Originally featured mainly in European racing machinery from Fiat, Peugeot and Alfa Romeo in the early part of last century, by the 1960s companies like Fiat and Jaguar had introduced OHC production engines. It was advanced technology for the time, but today OHC is the norm for even the least expensive production engine.
OHC cylinder heads, usually made of aluminum with multiple valves per cylinder, can offer engine builders opportunities to specialize in a niche market or to supplement services you already offer.
OHC heads have many advantages over older style overhead valve designs (OHV), but they are more complex and generally cost more to manufacture. This can actually be a benefit to cylinder head rebuilders, allowing enough room to make a profit on your labor. While new aluminum castings are available for certain applications, aluminum cores remain the starting point for most OHC head rebuilds.
OHC valvetrain configurations place the camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to OHV and pushrods. Compared to OHV pushrod cylinder heads with the same number of valves the reciprocating components of the OHC system are fewer and have a less total mass.
Though the system that drives the cams may have become more complex, most engine manufacturers are willing to make the trade off for better engine performance and greater design flexibility. Another performance advantage is gained as a result of better optimized port configurations found on overhead camshaft designs. Without the pushrods getting in the way, an OHC cylinder head can use straighter ports.
An OHC valvetrain can be driven using a timing belt, chain or even gears. While today’s multi-valve per cylinder engines are more often employing variable valve timing systems to improve efficiency and power, OHC designs also allow for greater engine speeds over pushrod engines as a result of having lower valvetrain mass.
Core availability for OHC late model cylinder heads doesn’t seem to be as big of an issue as it used to be, according to experts. That is because new castings have in many cases filled in where cores were once unavailable or not rebuildable. Better rebuilding techniques have also made cores that were once thought to be destined for the recycle pile easier to repair.
With the camshaft mounted in the cylinder head and not in the block like OHV designs, the distance between the valve train and the oil pump is further, requiring a healthy flow of oil to keep the cam and followers lubricated. If anything blocks or restricts the flow of oil to an overhead cam it will lead to big trouble soon. It is critical in OHC engines that the oil remains clean and the manufacturer-recommended viscosity be followed to maintain oil film strength between the cam journals and head, cam lobes and followers. Cam journal clearances must also be within wear limits to maintain good oil pressure, too.
The most common problem in OHC engines is worn and damaged cam journals in the cylinder head. In most late-model OHC engines, cam bearing inserts are not used. The camshaft usually runs directly on the machined bearing surface in the head. The aluminum casting provides a good bearing surface and requires a good film of oil between it and the cam to prevent wear and metal-to-metal contact. If the journal runs out of oil or the oil is dirty or oxidized, wear, galling and seizure are sure to happen sooner than later.
Cylinder head experts say that what is most in demand today in the OHC market is the multi-valve, 4-cylinder and V6 aluminum heads from VW, Mitsubishi, Honda and GM as well as some Ford 4.6L and 5.4L heads. Shops that stock the most popular heads can speed up turnaround times and increase their positive cash flow. Much of what a shop invests in equipment for OHC applications can be used on a wide variety of engines. Cleaning equipment such as spray cabinets and ultrasonic washers, for example, can be used for just about any job.
One of the biggest challenges in rebuilding late model cylinder heads is the cleaning process, according to one rebuilder. It’s a slower process with aluminum OHC heads no matter what you use to clean them. You must make sure you get them completely clean and that you don’t leave anything behind. If you use a glass bead blaster or other media you had better make sure you get all the media out of the passages in the head or you may get a comeback.
The preferred cleaning methods vary from engine builder to engine builder. Some rebuilders use ovens, blasting media, spray washers and aqueous systems with no problems. The best advice is to know the proper techniques to employ when using each type of cleaning equipment.
Disassembling OHC heads requires some special tools and equipment. If you’re doing a large volume of heads, a cylinder head workstation is a must. With a workstation you can greatly increase your productivity. A workstation can save up to an hour of labor per head, which in a year’s time can add up to a lot of savings. Many of these machines come with rotating tables, quick clamping devices and high-powered pneumatic cylinders to compress valve springs and remove or install keepers.
Next you have to get the heads straightened. There are several different ways to straighten aluminum cylinder heads and many rebuilders have their own techniques. Some rebuilders bolt the heads to a plate and put them in an oven; others press them; while others heat them.
Once they’re apart and clean, you have to deal with stripped threads, and because you’re dealing with aluminum you usually have some damage to the head. Spark plug holes often require thread inserts. According to experts, spark plug thread repair is a common problem with aluminum heads.
Line boring and an oversize cam may be necessary to properly restore the cam bores. Most late-model OHC engines don’t have cam bearing inserts. If the engine was overheated, cam bore damage is very common. Usually the middle bores are the most damaged because the head bows up in the middle. There are a number of ways to salvage a damaged head. One is to line bore or hone the OHC cam bores to accept a new cam with oversized journals, or to removed the cam journal caps and grind down the face of the caps and machine them back to standard size. If there are no caps, the cam bores can be enlarged to accept bearing inserts, if they are available for the application.
Another problem to watch out for is a bent cam. It only takes a few thousandths of an inch of misalignment for the cam to bind in the head. To check cam straightness, place the cam in a pair of V-blocks (one at each end), then rotate the cam with a dial indicator placed on the middle journal. If runout exceeds specs, the cam is bent and needs to be straightened or replaced.
Here’s a typical list of procedures for reconditioning OHC cylinder heads:
•Thoroughly clean disassemble heads
•Clean valves, springs, keepers and retainers separately
•Check spring pressure with mechanical tester
•Wear check with micrometer
•Stem end re-faced
•Valve face precision ground
•Overall length equalized
•Valve stem heights checked and set
•Assembled to OEM specifications
•Check for leakage on valves and seats
•Surface straightness checked
•Magnetic particle or pressure tested
•Guides checked for wear
•Seats checked and precision cut to proper width and angle
•Studs checked for damage and proper heights
•Cleaned again prior to reassembly
A few years ago the OHC market was mostly import heads that were being serviced, however, experts say that isn’t so much the case anymore, there is a more even mix of both import and domestic applications since most domestic manufacturers have gone to OHC heads. The domestic manufacturers had problems with castings early on similar to what the Japanese manufacturers went through when they switched to aluminum heads years ago. Many of the early domestic OHC heads, especially GM, developed cracks that needed repair. It is probably safe to say that many of these heads have been repaired at least once already.
A big advantage to doing head work, says one cylinder head expert, is that turnaround times are quick and therefore can help generate the all-important cash flow in a tough market. Doing this kind of work gives shops an opportunity to turn a job into cash in one or two days, which can be attractive for many reasons.
Special thanks to Dura-Bond Bearing Company for their help. To find a complete list of suppliers for OHC applications, click on the Buyers Guide tab.