Bolting up engine assemblies usually involves some sort of sealing aid, usually a gasket or chemical sealant. Companies in the sealing business spend millions on research and development, as do the automotive OEM’s, yet few Canadian driveways are free of an oil or coolant stain. What’s going on here?
Canadians tend to drive their vehicles for a long time, and operate under conditions that automotive manufacturers define as “severe service”, although few consumers understand this. Engine design and materials, inadequate maintenance by consumers and poor repair techniques by installers all are contributing factors. While we can’t do much about the engines themselves, technicians can combine good practices with a little customer education to reduce comebacks and improve customer satisfaction.
Why they’re there and how they work
Strictly speaking, gaskets and sealants aren’t necessary at all. If machined surfaces could be polished beyond mirror smoothness, almost to the atomic level, then assemblies would “cold weld” on contact, creating a bond that’s as strong as a welded assembly. Of course, that’s impossible in the real world, but the correlation between mating surface smoothness and sealing power is real. Gaskets and sealants allow engine assemblies to be machined to reasonable smoothness and flatness tolerances, with the sealing system taking care of the rest. For example, traditional cast iron engines use cylinder head and block deck mating surfaces with visible arcs from the fly cutting process used at the factory. The conventional wisdom was that the slightly roughened surface “held” the gasket in place and reduced creep. On top, cork and cork composition gaskets were used between stamped rocker covers and an often as-cast rail on the head. Combined with similar technology at the oil pan and “rope” rear main seals, oil leakage was almost a given as the gaskets hardened with age. The switch to alloy heads and complete engines, however, has changed both the gaskets and the surfaces they seal.
According to Bryan Schrandt, vice president & general manager for Detroit Gasket (a Freudenberg-NOK Company), the surface finish issue has added a new dimension to traditional gasket replacement, particularly the head gasket: “When you get to multi-layer steel, currently in production for numerous OE applications, it’s a different ball game. They require extremely flat surfaces, and more importantly, extremely smooth surfaces. It’s not quite a mirror surface, but it’s much smoother than the average (technician) is used to. The equipment to replicate that finish is pretty expensive. There is a move afoot to try to develop service gaskets for those engines so that the surface finishes don’t have to be replicated again. It’s expensive. At the OE level, (it takes) equipment that’s well into six figures. That’s fine when you’re building two or three thousand engines a day. It’s a little different when you’re doing a couple a day.” Wrapping sandpaper around a piece of two-by-four simply isn’t enough on modern alloy engines.
Assuming the mating surfaces are clean and flat, the other variable in the technician’s control is the clamping force holding the assembly together. “Torquing up” a housing or assembly would seem to be a straightforward way to clamp up mating surfaces, but the reality for the gasket is that as little as ten percent of the torque applied to a bolt or nut goes into establishing the clamp load. The rest is used to overcome friction at the fastener’s threads. This friction is one of the main reasons for the switch to angle measurement and bolt stretch when fastening critical bearing housings, but for most sealing applications, torque is still the most practical measurement at the bay level. Torque-to-yield head bolts are one answer to the problem of attaining consistent clamping force, but on other engine assemblies, the technicians are on their own. With so little of the torque going to clamp loading, technicians should consider the threads and bolt holes as important enough to justify at least a compressed air blast (be careful of the debris that flies out, however) if not a quick chase with a bottoming tap.
Bolt or stud threads can also be chased with the appropriate dies. Housings with steel studs threaded into aluminum are another source of trouble. Bent or distorted studs can “dimple” the alloy where the stud enters the mating surface, compressing the gasket unevenly, and possibly giving a false sense of the applied clamp load. A good practice where this is serious is to remove the stud and lightly chamfer the opening of the threaded hole. Is this kind of meticulous work possible when working by the “book”, or in a competitive environment? It depends, but at the very least, special attention to mating surfaces and fasteners should be noted on the work order, and brought to the customer’s attention.
Those torque-to-yield head fasteners are an excellent way to ensure good, clean threads since they’re new with each installation, but check the instructions in the package. Some are coated with a form of lacquer that requires special grease (often moly) for accurate installation. Oil alone may cause inaccurate readings, and if the gasket fails, there won’t be any evidence pointing to this mistake. A surprising number of techs still over torque gasketed assemblies, on the assumption that more is better. For rigid steel or cast assemblies and paper/cork/composition gaskets, it’s possible to get away with excessive torque, but problems can occur in two ways when a technician throws away the torque wrench.
The first occurs when alloy assemblies are bolted to alloy or cast parts. The softer alloy component literally bends, distorting the tension “field” at the mating surface and creating weak regions away from the fasteners themselves. Careful resurfacing is useless if the applied clamp loads warp the head. The other problem results from the difference in the “thermal coefficient of expansion” between aluminum alloys and iron. An alloy head expands much more than an iron block, and that motion can lead to lateral scuffing across the mating surface. While that’s not serious at a water outlet, where a thick composition gasket can be forgiving, for a head gasket it can lead to failures, especially if the cooling system is compromised. If a “hot spot” or localized overheating condition killed the gasket in the first place, then the cooling system must be thoroughly inspected.
Another possible problem area is the intake-head mating surface on V-type engines. Any head or deck surface work will alter the alignment of the intake; something for which aftermarket gasket manufacturers compensate with thicker gaskets that often feel softer when pressed with the fingers. “Softness”, however, is often determined by how the gasket is calendered, and bears no relation to how it will perform under clamp loads.
A little sealant goes a long way
Gasket sealants are an area where there seem to be as many opinions as there are technicians, and range from primitive “gasket shellac” to RTV systems that can replace almost every gasket in an engine. When it comes to the application of sealants to gaskets, however, there is one universal rule: apply sparingly. According to Bryan Schrandt, “the most common mistake is the over-application of supplementary sealers. I can’t tell you how many cases of engine failure we see where RTV is over-applied. It ends up in the oil pan, blocks the oil pump and starves the engine. We see a lot of failures where oil and water passages are plugged with sealers because of the “a little is good, a lot is better” philosophy. Most gaskets nowadays don’t require sealers.”
While major gaskets are trending toward all-elastomer (rubber) materials, one of the oldest comes out of a tube: RTV silicone. RTV stands for “room temperature vulcanizing”, and has been used by technicians for years sealing both coolant and oil. “Gel gaskets” have been used by OEM’s for years in production applications, where the product is screened onto assemblies much like the process used to print T-shir
ts, but in service applications, semi-solids have a potentially serious disadvantage: they’re easily wiped off when snaking a housing down into a crowded engine compartment, something manufacturers don’t worry about when “dressing” an engine on the assembly line.
Does this mean that they’re inferior to cut or molded gaskets? Properly installed, tube-applied products have excellent performance, and may be especially useful in cases where minor damage to a non-critical mating surface would challenge a composition gasket with the traditional light application of sealant.
Sealing technology has changed over the years, but the basics of good sealing technique hasn’t. Clean, flat and straight assemblies clamped by clean fasteners with the appropriate amount of sealant on the right gasket will reduce comebacks to essentially zero. Check next month’s SSGM for more on gaskets and sealing tech. SSGM