6/1/2002
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Ohc Journal Repairs
By Larry Carley
One of the advantages of an overhead cam cylinder head is that it eliminates the need for certain valvetrain components. Mounting the camshaft in the cylinder head rather than the engine block eliminates the need for lifters and pushrods. On some OHC engines, the cam lobes run directly atop the valve stems. On others, cam followers or rockers and hydraulic lash adjusters are used to operate the valves.
Getting rid of the lifters and pushrods reduces the reciprocating weight of the valvetrain and theoretically allows the engine to rev higher and produce more power. But it also requires a more complex cam drive system and limits the amount of metal that can be milled off the face of the head if the head needs to be resurfaced.
Excessive milling changes the installed height of the camshaft with respect to the cam drive, which in turn retards valve timing and ignition timing (if the cams also drive a distributor or crankshaft/camshaft position sensor).
Mounting a camshaft in the cylinder head also increases the risk of cam bore misalignment, which can lead to cam binding and breakage. The heads are usually the hottest part of an engine, so if the engine overheats the heads distort more than the block. Aluminum isn’t as rigid as cast iron, nor can it withstand heat as well as cast iron. As a result, head warpage, cracking, cam bore misalignment and cam failures are common problems with many OHC aluminum cylinder heads.
When OHC heads get hot, they swell most in the middle and bulge upward and outward. This has the greatest effect on the alignment of the #2, #3 and #4 cam journals. This can cause bore distortion, uneven wear, misalignment, possible wiping or even cam seizure and breakage.
Few aluminum OHC heads have bearing inserts. In most OHC heads, the machined surface of the casting supports the cam(s). This reduces the number of parts needed to assemble the head and saves the vehicle manufacturer money because separate bearings are not used. But the lack of replaceable bearings creates a challenge for engine builders when the head needs work and the cam bores are found to be worn, out of round, out of alignment or damaged.
Another factor that often contributes to cam journal bore wear in OHC cylinder heads is inadequate lubrication. OHC cam journals are a long ways from the oil pump, so it takes a few seconds for oil pressure to reach the journals after a cold engine is first started. Many OHC cam failures occur during cold weather because the viscosity of the oil in the crankcase is too thick. It takes too long for oil pressure to reach the cam journals and the journal bores suffer the consequences, i.e., wiping, galling and even seizure.
Most late model OHC engines today are factory-filled with 5W-30 for a reason. The lightweight multi-viscosity oil flows more easily when the engine is cold so it reaches the upper valvetrain components more quickly. Synthetic oil is even better, plus it improves fuel economy.
Using a heavier-viscosity oil can slow the flow of oil during cold weather, which may starve the cam for oil when the temperature drops below freezing. The risk of a dry start is even greater when the engine has sat for several days without running and most of the oil has drained back into the oil pan.
Oil neglect is another factor that can shorten the life of any engine, OHC or not. If the oil and filter isn’t changed regularly, viscosity breakdown, oxidation, acids and sludge can all hurt the oil’s ability to provide adequate lubrication throughout the engine. Any contaminants that end up in the crankcase can also have an adverse effect on lubrication. So if the cam bores show evidence of wiping, scoring or poor lubrication, the underlying cause should be determined and corrected to reduce the risk of the same kind of failure happening again.
OHC Cam Checks
After removing and disassembling an OHC head, two things should always be checked: flatness and cam bore alignment. If one is off, the other will usually be off too. The camshaft should rotate freely in the head once the rockers, followers or valve springs have been removed.
If the cam does not turn freely or binds, remove the cam, place it on V-blocks and use a dial gauge to check it for straightness. Measured runout at the center cam journal should be .001˝ or less. If the cam is straight but won’t turn, the head is warped and the cam bores are out of alignment.
Aluminum OHC heads can be easily checked for flatness in two places: across the face of the head with a straight edge, and down the OHC cam bores with a straightedge or bar. Specifications vary depending on the application, but as a rule most aluminum OHC heads should have no more than .002˝ out-of-flat in any direction on the combustion chamber surface. Cam bore alignment should usually be within .004˝ or less.
If the cam bores are still straight and only the face of the head is out-of-flat, resurfacing should be all that’s needed. On the other hand, if the cam bores are also out of alignment, the head should be straightened before the cam bores are align bored or honed. Straightening will often eliminate the need to align bore or hone the head. But if the cam journal bores are worn or damaged, align boring will be required to install a cam with oversized journals or to install cam bearing shells or inserts with a standard size cam.
Aluminum heads can be straightened a variety of ways. Some shops use a hydraulic ram to cold press a head, but there’s a risk of cracking the head with this technique. The preferred method is to bolt the head to a heavy steel plate using shims to offset the distortion. The head and plate can then be placed in an oven and heated up to about 450 degrees for three to six hours, then allowed to slowly cool back down to room temperature. Another method, which is faster but requires more skill, is to use a rosebud torch to spot heat the head and bring it back into shape. Either way, heat relaxes the head and reduces the risk of cracking. Just be careful not to get the head too hot. Aluminum begins to soften at around 550°-570° F and will anneal at 662° F.
Journal Repair Options
If the OHC cam bores are still round, within normal specifications for wear and free from damage (no scoring, gouges, etc.), no further work should be needed unless the bores are out of alignment. But if the bores are not straight, the head will have to be align bored or honed to bring the bores into proper alignment.
If the cam bores are out-of-round, worn or damaged, which they often are, what happens next will depend on the value of the head, the cost to replace it with another salvage core or a new casting versus the cost to repair it, and how much money your customer is willing to spend on repairs.
In cases where the cam bores are really bad and the head is a relatively common one, the most economical repair may be to find another head that’s in better condition. If the head has other problems, too, such as cracks, loose or damaged valve seats, badly worn or loose valve guides, etc., it may not be worth the time and effort to fix.
If the old head isn’t repairable, isn’t worth repairing or you can’t find a good salvage core to substitute for it, you may have to replace it with a new casting. Aftermarket heads are available for many engines at competitive prices, and cost much less than new OEM heads.
If the head is worth fixing, one repair option for worn or damaged cam journal bores is to machine the bores to .015˝ or .030˝ oversize and install a cam with oversize journals. It’s an expensive fix, but it does allow you to save heads that otherwise might not be usable. The only drawback with this approach (besides the added cost of the oversize cam) is availability.
Though oversize cams are available for a number of OHC engines, coverage is limited. For many applications, oversize cams are not available. Nobody makes them because there isn’t enough demand. So if you want to use an oversize cam and can’t find one, you have to make your own by spray welding the cam journals and then grinding the cam to the desired oversize. Some engine builders use this approach very successfully. But it does add extra steps to the repair process and requires additional equipment and expertise.
On OHC heads that have removable cam journal caps, many shops simply grind down the faces of the caps and machine the bores back to standard size to restore clearances. The process is similar to reconditioning the large end of a connecting rod. But with OHC cam bores, you have to maintain the original centerline of the camshaft. Changing the height of the cam with respect to the head and valves will alter valve timing and lift.
In cases where the cam bores are damaged or are too badly worn to grind down the caps and align bore the head (or there are no removable caps), another repair option is to weld the bores and remachine them back to standard size. This requires a TIG welder and a skilled operator. The head should also be preheated to minimize the risk of cracking, and slow cooled for the same reason. But this adds time and cost to the repair, and may not work well with heads that have thin caps or limited access to the bores.
The most popular repair method today for badly worn or damaged cam journal bores is to install bearing shells or inserts. On heads that have removable caps, bearing shells are relatively easy to install and require minimal machining to restore proper journal clearances. Most of these bearings are for standard sized cams but some are available for use with reground undersized cam journals.
Most heads can be machined in 25 minutes or less. With bearing shells, each half of the bearing has a locating tang so notches have to be cut in the head and cap. This can be done with a file, die cutter or special tool. On heads that do not have removable caps, the cam bores can usually be machined to accept full round bearing inserts. The bearings are then pressed into the cam bores.
Bearing shells and full circle inserts are available for a wide variety of OHC engines and usually cost less than buying or making oversized cams. Dennis Espinoza, an engineer with Dura-Bond, Carson City, NV, says his company has bearing shells and inserts for many OHC applications. The company’s catalog can be downloaded from its Web site at www.dura-bondbearing.com/prodinfo/index.htm.
Espinoza said Dura-Bond is constantly developing bearings for new applications as demand dictates. New bearings that are just now being introduced include ones for the 1997-2000 Ford 4.0L SOHC V6 (FA-6), the 1996-2000 Ford Taurus 3.0L DOHC Duratech (FA-7R, FA-7L & FA-7LA), the 3.0L DOHC V6 Duratech engine in the Lincoln LS (FA-8R & FA-8L), the 1992-98 Acura 2.5L SOHC 5-cylinder engine (HA-5), and the 1993-94 Mitsubishi /Chrysler Laser/Eclipse 2.0L DOHC engine (MIA-6).
"Dura-Bond uses babbit faced bearings with steel backings. The microbabbit provides good conformability and we feel is a much better bearing surface than aluminum. They won’t seize like aluminum will," said Espinoza.
Be sure to check with your replacement bearing supplier for technical and application information to ensure proper installation and performance.
Installing cam bearings in OHC heads obviously requires enlarging the original cam journal bores to accept the half shells or inserts. On some engines, the caps are fairly thin and were never designed to accept a bearing shell. But the slim shells will usually fit most engines with few problems.
Heads can be machined for bearings or oversized cams with large, stationary OHC align boring machines, or with drill-powered portable equipment. The latter seem to be popular with many shops because of the cost. A typical portable head boring system costs $500 to $700 compared to $12,000 up to $20,000 or more for a large align boring machine. Your choice of equipment will depend on a number of factors including quality, efficiency and required operator training and expertise.
According to Dale Wagner of Windy City Cylinder Heads in Chicago, IL, one of the most popular boring bars for installing bearing shells and inserts in OHC heads is one made by Silver Seal in Trenton, MI (800-521-2936). Another is one made by K-Line Industries in Holland, MI (800-528-9138). Portable boring bars are also available from DCM Tech, Winona, MN (800-533-5339).
"We see a lot of heads that have to be fixed with bearing inserts," said Wagner. "One head that often needs this type of repair is the Chrysler 2.0L Neon head. We also do a lot of Chrysler 2.2L and 2.6L heads as well as Ford 4.6L heads.
"One problem we have is that some of our customers don’t want to spend the money to fix the head if the cam journals are bad. When they find out how much it costs, they try to find another head. We charge $75 to do one bearing, or $125 to install the full set," said Wagner.
David Umbel of Mississippi Cylinder Heads in Columbus, MI, says he takes a different approach to pricing.
"We charge $50 to repair the first journal and $25 for each additional journal. But when we price a job on an OHC head, we include the cost to repair the cam bores in the overall price rather than break it out separately.
Umbel says he finds that many OHC heads need to be line bored. Umbel says one of the harder heads to fix is the 5-cylinder Acura head because it has seven cam journals and one is a different size than the rest. The Ford Escort 1.6L head is another where some journals have a different inside diameter.
Mike Rennie of International Engine Works in San Diego, CA, says slightly less than half of the OHC heads they rebuild need some type of cam journal bore repair. Some common problem heads include Honda F22, H23 and D15B heads.
"On some heads we install oversize cams. But cams are hard to find. On most we either cut the heads to accept Dura-Bond bearings or we weld the heads and machine the bores back to standard size. We use a Sunnen OHC boring machine with a single cutter that’s adjustable to any size," said International Engine Works’ Rennie.
Before cutting the cam bores to restore clearances or to install bearings, a die grinder should be used to remove any camshaft material from the bore surface if the cam seized or wiped against the bore. This is also necessary if any welding in the bore area will be done because iron residue will contaminate the weld.
If cam bearings are being installed, check the installed clearances to make sure tolerances are within specifications. For many OHC applications, the recommended bearing clearance is .002˝ or less.
When installing full round bearing inserts, make sure the oil hole(s) in the bearing line up with those in the head and that the bearing is properly centered in the bore.
Cam bearing and journal surfaces should both be coated with assembly lube when the cam is installed to prevent a dry start. The oil system should also be primed prior to first starting the engine for the same reason. Once the engine starts, it should be run at 2,000 rpm for at least 20 minutes for initial break-in.
Cylinder head repair work, on average, accounts for nearly 43% of the total rebuilding business the typical automotive machine shop undertakes. Multi-valve, aluminum OHC heads are becoming more prevalent every year. Be prepared to profit from this market segment!