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The Challenges of Rebuilding Late Model OHC Aluminum Heads
By Larry Carley
Take it apart. Put it together. Take it apart. Put it together. If taking apart and reassembling cylinder heads is eating up too much of your valuable shop time, keep reading. We have some tips on how to slash your disassembly/assembly labor costs, as well as other ways to streamline your OHC aluminum head rebuilding processes.
As many of our readers report, the cylinder head market is changing. For better or worse, the mix of heads being rebuilt today is shifting more and more to late model OHC aluminum heads. Five or six years ago, aluminum heads were more of a novelty than a mainstream item.
In the past, most traditional rebuilders didn’t see a lot of aluminum heads, maybe 10% or less of their total head business. A relatively few heads accounted for most of the aluminum head work that was being done: the Ford Escort 1.6L, Chrysler 2.2L, Mitsubishi 2.6L, Olds 2.3L Quad 4, etc. But over the last 10 years, the proliferation of aluminum heads in so many late model vehicles has brought aluminum head work to the forefront. Some shops now do more OHC aluminum heads than cast iron pushrod heads.
What does this mean to the average rebuilder? It means tooling up for the new generation of heads if you want to remain competitive and profitable.
Many shops that are doing late model OHC aluminum heads have discovered head disassembly and assembly creates a bottleneck in their work flow. Some OHC heads do not have removable caps on the cam towers, and the cam is directly over the valves. This makes it harder to disassemble the head, especially if the head also has recessed valve springs and/or four valves per cylinder.
A traditional c-clamp style valve spring compressor can be awkward to use on such applications. Consequently, it can take up to 30 minutes or more to disassemble some of these heads with ordinary hand tools and a spring compressor. Assembly can take even longer — up to an hour or more on some heads.
Save time with a cylinder head work station
The only way some shops can rebuild late model OHC aluminum cylinder heads profitably is to reduce the amount of labor they’re putting into disassembly and assembly. The more complex the head, the more labor it takes. And, the higher your labor cost, the more you have to charge the customer for the head. Get too much labor invested in a cylinder head and you may price yourself right out of the market.
One way to reduce disassembly/ assembly labor is to use a work station designed exclusively for that purpose. A ready-made head bench typically has a table or fixture on which the head is placed. A quick clamping system holds the head steady while the cam, followers, springs and retainers are removed. The valves are supported from underneath so the springs can be compressed mechanically, pneumatically or hydraulically from above.
The suppliers of cylinder head disassembly/assembly benches say in most cases disassembly time can be cut to 10 minutes or less on a typical OHC aluminum head — even ones with four valves per cylinder. You can also save time using the same equipment on pushrod heads, too.
As for reassembly, a head bench can usually reduce your labor time by up to half or more. A late model OHC head that might otherwise take up to an hour to reassemble can often be reassembled in 20 to 30 minutes — and with one pair of hands. You don’t need an extra pair of hands to operate or hold a spring compressor while the valve spring keepers and retainers are being installed.
The combined labor savings on both disassembly and assembly can often add up to an hour or more, say rebuilders who use this type of equipment. Multiply the labor savings times the number of heads you rebuild, and the savings add up quickly. The labor savings can go in your pocket, be passed along to your customers in the form of lower prices, and/or used to boost your shop’s overall productivity.
Fewer hours spent taking heads apart and putting them back together leaves more hours for more profitable machine work in a small shop. And, in a large shop or production rebuilder, anything that saves labor boosts productivity and allows you to increase your volume of work.
One rebuilder who had procrastinated on buying a cylinder head work station and finally bought one said he could "kick himself for not getting one sooner." He now uses the equipment on every head he rebuilds, including pushrod heads, and he has bought a second unit so one can be used full time for disassembly and the other full time for assembly.
Another plus that this type of equipment offers is that it makes it easier for an operator to assemble heads. This, in turn, reduces the risk of making mistakes that can result in costly comebacks (things like misplacing a valve stem keeper, forgetting to install a valve guide seal on one of the guides, or damaging the seal during installation).
There is also less risk of injury because the equipment captures the valve spring and keeps it compressed until the operator releases the tension. He doesn’t have to worry about a c-clamp slipping and releasing a valve spring prematurely.
Cylinder head work bench prices range from about $1,700 for a basic entry level bench up to $5,000 for a top-of-the-line work station. Options that typically add to the base price include a larger, more powerful pneumatic cylinder, a separate work table or stand, additional cup holders and special valve keeper tools.
How quickly the equipment pays for itself will depend on the volume of heads being rebuilt, your labor costs and the complexity of each job.
But according to one equipment supplier, a typical cylinder head work station should be able to pay for itself in three months or less in a shop that averages about six OHC heads a day. The higher the volume and/or labor cost, the faster the payback.
All of the head rebuilders we’ve interviewed over the past two years say a head bench is one of the best time-savers in the shop. No one had any regrets about having made the investment, and all agree a head bench is just as vital as any other piece of shop equipment for working on late model OHC aluminum heads.
Consider this: If you’re rebuilding a dozen OHC heads a week and save 40 minutes on each head by using a work bench, you’ll save $120 in labor every week if you’re paying someone $15 an hour to do this type of work. Multiply times 52 weeks a year, and you realize savings of $6,240 for the year.
The important thing to remember is that increases in productivity work to your benefit a number of ways. It reduces out-of-pocket labor costs, allows faster turn around, especially on custom jobs, allows your employees to accomplish more work in less time, and allows you to be more price competitive if you are losing machine shop labor sales to more efficient competitors.
In areas like the Northeast, where the cost of doing business is higher than in other parts of the country, anything that reduces labor is a plus — especially in today’s market where heads are shipped overnight almost anywhere in the country. No local market is "safe" from outside competition these days.
Another issue head rebuilders face today is finding good rebuildable cores. Cores that can’t be rebuilt have to be replaced — either with another used core or a new casting.
For some late model applications, cores are very scarce and pricey. It’s the law of supply and demand. The more a head is in demand, the harder it is to find, and the more you have to pay a core supplier or salvage operator if he has one.
Buying another core is always risky because the second head may be in no better condition than the first. According to some rebuilders, up to 70% of certain late model aluminum heads are usually found to be cracked.
New OEM castings can be quite expensive, though some vehicle manufacturers have lowered their prices to be more competitive with new aftermarket castings. Aftermarket heads typically sell for a third to half as much as a new OEM casting, but can still cost up to several hundred dollars for a bare casting — which is a lot more than a complete core in most cases. On the other hand, as competition between OEM and aftermarket head casting suppliers continues to grow, we may see prices come down somewhat.
The availability of aftermarket heads keeps expanding every year with more new heads being added to a growing number of engine applications. Yet for many engines, there are still no aftermarket replacement heads and never will be. Tooling up to produce a replacement head casting is very expensive.
Unless an application is a high volume, high demand head, it probably isn’t worth the investment to tool up for it. So one alternative is to hunt around for a rebuildable core or buy a new OEM casting (assuming one is available). The other alternative is to salvage the old core by repairing the cracks.
Almost every rebuilder who does any significant volume in aluminum heads today has some type of welding equipment to repair cracks, e.g., a TIG or MIG welder for aluminum heads, a spray welder or furnace welding setup for cast iron heads, and/or pins for repairing cracks in both cast iron and aluminum heads.
The combination of core scarcity combined with high replacement costs has made crack repair a necessity rather than a luxury. Many cracked heads that were once discarded because they were not worth the effort to repair are now being repaired successfully and reused — some more than once.
Repairing a crack also allows you to recover the labor that would have otherwise been lost if a head that turns out to be cracked is discarded. Some cracks are obvious and can be easily seen, so no time is wasted in disassembly, cleaning or inspection. But, many cracks can’t be seen until the head is taken apart, cleaned and tested with penetrating dye or a pressure tester.
If you end up discarding the head, you also throw away all the labor you’ve put into it up to that point. Being able to fix the cracks keeps the head in process and saves the labor already invested in it.
Though TIG (tungsten inert gas) welders are popular for repairing cracks in aluminum heads, the head has to be preheated and then slow cooled after welding to reduce the risk of new cracks forming.
One rebuilder we interviewed said he is using a "Miller Aerowave" welder that was originally designed for the aircraft industry. He says the Aerowave welder costs about twice as much as a conventional TIG welder but produces a more concentrated arc.
This prevents the heat from spreading as quickly into the surrounding metal and eliminates the need to preheat and post-cool the head. This eliminates two steps in the welding process and saves a lot of time.
Special advanced design pins developed by such companies as Lock-N-Stitch are also available to not only repair but strengthen castings which have had cracks repaired using the pinning process.
Regardless of the repair method used, heads should always be pressure tested afterward to check for leaks — especially porosity leaks that may not show up with penetrating dye.
Near perfection is required when it comes to head and deck flatness in many of today’s engines. Out-of-flat lengthwise should not be more than .003˝ (0.076mm) in a V6 head, .004˝ (0.102mm) in a four cylinder or V8 head, or .006˝ (0.152mm) in a straight six head. The maximum allowable limit for out-of-flat sideways in any head is .002˝ (.05mm) — with no sudden irregularities that exceed .001˝ in any direction.
Yet, many late model cores are badly out-of-flat. Aluminum is not known for its rigidity and can easily warp if the engine gets too hot. Many OHC camshafts are also bent or won’t turn in the head because the cam bores are out of alignment (as a result of warpage on the head face).
Checking and straightening bent cams adds time and labor expense, and replacing bent or broken cams is even more expensive. But there’s no easy solution for this aspect of OHC aluminum head rebuilding.
The need to straighten
Straightening heads before resurfacing can minimize the amount of metal that has to be removed from the head’s surface. Most late model castings are relatively thin and can’t handle much more than a light cut. Milling the head also reduces the head’s installed height, increases compression and retards OHC cam timing. This may require using a thicker head gasket or a shim to restore normal head height, compression and valve timing when the head is installed.
Heads can be straightened by counter-shimming the head and bolting it onto a thick steel plate. The head can then be placed in an oven, cooked for several hours and allowed to slow cool.
If the right combination of heat and shims are used, most of the warpage can usually be eliminated, which may also eliminate the need to align bore or hone the cam bores and/or resurface the head. But, sometimes the metal doesn’t move exactly as planned, and the job has to be done over.
One trick that can save some time is to preheat the head in an oven, then remove it and straighten it in a press while it is still hot.
Some rebuilders say they’ve had success straightening heads using a torch to spot heat the top of the head and "counter-warp" it back into shape. It’s a trial-and-error process at best, but with some practice, you may actually end up saving some time once you’ve perfected a technique for a particular head application.
For years, most aftermarket gasket manufacturers have recommended a surface finish of 55 to 110 microinches (60 to 125 RMS), with a preferred range of 80 to 100 RA.
But, on many engines today, they now recommend a surface finish of 30 to 110 RA for cast iron head and block combinations, with a preferred range of 60 to 100 RA. For aluminum heads, the numbers are even lower — 30 to 60 RA, with a preferred range of 50 to 60 RA or less, depending on the special requirements of the application.
Bimetal engines are extremely hard on head gaskets. The difference in thermal expansion between an aluminum head on a cast iron block is considerable and exerts a sideways shearing force on the head gasket. If the surface finish is too rough, the head bites into the gasket and rips it apart as it scrubs back and forth with every drive cycle of the engine.
To increase gasket durability, some OEMs have gone to expensive MLS (multi-layer steel) head gaskets. But MLS gaskets typically require a very smooth surface finish: 8 to 15 RA.
Ford’s 4.6L V8 uses an MLS gasket and requires a very smooth surface finish. However, at least one aftermarket gasket supplier, Victor-Reinz, now offers a standard graphite replacement gasket that can tolerate a more traditional surface finish.
Achieving such a smooth surface finish requires proper resurfacing equipment and techniques. Most rebuilders say they prefer dry milling heads because it eliminates the mess and maintenance that goes with wet grinding. Dry milling allows very precise control over the stock removal, and is faster than grinding because more metal can be removed in a single pass, eliminating the need for multiple cuts and saving time.
The key to achieving the smoother finishes required by many of today’s aluminum heads is to use the right combination of table feed and rpm when milling the head.
This requires a variable speed table and/or multi-speed or variable speed milling head. Increasing the rpm of the cutting head and/or slowing down the feed rate makes it possible to achieve a much smoother surface finish.
A milling machine with a multi-bladed cutter head with PCD (polycrystaline diamond) or CBN (cubic boron nitride) tooling will give the fastest cutting speed.
One equipment manufacturer recommends a feed rate of two inches per minute at 1,000 rpm to achieve a surface finish of 12 RA with a two-bladed cutter. Carbide or PCD tooling is usually recommended for aluminum, though some shops say they’ve also had good results on aluminum with CBN tooling.
Though most shops that mill heads do not use a coolant, milling with a coolant has some advantages. Coolant helps wash away swarf for a cleaner, more consistent cut.
It also keeps the tooling and work piece cooler which reduces heat build-up, distortion and tool wear. Consequently, milling with a coolant can help improve the overall process and improve the surface finish.
When resurfacing an aluminum head that has a lot of lime built up in the water jackets, the hard calcium deposits around the water jacket openings can sometimes be picked up by the tooling and dragged across the surface leaving a groove. Removing the deposits before the head is resurfaced can eliminate this problem and the need for a second cut.
Removing grease and grime from aluminum heads is probably more of a challenge than cast iron heads because most customers want a bright, shiny finish. A dull, discolored casting isn’t very attractive and may give a customer the false impression that the head is somehow lacking in quality as well as appearance. Most rebuilders who do aluminum heads are therefore very serious about restoring a like-new appearance to the casting.
How this is best accomplished varies depending on whom you ask. Some prefer an oven for baking off grease and dirt while others use a spray washer to wash the castings clean.
For follow-up, some use different media for blasting such as glass bead, stainless steel cut wire and zinc pellets, or soft media such as walnut shells or even baking soda. Some leave the finish natural while others apply a coating of bright aluminum paint.
Since all aluminum heads have replaceable valve guides, many rebuilders simply replace the guides if they are worn. But, to save time, others use one of two techniques to restore worn guides: they ream out the guides and install liners, allowing them to reuse the original valves or rechromed valves, or they ream out the guides to oversize and install valves with oversized stems.
Each approach has certain advantages. Replacing guides with new ones allows the original valves to be reused or allows for the use of valves with standard sized stems. But, replacing guides also runs the risk of galling the guide hole when removing and installing the new guide if not done properly. The same risks apply when replacing seats. Preheating the head and/or chilling the guides (or seats) prior to installation can lessen the risk of damaging the head. But each additional step adds time.
With cast iron heads that have integral guides, most rebuilders either install guide liners or ream out the guides for oversized valves if the guides are worn. Either technique can also be used with aluminum heads — which may save time and trouble depending on the application.