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Heads Up: Rebuilding Aluminum Cylinder Heads
By Dick Schaffner
In our last article (Automotive Rebuilder, July, 1999 issue page 54) we discussed how to check valve stem to guide clearances and the methods used to replace valve guides and seats. Camshaft clearances and techniques used to bring clearances to within specification were also covered. In this article we will cover the final assembly of the head.
The final step in our remanufacturing process is the assembly of the head. Proper assembly is just as important as any other step. The assembly process brings the casting and all of the parts together after all of the inspection, machining and cleaning has been finished.
All of the parts need to be ready when you begin the assembly. The parts required to assemble a cylinder head are: valves; keepers; retainers; springs; rockers; rocker shafts; cam(s); lifters; shims; seals; studs; and freeze plugs. The following are tips that we recommend to prepare each of the aforementioned components for final assembly.
Valves are first glass beaded to remove carbon and corrosion. Next, the valve stem is checked for wear and/or damage. The keeper groove and butt of the stem are also inspected for wear. Next, the valve is reground to the corrected angle; most valves are 45°, however, some applications require a 30° angle.
After the valve is ground, the margin of the valve must be checked. The margin of the valve is the thickness of the edge of the valve head between the angle and the bottom of the valve. If the margin is too thin the valve will burn.
Spring keepers and retainers are cleaned and checked for obvious damage to the coils. Then the spring pressure is checked and the springs are arranged in sets of equal pressure. Springs found to be just under the correct spring pressure at the specified heightx may be brought into spec by using correct spring spacers.
Rocker arms on overhead camshaft heads ride directly on the camshaft, so wear is more common than on OHV engines. If the pad that rides on the camshaft has wear, it can be resurfaced with a stone. Some rocker arms use a roller tip that rides directly on the cam. This greatly decreases any wear on the rocker arm or camshaft. The roller is inspected to ensure there are no defects in the surface, and that the roller is spinning freely.
Hydraulic lifters are used in some cylinder heads. Due to the cost of new lifters, our lifters are sent to a rebuilder for a fraction of the cost of a new one. A reman lifter has been disassembled, cleaned and inspected for wear before being reassembled with oil. In the past, our experience with cleaning or reusing used lifters often led to complaints of lifter noise, since a bad lifter is hard to detect by visual inspection.
After the bare cylinder head casting is machined and cleaned, it is painted an aluminum color and made ready for assembly. Once the casting and the parts are machined, inspected, and final cleaning is completed they can be brought together on the assembly bench.
The first step for the assembler is to reinspect the head for any defects in workmanship which may have been overlooked. Guides, cam bores, seats, surface and threads are common areas to look at when assembling a head.
Install the valve seals first to hold the valves in the head while checking the stem height. Put the seals on dry (without lubricant) then use a light oil in the guides and on the seal.
After the valves are installed, the stem height is checked using a dial indicator. This is the distance from the spring boss to the tip of the valve. You should look up all of the specifications on the head before starting to assemble the head.
If the valve is too long (over maximum specification) the valve tip must be machined until it falls within the allowable specification. The tip resurface limit varies from valve to valve but usually .010ý is all that should be removed from a valve tip. If the valve is severely out of spec, the valve and seat should be inspected for excessive machining.
After the correct stem height has been obtained, install the valve spring shims, springs and retainers. On OHC and DOHC heads with pad-type adjustment, we put the buckets and pads in with the cam. Then we slide the valve up through the guide, without valve springs installed.
Next we check the clearance between the pad and the cam. If the clearance is close, but slightly tight, we butt some off the end of the valve and recheck for proper clearance. If clearance is too much, the pad can be changed to obtain the proper clearance. This step is simple if the valve spring isn’t installed.
Once the valve lash is within specification take the cam and buckets back out, keeping them in order, and install the valve springs. Next, reinstall the buckets and cam and verify that correct clearance has been maintained. This may sound like a lot of work, but it works very well and facilitates attaining correct valve lash clearance.
The method of compressing the springs varies depending upon the type of head. We have three ways to compress the springs and install keepers. The first way is a conventional C-clamp valve spring compressor, either hand operated or pneumatic. Some cylinder heads with overhead cams will not allow the use of the C-clamp compressor. In these cases we use an air-operated cylinder assembly bench.
On certain heads we can use a hand held, spring loaded keeper installer tool. Use of this tool requires the keeper to be placed inside the relief hole in the retainer. The tool has a spring loaded probe which is inserted between the keepers. Holding the valve head with one hand and pushing down on the tool handle, the valve spring is compressed and the valve stem pushes up on the probe allowing the keepers to be correctly positioned in the grooves on the valve end.
Camshafts are installed next using an extreme pressure lubricant on all the journals and lobes. This special lube adds extra protection over regular motor oil, especially on initial start up.
If the head has an OHC rocker arm assembly that is integrated into the camshaft caps, this is assembled next. This assembly includes rocker arms, rocker arm shafts, springs, shims, and sometimes hydraulic lifters. The rocker arm shafts should line up with the oil holes in the camshaft caps to properly oil the rocker assembly. Failure to install the shafts correctly will result in a lack of oil to the rockers which will cause premature camshaft damage.
The rocker arms may be designated for right or left and intake or exhaust, although some heads have one rocker arm for both sides. Rocker arms with adjustment screws in them should be placed into a soft jaw vise to loosen the adjustment nut before installing them on the rocker shaft. This makes it easier to adjust the valve lash later. If the rocker has a hydraulic lifter integrated into it, be sure the lifter is installed correctly (right side up).
After the rocker arm assembly is completed it should be bolted on and tightened down in a sequence similar to the cylinder head torque procedure. There are many different variations of head designs so the steps in this procedure may be altered or rearranged to fit the particular head being assembled.
Once the head has been assembled, the valve clearances must be set (if the head is adjustable). Turn the camshaft several revolutions with a cam gear and bolt. Then adjust the valves as the cam is rotated according to the firing order.
Next, install any manifold studs that are missing. Heat tabs should be installed on freeze plugs and near water outlets. Care is always taken not to install heat tabs near the exhaust manifold or on the end where the timing belt rides. Placement of the heat tab away from the timing belt is critical to prevent damage which could result should the heat tab fall off the head and lodge in the timing belt and pulley interface.
Finally our heads are stamped with our semi-permanent ink stamp to identify them as our products. We then bag the heads, assign them a part number and enter them into our inventory system.
Our employees continually seek better ways to improve both quality and processing time of product. We welcome any ideas or comments on how other machine shops perform the remanufacturing of cylinder heads.
In this article we have discussed the preparation for final assembly which includes gathering and final inspection of parts, lubrication and packaging. We hope you have found the four articles in this series to be interesting and of value. We at Aluminum Head Service have enjoyed sharing this information and believe that through sharing ideas between automotive engine remanufacturers our overall business will benefit and our customers’ needs will be satisfied to a greater degree.
Also at this time we would like to give special thanks to the people who helped on this article. All four articles were written by employees Scott Schaffner, Jeff Spiller Sr., Ryan Spiller and Dick Schaffner, owner. Photographs and technical aspects were completed by Jeff Spiller Jr.