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Race Engine Technology Magazine

Whenever a speaker or writer is trying to convey technical information to an audience, the presence of obvious errors, no matter how small, tends to detract from the credibility of the rest of the material being presented.

The misuse of the terms "clearance" and "tolerance" in contemporary technical writing is a blatant example of that sort of error, and appears to be an increasingly frequent occurrence. That error interferes with the clear communication of technical information. However, it is increasingly doubtful that today's "technical audience" will even recognize the error.(For further evidence of our deficient and degenerating education system, see the rant BELOW.)

"Writers" who should know better often treat the two terms ("clearance" and "tolerance") as if they refer to the same concept, and are therefore interchangeable. They frequently suggest that a component or mechanism can be improved by the imposition of "higher tolerances" - when, in fact, the exact opposite is true.

More than a few of these writers blather on with self-bestowed authority, while perpetuating manifest technical and grammatical errors. This short article is an attempt to clarify, within the context of mechanisms and manufacturing, the meaning of the two terms "clearance" and "tolerance" and the obvious differences between them.

The Merriam-Webster® dictionary contains four different definitions for the word "tolerance". The definition ( non-politically-correct ) which is applicable to this context is: "the allowable deviation from a standard; especially the range of variation permitted in maintaining a specified dimension in machining a component."

That same dictionary contains three basic definitions (and multiple examples) of the word "clearance". The definition which applies to this context is: "the distance by which one object clears another, or the clear space between them."

Stated simply, CLEARANCE is the distance between the adjacent surfaces of mating parts (how much the mating parts CLEAR each other), while TOLERANCE is the allowable variation of a dimension from its nominal (desired) value ( the amount of error one will TOLERATE ).

As an example, let's discuss the fit between the main journals of a crankshaft and the engine bearings which support that crankshaft (and allow it to rotate with relatively low friction). Suppose we have a crankshaft in which all the main bearing journal outside diameters (OD) are exactly the nominal (desired, ideal) diameter of 2.4488 inches. Further, suppose that the inside diameter (ID) of all the main bearing bores in the engine block arel exactly the nominal of 2.6411 and that all the main bearing shells are exactly the nominal thickness of 0.0953.

Basic arithmetic reveals that the ID of the main bearings (as installed in the block) will be 2.4505.

2.6411 - (2 x 0.0953) = 2.4505

Further basic arithmetic reveals that the crankshaft journal OD's are 0.0017 smaller than the main bearing ID's.

2.4505 - 2.4488 = 0.0017

That difference (0.0017) is the bearing CLEARANCE, (the distance by which the journal outside surface CLEARS the bearing inside surface). That clearance, as explained in the engine bearings page, has a dramatic effect on the load carrying and frictional properties of the journal-bearing system. That 0.0017 nominal value, BTW, is a good number for this size bearings in a production engine for a road vehicle.

In real life, however, it is a rare case when the dimensions of a manufactured component are all exactly nominal. Contemporary manufacturing processes enable parts to be made with ever smaller variations from nominal at an acceptable cost; however, variations still do occur. Those variations from the nominal dimension are the TOLERANCES applied to the part ( the variation from nominal that will be TOLERATED ).

Going back to the example, the production TOLERANCE on the 2.4488 crankshaft journal OD is +/- 0.0005 (plus-or-minus 0.0005 inches), which means that any particular journal can have a diameter anywhere from 2.4483 to 2.4493 (a variation of 0.001 total). Similarly, the manufacturing tolerance on the main bearing bores in the block (in this example) is +/- 0.0005, which means that the ID of any main bearing bore can vary from 2.6406 to 2.6416. If we ignore the tolerance in the bearing shell thickness (assume that all bearing shells have the nominal 0.0953 thickness), then the journal and bore tolerances mean that the production clearance between any main journal OD and its mating main bearing ID can vary from as little as 0.0007 (2.6406 bore and 2.4493 journal) to as much as 0.0027 inches ( a 2.6416 bore and a 2.4483 journal).

Naturally, in a precision high-performance engine, the bearing CLEARANCES (typically LARGER than OEM) are held to a much SMALLER TOLERANCE (+/- 0.0001) than the OEM configuration. Those very small tolerances, coupled with appropriate clearances and exact axial alignments, can produce an engine in which the crankshaft, with all 5 main bearings torqued to spec, and with the front and rear oil seals installed, can be spun with the flick of the wrist and the result would make you think the crank was running in rolling element bearings instead of journal bearings.

For an example, see THIS VIDEO, which shows the crankshaft being spun with minimal hand-effort in a fully-torqued block assembly in our V-12 engine. And this is not even close to what the NASCAR engine guys achieve on a daily basis.

ADDITIONAL OBSERVATIONS ( another Rant )

The subject matter of this page (clearance and tolerance) is directed toward one specific error that occurs all too frequently in contemporary "technical writing". The fact that there are SO MANY OTHERS is a depressing statement about our degenerating educational system.

For example, it is commonplace today to read published articles on a wide variety of subjects in which the author apparently does not even know the difference between THERE, THEIR and THEY'RE. (In fact, I have met TEACHERS who did not know the difference.)

......Or between BARE and BEAR; or BRAKE and BREAK; or COURSE and COARSE; or CHORD and CORD; or CAPITAL and CAPITOL; or DUAL and DUEL; or FAIR and FARE; or MUSTARD and MUSTERED; or PLANE and PLAIN; or PRINCIPLE and PRINCIPAL; or PAIL and PALE; or PORE and POUR; or RAIN, REIN and REIGN; or ROLL and ROLE; or SHEAR and SHEER; or STEEL and STEAL; or VERSUS and VERSES; or VICE and VISE....... the list goes on.

It is apparently an elusive concept that the spelling of a word can dramatically alter the meaning of the word..... that a different word that SOUNDS the same does not convey the same meaning. That often leaves the reader to try and figure out what the writer actually means---or experiencing disgust at the ignorance being manifestly displayed..

It is abundantly evident that there are many other elusive language concepts, such as the identity, structure and use of a subject and a verb in a sentence; the agreement between nouns, verbs and adjectives with regard to number and gender, and other pre-high school language basics.

We also see the increasingly-common usage of "dominate" in place of "dominant", which then extends to the erroneous use of "predominate" in place of "predominant", as in ".....iron is the predominate element in this steel alloy "...(instead of the correct... predominant element... ) .

And that brings us to the phenomenally ignorant practice of using "apostrophe-s" to create a plural form (as in "....this assembly requires eighteen bolt's...").

OIL "GALLEY"......?

While I am ranting, I might as well add the all-too-common use of the term "oil galley" to describe a drilled passage (usually in an engine block) which conducts oil from one place to another.

A GALLEY is the kitchen on a ship; an oil GALLERY is the oil passage.

But then, the discussion continues to expand and gather energy (snowball rolling down a steep hill)..........

STIFFNESS vs. STRENGTH

A common example technical writing which is totally-technically-wrong occurs in articles discussing improved engine parts, in which the author (or promoter) claims that the STIFFNESS of a product has been increased by the use of a higher grade of steel alloy.

However, IF, in fact, the "improved" part has the same internal and external dimensions as the original part, then the stiffness is IDENTICAL to that of the original part, regardless of the steel alloy. The yield and tensile strengths of the component MIGHT be higher, depending on the new material selected and heat treatment process applied.

The basic difference between strength and stiffness is explained further HERE.

CONCLUSION

In closing, it appears to me that one can become a "technical writer" with only a smattering of technical knowledge and a 4th-grade comprehension of the English language.

I won't even BEGIN to discuss the abysmal comprehension of fundamental arithmetic being conveyed in our schools.

BUT FURTHER...

The bigger issue, in terms of the survival of our society, is the total ignorance and moral degeneration being promoted at the earliest levels of public education, including (but not limited to): the murder of full-term babies, legitimized by the idiotic and appalling Roe v. Wade decision and the subsequent judicial perversions of logic; legitimizing of numerous other perversions; the promotion of 55 or 72 (or whatever) "genders" in complete denial of physical evidence; the fallacious "theory" of evolution, being presented as a factual framework instead of the scientifically discredited THEORY that it is; and the biggest joke of all---the "consensus" on CLIMATE CRISIS.

First, the doomsayers claimed the world was going to end by 2000 due to "global warming".....then by 2010....then by 2020...all based on fraudulent "data" purveyed by deceitful, fact-averse libtards whose hidden agenda is the destruction of the most productive, most affluent society in the history of this planet, that was formed and prospered based on a Constitution shaped largely by Judeo-Christian princples.

The perversion of the First Amendment of our Constitution in the issue of "separation of church and state" is so very typical of the deceit and deception promoted by the left. Anyone with a room-temperature IQ can read that Constitutional text and conclude that the language is intended to protect the individual from the intrusion of government into the individual's practice of religion. It was NEVER conceptualized as eliminating the influence of religion on government-(which it so desperately needs now). The original intent is further evidenced by the writings of the various Founders at the time, and the fact that most of the Founding Fathers were practicing Christian men.

Then we have the perversions being attempted on the Second Amendment, promoted by hypocrisy-addled politicians and hollywood idiots who are surrounded by gun-toting security, but who would deny the reality that self-defense is a God-bestowed right of ALL human beings, not just the self-appointed, morally-superior "elites".

How about the perversion of the Tenth Amendment by the self-proclamed morally-superior left-wing judiciary. That amendment CLEARLY STATES that the Federal Government posesses ONLY the powers granted to it by the Constitution, and that ALL other powers are reserved to the states. The brain-dead judiciary has enabled the de-facto nullification of that entire provision of our basic structure.