Tolerance Class
/glossary/tolerance-classNamed dimensional-deviation band under a cited product standard, size range, and drawing limit.
A tolerance class is the named band of permitted deviation on one dimension under one standard. Dada Nagar cylinder shops, about 8 km from the Bartan Bazar yard, write DIN 2391 T-class on the OD and ISO 286 H8 or H9 on the bore. IS 1239 Class B is Medium wall: 3.2 mm at 25 NB on the site class chart, not a ±0.2 mm OD band. Put those millimetre limits on the RFQ.
In detail
A tolerance class names the permitted variation on a stated dimension (OD, wall, length, or bore) under one product standard and size range. IS 1239 Class A, B, and C on this site are Light, Medium, and Heavy wall series, not an OD millimetre band. Kanpur hydraulic drawings still write DIN 2391 T1 through T4 as OD-class labels. EN 10305-1:2016 instead publishes size-and-condition tables for seamless cold-drawn tubes with outside diameter D of 380 mm or less. Copy the millimetre limits from the invoked edition. A class letter from welded utility tube does not travel onto a precision barrel.
What a tolerance class is, and what it is not
A tolerance class is a label that points at a table in a named standard edition. The table then gives upper and lower deviations for a stated size. The label without the table is a nickname.
On this site, IS 1239 Light, Medium, and Heavy (Class A, B, and C) set wall thickness for each NB. They do not set an OD plus-or-minus in millimetres. The IS 1239 class thickness chart is the wall table Kanpur PWD and JJM buyers already use.
DIN 2391 drawings in the Dada Nagar hydraulic cluster still print T1, T2, T3, or T4 next to the OD. The precision-tube cluster treats those letters as drawing references. The millimetre window still comes from the invoked DIN 2391-1:1994 edition and the ordered size.
EN 10305-1:2016 is the current CEN delivery condition for seamless cold-drawn precision tubes of circular cross section with specified outside diameter D of 380 mm or less. Its public abstract says the tubes have precisely defined dimensional tolerances and a specified maximum surface roughness. Typical fields named on that abstract are automotive, furniture, and general engineering. The abstract does not reprint T1 through T4.
A tolerance class is not a pressure rating. Size, wall, material allowable stress, and the design code set pressure, not a class letter. H8 and H9 are ISO 286 bore zones, not OD classes and not wall classes. BK+S is a delivery condition (cold drawn and stress relieved). It does not pick T3.
Who should name a class, and who should not borrow one
Name a class when the mating part will feel that dimension. A Dada Nagar barrel that locates on the OD needs the DIN 2391 T-class from the cylinder drawing. A honed or skived bore needs H8 or H9 from the same drawing plus a Ra limit.
PWD and JJM water lines in Kanpur stay on IS 1239 Light, Medium, or Heavy. Fire-system installers on GI pipes in Kanpur stay on Heavy GI to IS 4736 where that coating applies. Borewell contractors stay on IS 4270 or IS 12818 column pipe. Those buyers write NB and wall class, not T3.
Sugar-mill and Panki boiler buyers write OD and minimum wall on SA192 or SA210. They do not write DIN 2391 T-class on a superheater tube. Printing-cylinder shops write a different H-class and a rougher Ra than hydraulic SRB; keep those POs apart.
Do not borrow Class B from an IS 1239 hydrant BOQ and treat it as ±0.2 mm on a 50 mm precision OD. Do not borrow T3 from a hydraulic drawing and stamp it on Medium ERW. Do not treat ±0.1 mm as a default just because the product name says precision.
Class families used on Kanpur drawings
The table keeps wall class, precision OD class, and bore zone on separate rows so a buyer can see they are not interchangeable. The first column names the standard and class family; the second column states what dimension that family actually controls.
| Family | What the label controls | Typical Kanpur buyer | Write on the PO |
|---|---|---|---|
| IS 1239 Class A / Light | Wall series for that NB | PWD / JJM plumbing | NB, Light, wall in mm |
| IS 1239 Class B / Medium | Wall series for that NB | Water and gas utility | NB, Medium, wall in mm |
| IS 1239 Class C / Heavy | Wall series for that NB | Fire riser, buried GI | NB, Heavy, wall in mm |
| DIN 2391 T1 to T4 | OD class on a legacy drawing | Dada Nagar cylinder shop | T-class plus mm limits |
| ISO 286 H8 or H9 | Finished-bore zone | Hydraulic or printing barrel | H-class, ID, Ra, method |
EN 10305-1:2016 uses its own size-and-condition tables rather than the DIN T-letter set. Treat a DIN-to-EN swap as a substitution that engineering must approve against the millimetre limits, not as an automatic rename.
How to write the class on an RFQ
A line that reads "50 mm precision tube, tight tolerance" will attract four incompatible offers. Split the tube standard from the class, then split OD from bore.
- Product standard and edition: IS 1239 Part 1:2004, DIN 2391-1:1994, or EN 10305-1:2016.
- Grade and delivery: ST52 / E355, BK, BK+S, or NBK if the drawing is precision; Light / Medium / Heavy if it is IS 1239.
- Size: NB or OD × wall or OD × ID, each in millimetres.
- OD class: DIN T1-T4 with the millimetre window, or the EN 10305 table the mill will inspect to.
- Bore: as-drawn, honed H8/H9, or SRB. ISO 286 zone and Ra as separate fields.
- Straightness, ovality, length, and the measurement method (micrometer, three-point, CMM).
- Documents: EN 10204 3.1 MTC plus a dimensional report that reprints the class limits.
- Delivery: ex-warehouse Kanpur. GST 18% extra. Packing that protects the gauged surfaces.
Worked mass check when Class B means wall
Run the site mass formula once on this page: W = (OD − t) × t × 0.02466 kg/m, OD and wall in millimetres. That 0.02466 factor is the site carbon-steel constant at 7850 kg/m³.
Take 25 NB Medium (Class B) from the class chart: OD 33.7 mm, wall 3.2 mm. W = (33.7 − 3.2) × 3.2 × 0.02466 = 30.5 × 3.2 × 0.02466 = 2.41 kg/m. A 6 m length is 14.5 kg on the generated weight table.
The MS ERW band for 25 NB Medium is ₹56-63 per kg ex-warehouse Kanpur as of 24 May 2026. That 6 m length sits between 14.5 × 56 = ₹812 and 14.5 × 63 = ₹914 before GST and freight. Class B here changed the wall, the mass, and the rupee line. It did not set an OD tolerance of ±0.2 mm.
A 25 NB Light wall on the same OD is a different kg/m and a different band. Weigh the bundle against 2.41 kg/m before accepting a cheaper per-length offer that is actually Light class.
Common incoming-inspection mistakes
In practice, a Fazalganj inspector measures one OD spot and stamps "T3 OK." Ovality and the far end can still sit outside the drawing window. Ask for readings at both ends and mid-length.
A second failure is reading Class B on an IS 1239 challan as a precision OD class. Medium wall at 25 NB is 3.2 mm. Gauge the wall, not a borrowed ±0.2 mm story.
A third failure is accepting H8 because the tag says hydraulic. H8 is size-dependent. Copy the ISO 286 upper deviation for that ID, or the drawing millimetre pair, onto the inspection sheet.
A fourth failure is a 3.1 MTC offered as the only dimensional evidence. Chemistry and tensile results do not prove T3 or H8. The dimensional report must reprint the class limits and the measured values.
A fifth failure is substituting T2 for T3 to cut price. The precision cluster is explicit: compare the millimetre limits and get engineering approval before the PO changes.
Standards that publish the millimetre tables
IS 1239 : Part 1 : 2004 (reaffirmed 2019) covers welded and seamless tubes for water, non-hazardous gas, air, and steam from 6 mm NB to 150 mm NB on the BIS preview fetched for this site. Classification is by class, grade, NB, process, end, and surface. The product manual (PM/1239 (Part 1)/4 Feb 2022) is the public certification checklist. Part 2:2011 covers wrought-steel pipe fittings, not larger tubes.
DIN 2391-1:1994 is titled Seamless precision steel tubes, Part 1: Dimensions. The Intertek catalogue lists it as superseded by DIN EN 10305-1:2016-08. Keep DIN 2391 on the PO only when the controlled drawing still names that edition.
EN 10305-1:2016, Steel tubes for precision applications, Part 1, Seamless cold drawn tubes, is the CEN replacement. EN 10305-4:2016 is the hydraulic-and-pneumatic-power part. ISO 286-1:2010 is the ISO code system for tolerances on linear sizes that H8 and H9 live in.
ASTM boiler and pipe specifications carry their own dimensional clauses. Do not drop a DIN T-class onto ASTM A106 or SA213. IBR Form III-B is the tube certificate and Form III-C is the certificate for boiler mountings and fittings; neither form is a tolerance class.
Buying to a named class from Kanpur
Dada Nagar shops collect precision tube against a drawing that already names T-class and H-class. Intra-city pickup is about 8 km from Bartan Bazar. Panki, about 12 km, takes ST45 / E235 for steering and shock tubes more often than a finished H8 barrel.
Fazalganj and Coolie Bazar traders still sell IS 1239 by NB and class. A JJM house-connection lot that says "Class B" wants Medium wall, BIS licence, and an MTC, not a DIN T3 report. See MS ERW pipe in Kanpur and steel pipes in Kanpur.
The Kanpur steel market page places the yard at 83/124, Bartan Bazar, Basanti Nagar, Juhi, 208014. Offers go out ex-warehouse Kanpur. Trucks that leave that yard cover Uttar Pradesh, Bihar, Madhya Pradesh, Delhi NCR, and Uttarakhand on the FAQ service list. Lucknow sits 80 km out on NH-27 per the city page. Unnao is the 18 km hop. See steel pipes in Lucknow and steel pipes in Unnao.
Indicative DIN 2391 bands on the precision tube price list run ₹95-150 per kg by size as of 24 May 2026. A tighter OD class can move the mill onto a slower draw; that is a price and date line, not a reason to relax H8.
Measuring OD, wall, and bore as separate fields
Incoming inspection at a cylinder shop should treat OD, wall, ID, ovality, and straightness as five rows. One "class OK" stamp covers none of them.
Measure OD with a micrometer or three-point gauge at positions the drawing names. Record min and max. Ovality is the difference between those two readings on the same cross-section, not a second T-class.
Measure wall with an ultrasonic gauge or a ball micrometer if the end is accessible. For IS 1239, compare the reading with the class-chart wall for that NB. For a barrel that will be honed, record remaining wall after the cut.
Measure finished ID against the ISO 286 zone printed on the drawing. A single nominal ID of 50 mm is not H8. Put the instrument (bore gauge, air gauge, CMM) on the report so the mill and the shop use the same method.
IS 1239 Class A B C is wall series
Kanpur BOQs still mix the older Class A / B / C language with Light / Medium / Heavy. On this site those pairs are the same wall series. Class A is Light. Class B is Medium. Class C is Heavy.
At 25 NB the chart uses OD 33.7 mm: Light 2.6 mm, Medium 3.2 mm, Heavy 4.0 mm on the generated weight file. At 50 NB the chart uses OD 60.3 mm: Light 2.9 mm, Medium 3.6 mm, Heavy 4.5 mm. Those millimetres are walls. They are not OD tolerances.
A contractor who writes "Class B, tight OD" on an IS 1239 hydrant line is mixing two families. Write Medium, 3.2 mm at 25 NB, and stop. If the fitting actually needs a precision OD, that line belongs on DIN 2391 or EN 10305, not on the hydrant PO.
IS 3589 covers 168.3 mm to 2540 mm OD water and sewage pipe. It is not a larger IS 1239 tolerance class. Move that diameter onto the IS 3589 RFQ.
When a PWD line stays commercial and a barrel moves to precision
Keep commercial ERW on the water, gas, fire, and structural BOQ when the drawing only names NB and class. The cheaper mill route and the BIS licence matter more than a T-class the inspector will never gauge.
Move to cold-drawn precision when the part locates on OD or ID: hydraulic barrels, guide tubes, shock outers, and some printing shells. Then the class letters earn their keep because a millimetre error scraps the mating piston or seal.
A mixed truck from Kanpur can carry both families if the precision sits in lined cradles and the ERW bundles do not rub the gauged ends. Tag each bundle with standard, class, and heat. Do not let a Medium IS 1239 stencil travel onto a T3 crate.
Send the drawing or the BOQ page with the RFQ. RP Sales replies within 4 working hours with a technical and commercial offer that repeats the class the buyer wrote, or flags the mismatch.
Where you'll meet this in procurement
You will meet Tolerance Class on size charts, mill quotes, and fitment checks. Cross-check Tolerance Class against OD, wall thickness or schedule, length, and end finish so the delivered pipe matches flanges, fittings, and pressure duty—not just a familiar nominal label.
Related terms
Related resources
- Pipe schedule chart
- Steel pipe weight calculator
- DIN 2391 tolerance class table
- DIN 2391 vs EN 10305
- IS 1239 class thickness chart
- Precision tube price list
- MS ERW price list
- DIN 2391 ST52 hydraulic tube
- Steel pipes in Kanpur
- MS ERW pipe in Kanpur
- Kanpur steel market
- Hydraulic cylinder industry
- Request a quotation (RFQ)
- Back to full glossary
Key points
- A tolerance class is a pointer to a table in a named standard edition, not a universal millimetre band.
- IS 1239 Class A/B/C means Light/Medium/Heavy wall. At 25 NB, Medium wall is 3.2 mm on the site class chart.
- DIN 2391 T1-T4 are OD-class labels on legacy drawings. EN 10305-1:2016 uses size-and-condition tables for D of 380 mm or less.
- H8 and H9 are ISO 286 bore zones. BK+S is a delivery condition and does not pick a T-class.
- Mass for 25 NB Medium is 2.41 kg/m from (33.7 − 3.2) × 3.2 × 0.02466. Class-B walls from Kanpur still carry GST 18% extra on the invoice.
Frequently asked questions
Sources
- IS 1239 : Part 1 : 2004 (reaffirmed 2019). Steel Tubes, Tubulars and Other Wrought Steel Fittings: Part 1 Steel Tubes; BIS preview scope 6-150 mm NB (Bureau of Indian Standards)
- PM/1239 (Part 1)/4 Feb 2022. Product Manual for Steel Tubes, Tubulars and Other Wrought Fittings Part 1: Steel Tubes according to IS 1239 (Part 1):2004 (Bureau of Indian Standards)
- DIN 2391-1:1994-09. Seamless precision steel tubes: Part 1: Dimensions; superseded by DIN EN 10305-1:2016-08 (Deutsches Institut für Normung)
- EN 10305-1:2016. Steel tubes for precision applications: Technical delivery conditions: Part 1: Seamless cold drawn tubes (D ≤ 380 mm; precisely defined dimensional tolerances) (European Committee for Standardization (CEN))
- EN 10305-4:2016. Seamless cold-drawn tubes for hydraulic and pneumatic power systems (European Committee for Standardization (CEN))
- ISO 286-1:2010. Geometrical product specifications: ISO code system for tolerances on linear sizes (International Organization for Standardization)
- Indian Boiler Regulations, 1950. Forms index: III-A steam pipes, III-B tubes, III-C boiler mountings and fittings (Central Boilers Board, Government of India)