Fits

Designation of Standard Fits

Letter Symbols	Definition of Fits	Description
RC	Running and Sliding Clearance	Intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes.
RC 1	Close Sliding	Intended for the accurate location of parts that must be assembled without perceptible play.
RC 2	Sliding	Intended for accurate location, but with greater maximum clearance than class RC 1. Parts made to this fit move and turn easily but are not intended to run freely, and in large sizes may seize with small temperature changes.
RC 3	Precision Running	The closest fits that can be expected to run freely, intended for precision work at slow speeds and light journal pressures, but are not suitable where appreciable temperature differences are likely to be encountered.
RC 4	Close Running	Intended chiefly for running fits on accurate machinery with moderate surface speeds and journal pressures, where accurate location and minimum play are desired.
RC 5 & RC 6	Medium Running	Intended for higher running speeds, or heavy journal pressures, or both.
RC 7	Free Running	Intended for use where accuracy is not essential, or where large temperature variations are likely to be encountered, or under both conditions.
RC 8 & RC 9	Loose Running	Intended for use where wide commercial tolerances may be necessary, together with an allowance on the external member.
LC	Locational Clearance	Intended to determine only the location of the mating parts; they may provide rigid or accurate location, as with interference fits, or provide some freedom of location, as with clearance fits.
LC	Locational Clearance	Intended for parts that are normally stationary, but that can be freely assembled. They range from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as spigots, to the looser fastener fits where freedom of assembly is of importance.
LT	Locational Transitional	A compromise between clearance and interference fits, for applications where accuracy of location is important, but a small amount of clearance or interference is permissible.
LN	Locational Interference	Used where accuracy of location is of prime importance, and for parts requiring rigidity and alignment with no special requirements for bore pressure. Not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit. These conditions are covered by force fits.
FN	Force or Shrink	A special type of interference fit, normally characterized by maintenance of constant bore pressures throughout the range of sizes. The interference therefore varies almost directly with the diameter, and the difference between its minimum and maximum value is small, to maintain the resulting pressures within reasonable limits.
FN 1	Light Drive	Requiring light assembly pressures, and producing more or less permanent assemblies. They are suitable for the thin sections or long fits, or in cast-iron external members.
FN 2	Medium Drive	Suitable for ordinary steel parts, or for shrink fits on light sections. The tightest fits that can be used with high-grade cast iron external members.
FN 3	Heavy Drive	Suitable for heavier steel parts or for shrink fits in medium sections.
FN 4 & FN 5	Force	Suitable for parts that can be highly stressed, or for shrink fits where the heavy pressing forces required are impractical.
B	Bilateral Hole (Modified Standard Fit)	The symbols used for these fits are identical with those used for standard fits; thus, LC 4 B is a clearance locational fit, Class 4, except it is produced with a bilateral hole.
S	Basic Shaft (Modified Standard Fit)	The symbols used for these fits are identical with those used for standard fits; thus, LC 4 S is a clearance locational fit, Class 4, except it is produced on a basic shaft basis.