Heads Up/Rebuilding Aluminum Cylinder Heads
By Dick Schaffner
Do you sometimes wonder what processes and procedures other rebuilders use when remanufacturing aluminum heads? We sure do! It’s because of our quest to improve the quality of a remanufactured aluminum cylinder head that we are writing a series of articles on how our shop, Aluminum Head Service (AHS), Belton, MO, performs aluminum head remanufacturing. In this four-part series we’ll examine the process, in detail, that our shop uses to get the job done the best way we know how.
Over the past 10 to 15 years engine designs have changed quite a bit, mostly due to increasingly strict clean air and emissions regulations. The most obvious way to meet these requirements has been to reduce weight, add fuel injector systems, and monitor fuel delivery and engine performance through on-board electronics. Unfortunately, most consumers don’t want to give up horsepower, so even larger production engines have had to be lightened in order to save weight.
OEM vehicle and powertrain engineers have had their work cut out for them. But it’s also obvious that some of their design ideas didn’t quite live up to performance expectations – the 2.9L Ford, GM Quad 4, 2.6L Mitsubishi and 2.6L Isuzu being good examples. On the other hand, these engines have provided challenging repairs, as well as good business for rebuilders.
To meet these challenges a lot of new tooling and techniques have been developed by equipment and shop suppliers, as well as the everyday genius we call the shop machinist. Machine shops have increasingly had to buy or build new equipment to perform the machining operations required to make a successful repair. Tolerances have become tighter and more moving parts in the valvetrain have driven up the time and investment required to repair today’s OHC aluminum cylinder heads.
For the past 25 years our company has been involved with mostly European and Japanese engine work. The thinner castings, camshafts and valvetrains on these engines have presented us with ongoing challenges. Let’s take a closer look at the primary areas which need to be addressed in order to successfully get the job done. These include:
•Selecting a cleaning process;
•Component part inspection;
•Pressure testing castings;
•Checking all tolerances;
•Making a repair estimate; and
•Actual repair of the cylinder head.
In many ways, teardown is the most important aspect of rebuilding an aluminum cylinder head. In most cases, when tearing down the head you can see where problems lie. It is at this point that you need to make a decision as to which direction you should take when it comes to broken parts such as cams, washers, springs, rocker arms and cam lobes.
You can also usually check cam alignment, providing the journals are not scored from lack of oil or debris from the engine bearings, broken studs, or missing parts such as cam caps and alignment dowels. Be sure to check for large and unrepairable cracks which are mostly under cam journals, and also check for water etching. It is usually at this stage that you can determine if the head is going to be repairable, or whether it will be necessary to purchase a replacement casting.
Choosing a cleaning process is very important when it comes to rebuilding aluminum cylinder heads. We have tried quite a few methods to clean heads, everything from solvents and carb cleaners to ovens and vapor degreasers. Through experience we have settled on a few different methods – aqueous jet washers with special cleaning powders, glass beading and an oven system.
The cleaning process used depends on the severity of build-up and the particular head being repaired. We find that some heads tend to lose temper in the oven at temperatures above 500° F. So where a head exhibits just varnish and light carbon we will jet wash, dry and glass bead.
A head with heavy deposits and gunk, on the other hand, would be baked in the oven at 475° F for six hours and then be glass beaded. We have not perfected our shot blaster for aluminum, but have found that it does a great job on cast iron.
However, there are some shot blasters that can be set up to specifically clean aluminum and do the job well. So far our research indicates it requires a larger investment and you have to train your employees to use them properly. The upside to the baking and blasting system is that your hazardous waste is at a minimum.
Once the head has been degreased, we bead blast the ports, combustion chambers and outside for a clean appearance. It is to your benefit to thread bolts two or three threads by hand into exposed bolt holes before glass beading. This will allow you to keep out any small glass bead particles, especially if there are any "gummy" deposits in the bolt hole.
Glass bead which becomes stuck in bolt holes and is not removed can cause stripped, broken, or stuck bolts in the holes. If a head has oil passages we pull all of the plugs; if plugs are not present we drill holes into the passages so we can run brushes through them to be sure they are clean and free from debris. The drilled holes will be welded or plugged later.
We wash the head before it goes to the machining area; cleaning the parts for the head can take some time. Many late model heads have hollow cams that act as the oil distribution system. They have plugs or steel balls in each end of the cam that must be removed to properly clean the oil passage that runs the length of the cam.
We have found that if you don’t clean the camshaft passage there is a good chance the debris inside will come out after it has been disturbed by washing, immediately destroying the cam and journals in the head. The best way we have found to deal with the plugs (core plugs) is to knock them out with a small punch. Some cams have a threaded pipe plug and are easy to remove.
Steel balls are the hardest to remove. Heating the ball with a TIG welder to a bright red and then shrinking it with cold water or letting it cool will usually allow the ball to come out with a wrap or two on the end of the cam. Steel balls are available from most industrial tool supply houses and are not very expensive.
Cleaning hydraulic lifters is possible on some models, but not all. The lifters we do clean are the ones with clips that are easy to remove. The crimped ones are replaced or sent to a rebuilder.
The lifters we clean are taken apart and all of the parts are kept with each lifter and then dipped in carb cleaner to remove varnish. Following varnish removal they are rinsed in clear mineral sprits and finally put back together and checked for pump action.
Springs, retainers, spring seats, rocker arms, shims and miscellaneous parts are jet washed, dried, then tumbled in a tumbler with clear mineral spirits. Use caution with bolts; they should only be tumbled for five minutes or less so the threads do not become damaged.
Valves are not recommended for the tumbler because of the potential for valve stem damage. Valves are jet washed and dried. We then glass bead the heads, deburr and polish the stems. NOTE: Care must be taken not to polish off any coatings on the valve stem or the wear resistance will be greatly reduced.
Rocker shafts and cams are first jet washed, followed by a brush run through the oil passage, and then finally dried. Pad buckets and pads are jet washed, dried, and polished. Now that the head and parts are cleaned it’s time to inspect them a little closer.
Any major defects might render the head unrebuildable from the start. It’s a good idea, as we mentioned earlier, to thoroughly check the head for obvious defects before spending a lot of time on complete teardown and cleaning only to find that the core is non-rebuildable.
It’s always important to look for broken items such as broken bolts, studs, drill bits or taps, broken or bent valves, and damaged cam caps or ears of the casting. Major cracks in the casting, both in the combustion chamber, under the cam journals, or around the outside of the casting may also be good cause for scraping the head.
After removing the OHC assembly, inspect the cam journals and lobes for wear such as grooves, scratches, gauling, or flat spots. Next inspect the head for excessive warpage or camshaft bore distortion. Lay the camshaft in the cam bores and spin it to ensure it is free wheeling. Then lightly put pressure on both ends of the cam with your thumbs as shown in the photo at the top left corener of page 34, checking for "rock" (a high spot in the center of the cam bores). If excessive warpage or camshaft bore distortion is found, straightening and/or align boring will be needed. Clean the camshaft and make a determination if the cam can be polished, reground, or will need to be replaced.
Removing springs and valves
Valves may be bent to such a degree that they are holding the spring in a fully compressed state. This could make it difficult or impossible to remove the keepers. Since the valve is bent, it is going to be replaced anyway, so carefully use a small hammer to tap valves back into a flattened position. This should allow the valve spring to be removed.
Their are two primary methods of valve spring removal. Some heads can be torn down by either a hand spring compressor or a disassembly machine; usually air or electrically operated. Other heads, which utilize a deep "bucket type" bore, e.g., Toyota 3VZ-E or VW 1.6L, may conceal all but the valve spring retainer. This makes it very difficult, if not impossible, to remove with a conventional type manual valve spring compressor.
In these cases, a disassembly bench is often the only way to go. Note: If working on a head which is unfamiliar to you or somewhat strange by design, it is a good idea to keep one of each of the intake and exhaust valve seals wired to the rest of the parts for identification so they can be matched to the new replacement valve seals.
Valves should slide out the bottom of the guide. If they don’t, use a rubber mallet to tap on them lightly. If they still do not move, the stem might be bent. If a valve will move partially out of the guide but not all the way, then the tip could be "mushroomed," i.e., the rocker has flattened the tip of the valve. To solve this problem, use a file on the corner edge of the tip of the valve while spinning the valve by its head. This will help remove any burrs or mushrooming, allowing the valve to slide freely out of the guide.
Next, remove lower spring washers or shims. This is often one area many inexperienced people forget to attend to. Sometimes the shims are covered in oil or look like they are part of the head. Make sure they are removed and wired together. This is also a good time to check for cylinder head bolt washers which might have been forgotten or stuck.
Now you are ready for inspection of the casting itself. Most visible cracks will need to be marked for repair later. There are two basic types of crack detection processes – pressure testing or dye testing. We will go into further detail on this subject in next month’s article. If the cylinder head is or isn’t cracked, send it on to the next appropriate process.
Examine all threaded holes for obvious defects, such as stripped threads and broken bolts or taps. If holes are stripped, an insert will be required. Most bolt hole inserts should be installed on a mill or with a fixture to ensure straightness, e.g., an intake or exhaust manifold surface. Broken bolts will need to be drilled out and possibly inserted as well.
In our next article we’ll examine how we properly pressure test and begin to rebuild the cylinder head casting. Questions concerning our rebuilding process and procedures may be directed to this magazine by writing to: Heads Up, Automotive Rebuilder, 11 South Forge Street, Akron, OH 44333.