How to measure a large
BSP flat gasket
when the original
is missing
At 1¼" to 2½" BSP, variation between fitting manufacturers in the recess dimensions can be several millimetres. A gasket specified by thread size alone may be too small, too large or the wrong thickness. The correct dimensions normally come from verified equipment data, a confirmed part reference, or direct measurement of the fitting face recess. When the original gasket and part reference are missing, the recess measurement becomes the primary input.
Why this is a bigger problem above 1" BSP
At ½" or ¾" BSP, a small dimensional error in OD may not be noticed immediately — the small sealing area may make a minor mismatch less obvious at first inspection, although the fit is still incorrect. The consequences of ordering by thread size without measuring are lower at small sizes.
Above 1", this changes. A gasket that is 3 mm undersized in OD at a 1½" BSP connection leaves a visible unsealed zone around the outer edge. A gasket that is 3 mm oversized cannot sit flat in the recess and prevents the union from closing correctly. The same absolute error that may go unnoticed at ¾" is more likely to produce a failed joint at 1½". And at commercial and industrial connection sizes, the cost of a repeat repair — disassembly, sourcing the correct gasket, reassembly — is significantly higher than at a domestic heating union.
The thread size tells you what range to expect. The fitting face tells you what to order. Use the BSP nominal size as a sanity check — if your measured OD is wildly outside the expected range for that thread size, something is wrong. But for the actual gasket specification, the recess measurement is the primary input.
Scope note. Measurements should only be taken when the system is safely isolated, depressurised, cooled where relevant, drained as required and accessible under the equipment manufacturer's procedure and site rules. Work on pressurised, hot or chemically treated systems should be carried out by a competent person.
Step 1 — Confirm the joint type before measuring
Before measuring anything, confirm that you are dealing with a flat face BSP threaded union — not a flanged connection or a BSPT (taper thread) fitting. Getting this wrong means you are ordering the wrong type of sealing element entirely.
Flat face BSP union — flat gasket is correct
The fitting face is flat. There is a visible shallow recess ring machined into the face — an outer step and an inner step, forming a channel. The thread is parallel (often marked G). The union screws together with a union nut that draws the two faces together.
These are NOT flat face BSP unions
Flanged connection: has bolt holes around the perimeter — a different joint type requiring a flange gasket, not a BSP union gasket.
BSPT taper thread: seals on the thread taper itself with PTFE tape or thread sealant — no flat face gasket. The thread is tapered, not parallel.
At 2" and 2½" BSP, the boundary between flat face threaded unions and flanged connections varies by equipment manufacturer and application. If the fitting has bolt holes, it is a flanged joint. If it screws together with a union nut and has a flat machined face with a recess, it is a BSP flat face union. If unclear, check the fitting documentation or manufacturer's catalogue before proceeding.
Step 2 — The three measurements to take
Outer diameter of the recess
Set your calliper to measure across the outer edge of the recess ring — the larger of the two steps on the fitting face. This is the gasket OD.
At larger BSP sizes, take two OD measurements at 90 degrees to each other. If they differ by more than 0.5 mm, check that the fitting face is undamaged and that you are measuring the recess, not the fitting body OD or another feature.
Inner diameter of the recess
Measure across the inner edge of the recess — the step closest to the bore of the fitting. This is the gasket ID.
Be careful not to measure the bore itself, which is larger than the recess inner edge. The recess inner wall is a vertical step. The bore is the through-hole in the fitting. At larger sizes, these are more clearly separated and easier to distinguish than at ½" or ¾".
Recess depth — guides the gasket thickness
Use the depth gauge function of your calliper, or a separate depth gauge, to measure the step depth of the recess — the distance from the face surface to the bottom of the recess.
The recess depth helps identify the thickness range the joint was designed to accept. Final thickness should be checked against the equipment specification, gasket material data and the required compression condition. For most domestic and commercial BSP flat face unions, standard gasket thicknesses are 1.5 mm or 2 mm. If your measured depth is significantly outside these values, recheck — some fittings have shallower recesses and use thinner gaskets.
Step 3 — Cross-check against the BSP nominal size
With your three measurements taken, cross-check the OD value against a typical market range for the BSP size marked on the fitting. The table below gives indicative typical OD ranges for common market patterns:
| BSP nominal | Thread OD (approx.) | Typical recess OD range |
|---|---|---|
| 1¼" | 41.91 mm | 42–45 mm |
| 1½" | 47.80 mm | 48–52 mm |
| 2" | 59.61 mm | 60–65 mm |
| 2½" | 75.18 mm | 75–80 mm |
These are common market-orientation ranges for sanity checking only — not ordering dimensions and not universal standards. The replacement gasket should be specified from verified equipment data, a confirmed part reference or direct measurement of the fitting recess. If your measured OD falls within the range for the BSP size marked on the fitting, your measurement is likely correct; if it falls significantly outside, recheck.
The three most common measurement errors
When measurement is not the problem
If the fitting face is damaged — corroded, pitted, raised from a previous overtightened gasket, or scored by a removal tool — correct measurement of the recess dimensions does not fully solve the problem. A correctly sized gasket on a damaged face may still leak.
Before ordering the replacement gasket, inspect the face after taking the measurements:
- Pitting or corrosion on the sealing face — particularly at the recess OD edge or the inner bore edge — may need to be assessed for whether it prevents reliable sealing. Significant pitting may require face repair or fitting replacement.
- Raised burrs from a previous gasket or removal tool — these should be corrected under the relevant component procedure before the replacement is fitted. A raised edge prevents the new gasket from seating flat.
- Residue from the old gasket in the recess — even without a gasket to measure, old residue may be present. It should be removed under the relevant procedure before final measurement and fitting.
A correctly dimensioned gasket on a damaged or contaminated face will not seal reliably. Correct measurement determines the gasket dimensions. Face preparation determines whether those dimensions can seal reliably. Both are required. At 1¼" to 2½" BSP, face damage is more consequential than at smaller sizes — a damaged zone that would have been a nuisance at ¾" is a meaningful proportion of the sealing area at 2".
Frequently asked questions
How do I find the right gasket size if the original is missing?
First confirm the joint type — that you are dealing with a flat face BSP threaded union, not a flanged or taper-threaded connection. Then measure the sealing face recess directly on the fitting: measure the outer diameter of the recess (this gives the gasket OD), the inner diameter of the recess (gasket ID), and the depth of the recess step (this guides the gasket thickness). Cross-check the OD dimension against the typical range for the BSP nominal size marked on the fitting. If the measured OD is significantly outside the typical range for that BSP size, recheck your measurement or confirm the thread standard. Do not estimate from the thread outer diameter — the recess dimensions are set by the fitting manufacturer, not uniquely determined by the thread size.
Can I use the BSP thread size to determine the gasket size if I have no original?
The BSP thread size gives you a useful cross-check range, but it does not uniquely determine the gasket dimensions. Different manufacturers machine the flat face recess to different OD and ID values for the same nominal BSP size. At 1¼ to 2½ inch BSP, the variation between manufacturers and product ranges in the absolute recess dimensions can be several millimetres — enough to produce a gasket that does not seat correctly in the recess. The thread size tells you the approximate order of magnitude to expect. The actual gasket dimension must come from measuring the fitting face recess directly.
Why is measuring more critical at larger BSP sizes when the original is missing?
At ½ or ¾ inch BSP, a small dimensional error may not be noticed immediately, because the small sealing face can make a minor mismatch less obvious at first inspection, although it is still an incorrect fit. At 1¼ to 2½ inch BSP, the same absolute error can create a more visible unsupported or unsealed area, and the consequence of correcting it is usually higher: an undersized gasket leaves a larger unsealed zone around the bore, and an oversized gasket may prevent the union from fully closing. The margin for sizing error narrows as the connection size increases.
Three measurements, taken correctly, from the fitting face. When no verified part reference exists, that is the basis for the specification.
OD from the outer recess edge. ID from the inner recess edge — not the bore. Depth from the face surface to the recess bottom. Cross-check the OD against the BSP nominal. Inspect the face for damage and clean out any residue before fitting the replacement. At 1¼" to 2½" BSP, the margin for measurement error and face preparation shortcuts is narrower than at smaller sizes. The measurements take minutes. Getting them wrong means a second disassembly.