have found is galling of the guide and guide hole. This is caused by a broken or damaged guide, or by carbon build-up on the chamber end of the guide. Sometimes glass beading the guide to remove build-up and putting some lubricant on the guide will help it to come out clean. Other times nothing will help other than boring the guide out with a core drill, then chipping out the thin shell that is left.
If that fails, most guide manufacturers make oversized guides, (.001" -.002") for slightly worn holes. For some applications where the size is greater than .001"-.002", a .015" guide is used by reaming the hole oversized. If necessary, heating the head to about 250° to 275° F will greatly help in the guide removal and replacement process.
Bronze guides are a little difficult to work with compared to iron because the bronze is much softer and tends to distort and mash-out with heavy hammering. This causes breakage of the top and bottom guide bosses. We have found that if you turn the air down on the air hammer and jolt the guide for a couple of seconds it should move. If it doesn’t, stop hammering on the guide, check the I.D. of the guide, and find a tap that will cut threads in the guide.
Insert the tap from the top side of the head (the spring and cam side); tap it about a half-inch then screw a bolt in the guide. Then use a pin punch to drive the guide out. By driving against the bolt you won’t swell the guide in the hole, and it should come out with out galling the hole.
When installing bronze guides we do it two ways. Using a snug fitting guide driver, we lube the guide with a lubricant such as piston pin lube. We gently tap the guide in by hand using a light ball peen hammer. If the guide resists, heating the head may help the process.
It’s good to have a couple of extra guides just in case you damage one. When installing guides in European heads like VW, Audi, etc. we use an arbor press. Most of these heads are square and pressing doesn’t seem to distort the guide. In most cases, you can feel the guide move in the press handle. After installing the guide, we deburr the I.D. with a tapered reamer, then fit the valves with a guide reamer or a guide hone.
Counterboring the seat pockets for seats is done on our head machining center with a fixed-blade carbide seat pocket cutter. First we bore the weld out, undersized of the final cut, then we pressure test the casting again to be sure that after removing part of the weld, there are no leaks.
If no leaks are found, the seat pockets are cut to final size and made ready for seat installations. Occasionally an odd size seat is required. In these cases, we use an adjustable carbide tool to cut the counterbore.
The proper seat is selected and coated with a retaining fluid. Next, it’s driven into the counterbore with a pilot and seat driver. Once the seats are installed, a final pressure test is done to assure that the casting doesn’t leak as a result of the seats being driven into the casting.
Now it’s time to cut the seats. It’s important not to cut the seats too deep. This can cause improper valvetrain geometry and valve clearance problems. Our technicians check the stem height specifications in the repair manual and then set up a stem height gauge.
The seat is then cut with a carbide cutter; the valve is vacuum tested for seal and concentricity; and the stem height of the valve is checked in the hole. If the seat is within specification, the other seats are cut to the same depth and vacuum tested. Once all the seats are cut, the head is checked for broken studs, stripped bolt holes, etc. Miscellaneous repairs are then made to make the head ready to be surfaced.
Surfacing is done as a last step before final wash and assembly of the head. Take one last good look at bolt holes, spring seat and valve cover rails, seal surfaces, and water outlets for corrosion.
Once the head has been surfaced, you don’t want to scratch or ding the gasket surface. We use two types of cutters for milling. A single half-inch round C7 carbide insert is used on all aluminum heads and a CBN (Cubic Boron Nitride) half-inch round insert is used on aluminum heads that have steel inserts, such as the Toyota 1AC gas cylinder heads and VW diesel heads with pre-chambers.
It‘s important to machine aluminum heads to the correct Ra specification for the application. Some gasket manufactures will furnish the Ra specifications upon request from the technical department. Some machine manufactures also have the specifications. Most Ra gauges are expensive, so you might want to get an Ra chart from a gasket supplier or tool supplier.
Mounting the head in the mill is important if it’s an overhead cam head. Parallel cam alignment to the surface of the head is critical. If the camshaft isn’t parallel to the surface, misalignment of the timing belt can occur causing the belt, gears and cam thrust to fail or prematurely wear out.
Head thickness is critical on most overhead cam heads for a couple of reasons. One reason is that some engines have very tight valve-to-piston clearances and valve damage can occur. Also, camshaft timing is affected by lowering the camshaft toward the block because it will retard cam timing causing poor running, unexplained overheating and poor fuel mileage. A good quality head saver shim will repair this problem and is an economical correction.