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ASME B4.2-1978 (2009) — Preferred Metric Limits and Fits
📏 ASME B4.2-1978 (2009) (Preferred Metric Limits and Fits) is the American National Standard that brings the international ISO system of limits and fits into general engineering use in the United States. Developed by the ASME B4 Committee (Standardization of Allowances and Tolerances for Manufactured Parts), it establishes the designation symbols used to define dimensional limits on drawings, the preferred basic sizes (first and second choice), the preferred tolerance zones (first, second, and third choice), the preferred limits and fits for sizes up to and including 500 mm (first choice only), and definitions of related terms. Tolerance zones for basic sizes from 500 to 3150 mm are specified in Appendix B. Published in 1978, the Standard was reaffirmed several times and remains in force after the 2009 reaffirmation.
💡 The value of B4.2 lies in “preference”: instead of requiring designers to use every ISO tolerance zone, it selects a compact set of frequently used combinations from the 28 fundamental deviations and 18 grades of tolerance. This reduces the variety of tooling, gages, and spares and significantly lowers manufacturing cost and inventory.
1. Scope and Terminology 📌
This Standard describes the ISO system of limits and fits for mating parts and applies to hole-and-shaft pairs. “Hole” and “shaft” are used in a general sense: a hole may be the space bounded by two parallel faces of any part (such as the width of a slot), and a shaft the material bounded between two parallel faces (such as the thickness of a key). Key defined terms include:
- Basic Size: the size to which limits or deviations are assigned; it is the same for both members of a fit — the 40 in 40H7.
- Deviation: the algebraic difference between a size and the corresponding basic size.
- Upper/Lower Deviation: the algebraic difference between the maximum/minimum limit of size and the basic size.
- Fundamental Deviation: the deviation closest to the basic size; designated by the letter H in 40H7.
- Tolerance: the difference between the maximum and minimum size limits on a part.
- Grade (IT number): the international tolerance grade (IT01 through IT18) that sets the magnitude of the tolerance.
⚠️ Deviation and tolerance are different concepts: the deviation locates the boundary, the grade sets the width. 40H7 means fundamental deviation H (lower deviation zero) with grade 7. Miswrite one letter or digit and a clearance fit can silently turn into an interference fit.
2. Tolerance Designations and Preferred Basic Sizes 🔧
Hole tolerance zones use capital letters for the fundamental deviation (A through ZC); shaft zones use lower-case letters (a through zc). Grades are given as numbers (e.g., 7, 8, 9). The designation takes the form “basic size + fundamental deviation letter + grade,” as in 40H7 (hole) or 40h7 (shaft). In the hole-basis system the hole fundamental deviation is H (lower deviation zero); in the shaft-basis system the shaft deviation is h (upper deviation zero).
Preferred basic sizes are selected from ASME B32.4-1974 (preferred metric sizes for metal products); first-choice sizes are rounded from the Renard R10 series of preferred numbers. Finer or coarser steps are obtained logically by expanding or reducing to the R5 or R20 series. Using preferred series aligns design sizes with mill stock and warehouse supply, cutting the cost of custom sizes.
| Category | Example Designations | Characteristics | Typical Applications |
|---|---|---|---|
| Hole-basis clearance | H7/g6, H7/f7, H8/f7 | Hole H; shaft g/f; guaranteed clearance for sliding | Plain bearings, guide bushes, piston and cylinder |
| Hole-basis transition | H7/js6, H7/k6, H7/n6 | Clearance or slight interference possible; good centering | Dowels, gear-and-shaft alignment fits |
| Hole-basis interference | H7/p6, H7/s6, H7/u6 | Hole H; shaft p/s/u; interference transmits load | Gear hubs pressed on, bearing outer rings |
| Shaft-basis clearance | G7/h6, F8/h6 | Shaft h; hole G/F; clearance for sliding | Long shafts with multiple bearings, keys |
| Shaft-basis interference | P7/h6, S7/h6 | Shaft h; hole P/S; negative deviation | Parts pressed on a shaft, bushes |
💡 Prefer the hole-basis system in engineering: holes require dedicated tooling (reamers, broaches) while shafts can be turned flexibly, so fixing the hole as the reference minimizes tools and gages. Choose the shaft basis only when one shaft must mate with several holes (a long shaft with several bearing housings) or when cold-drawn standard shaft stock is used.
3. Grades and Fit Selection 📏
The Standard tabulates tolerance values for each IT grade and fundamental deviation by size step, primarily 0–500 mm with 500–3150 mm in Appendix B. Tolerance does not scale linearly with size; values step in size ranges, so a given grade yields a larger tolerance on larger parts. Common grades and their uses:
- IT01–IT4: gages, precision instruments, precision-fitting parts; requires lapping or fine grinding.
- IT5–IT7: general precision fits (such as rolling-bearing seats); achievable by precision turning and grinding.
- IT8–IT11: medium-precision general machined parts.
- IT12–IT16: rough-machined, stamped, and cast non-mating surfaces.
🚨 Specifying an unnecessarily tight grade (for example IT6 on a non-mating surface) multiplies machining cost without functional benefit. Define the functional requirement first (clearance/interference, alignment, assembly method), then consult the preferred-fit tables of B4.2, and finally verify with a tolerance stack that the worst-case combination still meets function.
4. Engineering Practice Essentials 🛠️
- 🔍 Correct notation: use the standard symbols (e.g., 40H7/g6) and state dimensions in mm; never mix inch values into metric drawings — a 25.4-factor error is a classic cause of scrap.
- 🔧 Prefer the preferred fits: use the first-choice fits in the B4.2 tables; avoid inventing non-preferred zone combinations. When a non-standard zone is unavoidable, use the full ISO 286 tables.
- 📊 Process matching: match the IT grade to shop capability — conventional lathes hold IT9–IT10, CNC lathes IT7–IT8, grinders IT5–IT6. Verify capability indices (Cpk) in process review.
- 📋 Inspection and tooling: choose measuring instruments by grade — length measuring machines or air gages above IT6, micrometers and plug gages for general fits; instrument uncertainty should be about one-tenth of the tolerance.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between 40H7 and 40h7?
A: 40H7 is a hole designation (fundamental deviation H, lower deviation zero); 40h7 is a shaft designation (fundamental deviation h, upper deviation zero). Both are “reference member” concepts, but one is for holes and one for shafts — they are not interchangeable.
A: 40H7 is a hole designation (fundamental deviation H, lower deviation zero); 40h7 is a shaft designation (fundamental deviation h, upper deviation zero). Both are “reference member” concepts, but one is for holes and one for shafts — they are not interchangeable.
❓ Q: What kind of fit is a transition fit such as H7/k6?
A: In a transition fit the hole and shaft tolerance zones overlap, so actual assembly may give slight clearance or slight interference. It is used for centering and alignment, usually with dowels or keys transferring load, and must not have a large interference.
A: In a transition fit the hole and shaft tolerance zones overlap, so actual assembly may give slight clearance or slight interference. It is used for centering and alignment, usually with dowels or keys transferring load, and must not have a large interference.
❓ Q: How does B4.2 relate to ISO 286?
A: B4.2 adopts the ISO system of limits and fits (ISO/R 286 and ISO 1029) and organizes preferred tolerance zones and fits for American engineering practice. Its numerical values agree with ISO 286, but the scope and selection rules are trimmed for direct US use.
A: B4.2 adopts the ISO system of limits and fits (ISO/R 286 and ISO 1029) and organizes preferred tolerance zones and fits for American engineering practice. Its numerical values agree with ISO 286, but the scope and selection rules are trimmed for direct US use.
🔍 Q: Why do preferred basic sizes matter?
A: Basic sizes come from the R10 preferred-number series and align with mill-product inventory (as in B32.4). Using preferred sizes cuts custom stock, tooling, and gage variety, shortens lead time, lowers cost, and improves part interchangeability.
A: Basic sizes come from the R10 preferred-number series and align with mill-product inventory (as in B32.4). Using preferred sizes cuts custom stock, tooling, and gage variety, shortens lead time, lowers cost, and improves part interchangeability.
