Tight Engine Tolerances Change Lubrication Needs

One quote I’ve seen lately in the field of automotive lubrication is “. . . due to tighter manufacturing tolerances . . .”. In this magazine and others, I have seen this phrase appear in articles about engine lubrication and bearing wear.

The reader is often led to believe one of the primary reasons for increasing performance in motor oil specifications is increased, or tighter bearing tolerances. After a while, I wanted to know how much tighter these tolerances actually were.

When it came to the automotive measurements, I had the resources at my fingertips. With my trusty Alldata automotive database, I started my research. The Big Three American automotive manufacturers (General Motors, Ford, Chrysler) have continually offered at least one model sporting a V-8 engine for the past 25 years. A quick look into the older books on my shelf proved that this data was accurate for about 40 years on the average.

The four-cylinder and V-6 engines in modern front-wheel drive cars have not been in existence more than 20 years. However, the tolerances remain the same from beginning to end.

The connecting rod bearing allowances for V-8, V-6 and four-cylinder engines have all consistently ranged from approximately 0.0001 to 0.00022 inches.

These tolerances are easily measured with a small strip of plastic placed between the bearing and the journal on a crankshaft. The plastic strip is crushed and deformed as the bearing is torqued to specifications. The rod or main bearing bolts are then loosened. The plastic is retrieved and compared to a visual scale that comes with the packaging.

The wider the crushed and retrieved strip is, the tighter the fit between bearing and journal. This simple system works accurately down to one ten-thousandth of an inch. It is a very accurate way for even the least-equipped home mechanic to check whether his rebuild for his vehicle is within manufacturer’s tolerances.

So where are the tighter tolerances I keep hearing about? If they are not tighter on the surfaces that motor oil is required to lubricate, how could tighter tolerances affect the manufacturing specifications of the oil used every day in cars, spread over a 40-year period?

Just as the velocity of light is a constant in the universe, it appears the manufacturing dimensions of the bearing surfaces on automobile engines have also been fairly constant. So why do we have the higher motor oil specs the past 25 years? I think the answer lies in the heat problems the more modern engines are encountering.

I have said before in this column that heat is the real killer in the modern automobile motor. Most filters do a good job, and most oils have the same rating as dictated by the various governing bodies in the auto industry. Filters perform sufficiently, providing the oil is changed regularly and it has not been overheated.

According to a technical bulletin1 from Chevron Oil, about 70 percent of the parasitic drag in the internal combustion engine is from the shearing of oil molecules in the clearance between bearing surfaces and bearing journals. Motor oil manufacturers have done a wonderful job in keeping this drag to a minimum over a large temperature range.

We are approaching ridiculously light oils (0W-30, 5W-30), and their ability to lubricate for the oil change interval is decreasing. If we could eliminate the need for the oil by eliminating the friction between the engine component surfaces, then we could eliminate a large portion of the horsepower losses in the modern combustion engine.

What has really changed over the past few years is the heat loads that both oil and coolants are expected to handle. I didn’t know there was a 195-degree thermostat when I was in high school. Because I lived in a semitropical environment, we often didn’t keep thermostats in our cars. They were for people who needed the heater to work in their cars.

All of this talk about lost horsepower, bearing clearances, oil viscosities and heat transfer is just a smoke screen for the real villain in the internal combustion engine - friction. Developing near or absolute frictionless surfaces that could handle the temperatures and scuffing of the modern combustion engine would reduce environmental damage and greatly increase fuel efficiency and engine longevity.

All of our solutions are trade-offs now. We haven’t reached the goal of an absolute frictionless engine. Electrical engineers haven’t reached their goal of a room-temperature superconductor. And all diet drinks do not taste as good as the original. But we are working on it.

Reference

1. Chevron Research Company, Technical Service Lubricants Division. (1989). Technical publication: Automotive Engine Oils. p. 10.