GM Once Tried To Use Rubber Oil Caps And It Was A Disaster

For automakers, cost cutting is crucial for keeping the lights on. It’s very much a race to the bottom—if your competitors can sell similar cars for a lower price, you’re going to find it hard to compete in the marketplace. Sometimes, though, the penny-pinching can go too far, resulting in sub-standard parts that aren’t quite up to scratch.

One of the largest automakers in the world, GM, has always been a pioneer in the field of saving a few bucks. In the 1970s, it had the grand idea of making oil caps simpler. Why bother with molded plastic or multiple pieces of stamped steel with a separate gasket when a simple chunk of rubber would suffice?

It was a cheaper option, to be sure, but it also prioritized cost over quality. These budget oil caps would eventually prove to be subpar as years of aging saw them harden and crumble in use, as you can see in the YouTube video below by Rare Classic Cars & Automotive History:

The rubber caps landed on a wide range of GM vehicles from the early 1970s through to the early 1990s. It primarily appeared on models using the classic small-block Chevrolet V8, as well as some four-cylinder engines and GM’s 60-degree and 90-degree V6s of the 1980s and early 1990s.

Ultimately, you’d find this cost-saving measure on a wide range of Chevy’s cars, including fancy models like the Corvette. You’d also spot them on Buicks and Oldsmobiles, and all sorts of trucks and vans. However, these caps didn’t only appear on the cheaper brands. You could even buy a Cadillac Cimarron with the 2.8-liter V6 in 1986 and find one of these press-on oil caps in the engine bay.

The reasoning behind the decision is easy enough to figure out from first principles. An all-rubber oil cap—more of a plug, really—can be made in a single production step. Squirt rubber into a mold, and it’s pretty much done.

Meanwhile, a plastic oil cap would generally take multiple steps, first with the molding of the cap, then molding a rubber seal, and then combining the two. Similarly, fabricating a metal cap would be much more time consuming too, and it would also typically require a rubber seal.

There’s also the benefit that a press-in rubber oil cap is effectively its own seal. The valve cover or tube that it covers simply needs a clean round aperture for it to mate with. In contrast, twist-on caps require a lip or thread of some sort, which tends to require a machining or tapping operation. Eliminating this operation saves money on tooling and time on the production line.

From a cost perspective, these rubber caps seem like a dream. It’s in the real world where they tend to fall down. Over time, rubber tends to break down, becoming hard and losing the elasticity it’s so prized for. In extreme cases, it can begin to crumble, shedding particles as it falls apart. This process is typically accelerated when the rubber is exposed to repetitive heat cycles, such as you might find in your engine bay.

Obviously, rubber holds up in many applications in the engine bay, but it always degrades over time—and in this case, in a way that tended to frustrate customers. While regular oil caps often have rubber seals, too, they’re generally built in such a way that there’s less risk of rubber chunks flaking off into the valve covers or oil filler itself. Not so with the all-rubber cap.

Some owners complained of aging oil caps failing to seal properly, leading to fumes in the cabin and oil mists desecrating the engine bay. In other cases, the rubber caps could fall apart, with the top section shearing off and leaving a plug in the oil filler tube on some models. In some cases, crumbling rubber particles could even end up in the oil pan—somewhat of a concern, as this risked blocking the oil pickup.

The one saving grace was that these rubber oil caps were easily accessible and cheap. They might degrade, but you could swap one out in five minutes for just a few dollars. Of course, most cars don’t have disposable oil caps that need regular replacement, but at least it wasn’t a tragic or expensive failure when these parts did harden or fall apart over time.

As seen in this DIY video, the rubber caps weren’t exactly easy to remove. This 1991 Chevy S10 features the 4.3-liter 90-degree V6. 

[Ed Note: There are so many different styles of oil caps; there’s the old-school metal breather cap that’s held on by some metal springs that grip the inside of the tube, there are some with dipsticks built in, there are single-piece threaded ones made of plastic or stamped steel, and then there’s the dastardly two-piece plastic design found on Jeep 4.0-liter engines, among others. It’s a strange ratcheting-style cap that, in time, will stop working, instead simply clicking when you try to loosen it. Oddly, if you let the engine cool down, the cap often works just fine, leading some to spray the cap with brake cleaner to cool off the valve cover so it will loosen its grip on the cap. Sometimes that doesn’t work, and you’ve got no choice but to break out pliers:

Why is this so complicated?! It’s an absurd and unnecessary design that many simply replace with a single-piece cap. Sometimes the answer is: Keep It Simple, Stupid. (But not too simple — see rubber cap issues). Anyway, back to Lewin. -DT]

Cost-saving measures will always be popular with automakers. If you can save even just a dollar per engine on a million engines, you’ve saved a million dollars, and that scales the longer you’re in production. However, these same measures can annoy customers when they impact the end user experience. Rubber oil caps did just that, turning a lifetime part into something owners actually had to worry about. When they left the market in the early 1990s, they were certainly not missed.

Image credits: via YouTube screenshot, GM