Auto Service World
Feature   January 1, 2003   by Andrew Ross

CounterTalk: Knowledge Building – New Engines, New Approaches

It’s not hard to notice that the world of automotive internal combustion has changed a great deal since Henry Ford took his Sweepstakes vehicle to victory more than 100 years ago. What isn’t so readily apparent, though, is the impact that more subtle changes can have on your engine parts sales.

Probably the most notable series of developments, though certainly not the only ones, have come in the sealing category. Though bimetallic engines–with their aluminum cylinder heads and cast-iron blocks–made their debut some decades ago now, developments have continued unabated at both the original equipment level, and the aftermarket to effectively seal these difficult applications.

The most common problem suffered by combining these two metals in a single engine package is the destruction of the head gasket as a result of the metals’ different thermal expansion rates. Aluminum expands more when heated than cast iron, which has the effect of applying a shearing stress on the head gasket.

Sometimes this effect is called “scrubbing” or “fretting,” and is behind the addition of graphite and Teflon friction reducing materials to head gaskets, though graphite also lends high temperature properties. Without allowing the gasket, head, and block surfaces the few microns needed to slide over each other during heating and cooling, the top surface of the gasket would move while the bottom surface, pressed against the relatively stationary block, would remain, well, relatively stationary. Try holding a piece of paper between your hands and sliding them back and forth. You can do it a few times, but eventually the paper is going to snag and tear, wrinkle, or just wear through. (Though your patience will likely have worn out before that!)

When that happens to a head gasket, a garage gets a customer, and you get a phone call.

The replacement head gasket you have to offer may or may not be of similar construction to the original equipment part, and with good reason.

Just as the adhesives used on the assembly line to secure windshields differ from those appropriate for aftermarket repairs, cylinder head gaskets can differ, to account for significant differences in field conditions versus those at the manufacturing level.

One important reason for changes is the fact that a brand new engine is perfect in every way–at least in theory–and so little compensation for wear and the effect of thousands of heat cycles on dimensions needs to be taken into account.

The second important reason is that aftermarket suppliers often re-engineer the head gasket to solve the problem that caused the failure in the first place.

Regarding the first issue, surface finish has been the bane of late-model cylinder head service for several years now. In some circles, it has become something of a controversy. Some camps believe that the OE surface finish must be reclaimed for anything approaching reasonable life from the head gasket. Others have said that the pre-existing standards were sufficient.

The truth is somewhere in the middle. Using the Ra scale, surface finishes have been called for as smooth as Ra 7-15 micro-inch for use with Multi-Layer Steel (MLS) gaskets, but since many shops can find it extremely difficult to get down to the low end of this, some gasket manufacturers allow up to Ra 30.

This compares to Ra 60 to 100 for cast iron heads, and a smoother, but now much more customary Ra 20 to 60 for aluminum.

It is important to remember in all of this that few people, even seasoned machinists, can “eyeball” the surface finish. I recall sitting in a room with a dozen or so machinists as they were challenged to select the smoother finish on sample head sections. They weren’t asked to estimate the Ra, just pick the smoothest finish. Nobody did.

The only sure way to check is with a profilometer, but those are very expensive so comparator gauges are the next best thing.

This is, of course, all stuff for the machinist to focus on, and as long as he does his job properly, the head should be surfaced properly for the application.

Which is exactly half the job. A head re-and-re leaves behind the block, and it is not uncommon for the technician to damage that surface just by employing improper cleaning techniques.

Technicians need to be aware that using mechanical grinding or abrasion tools is not appropriate for cleaning off gasket material. They need to take care with scrapers, too, but the rubberized discs used on drills can create severe surface irregularities that today’s gaskets cannot seal.

Abrasive pads or a proper scraper used with gasket removing compound is the best technique.

One issue that is not focused on enough, in my opinion, is proper torque-down techniques and values. As much as the change to aluminum cylinder heads and the thermal expansion issues have played a role in repair techniques, the continual thinning and lightening of cylinder heads and blocks by engine designers has injected added stresses on the technician.

Ford’s experience with the 3.8 V6 engine–the subject of one of the largest, longest-lived warranty crises of recent memory–is the best example of the impact that insufficient clamping force can have on gasket life.

When people discuss MLS gasket technology, it is usually in terms of this gasket construction’s ability to accommodate lateral movement under thermal stresses. What is not often discussed is MLS’s ability to seal under low clamp load, allowing for more head lift than other gasket constructions.

The reason for low clamp load is relatively low torque values, designed to minimize head and block distortion. This was never a problem when these parts were overbuilt as in the past. Now it is a primary consideration.

Every one of your customers should have a current torque tables chart, available from a number of sources. They should also be aware that the torque procedures should be followed in reverse when removing the cylinder head after a suspected gasket failure.

Aluminum cylinder heads will likely warp if they are not de-torqued properly. The problem in allowing this to occur, even if the head is being replaced, is that it may cause the technician to believe that a warped head caused the gasket failure when it may have been another cause entirely, such as detonation or preignition, incorrect torquing due to incorrect procedure or re-use of torque-to-yield bolts, or poor cooling system performance.

Yes, the head will be resurfaced, but failure to correct the initial problem could mean a comeback a few months or weeks down the road. Just try suggesting that it was the way the technician removed the head that caused the comeback then.

There are certainly myriad changes beyond these, but the point here is not to trust pre-existing rules of thumb. There have been changes in procedures and construction that may have slipped past technicians who do not perform engine service regularly.

Always check current specifications and procedures, ask for updates on installation instructions, and make sure that your customers are aware of these. The engine parts business may not look as high-tech as a schematic or diagnostic tree, but it requires no less attention to detail.