Two-stroke cylinder-head geometry
Squish Clearance Calculator
Compare nominal stack geometry with crushed-solder measurements, inspect squish-band taper and side-to-side variation, estimate running clearance, and calculate the gasket or stack change required for your chosen target.
Calculated output
CalculatedClearance summary
Dimension Preview
Hover pointsMeasured clearance profile
Focus or point to a chart marker for details.
Calculated Dimensions
DynamicSquish-band cross-section
Audit the result
Per cylinder unless notedDimension breakdown
| Item | Reference | Calculated value |
|---|---|---|
| Nominal cold clearance | deck + gasket + head recess + shim | — |
| Required cold clearance | running target + dynamic closure | — |
| Outer-edge mean | average of stations A and B | — |
| Inner-edge mean | average of stations A and B | — |
| Average taper rise | inner mean − outer mean | — |
| Opposite-side difference | largest A/B difference at matching edges | — |
| Squish-band inner diameter | bore − 2 × radial band width | — |
| Squish-band area | annulus area and percentage of bore area | — |
| Measured band volume | linear radial profile integrated over annulus | — |
| Total measured band volume | per-cylinder band volume × cylinder count | — |
| Geometry-only target gasket | required cold − deck − head recess − shim | — |
| Reference clearance | — | — |
Calculation method
Squish formulas and conventions
The tool keeps nominal stack geometry separate from direct solder measurements. A disagreement is useful evidence: it can indicate measuring error, gasket compression difference, surface geometry, piston rock, bore position, or an unmodelled step.
1. Nominal cold clearance
Cnominal = deck + gasket + head recess + shim
Positive piston protrusion is represented by a negative deck value. A head spigot or band projection can be entered as a negative recess.
2. Measured statistics
Cmin = minimum of outer A/B and inner A/B
The spread is maximum minus minimum. The solver uses the tightest solder result when measured reference is selected.
3. Band taper
angle = atan((inner mean − outer mean) ÷ band width)
A positive result opens toward the chamber; a negative result closes toward the chamber. This is relative geometry, not an absolute head-machining angle.
4. Running estimate
Crunning = selected cold reference − dynamic closure
Dynamic closure must be supplied by the user. It can represent an evidence-based allowance for thermal growth, bearing clearance, rod stretch, piston rock, deposits, and operating load.
5. Target stack change
Δstack = (running target + dynamic closure) − current cold reference
Positive means add cold clearance; negative means remove it. Candidate gasket assumes the complete change is made through compressed gasket thickness.
6. Squish-band area
Aband = π ÷ 4 × (bore² − inner diameter²)
Band volume integrates the measured linear clearance profile across the annular band and is reported as a geometry audit, not trapped chamber volume.
Measurement and safety
Squish clearance FAQ
How should squish clearance be measured with solder?
Use soft solder thicker than the expected gap. Place a strip across the complete bore so it reaches both cylinder walls, assemble the intended head and compressed gasket, rotate through TDC by hand, remove the solder, and measure each crushed band section with a reliable micrometer. Repeat on a second axis and on every cylinder.
Why measure both outer and inner band edges?
The outer and inner values reveal whether the head band opens or closes toward the chamber. A single measurement may find minimum clearance but cannot describe taper.
What do stations A and B represent?
They are opposite ends of one wall-to-wall solder strip. Their difference helps identify tilt, piston rock, off-centre machining, surface distortion, or inconsistent measurement. Repeat the process front-to-rear and side-to-side.
What sign is used for piston deck position?
Enter a positive number when the piston is below the cylinder deck at true TDC. Enter a negative number when it protrudes above the deck.
What is head-band recess?
It is the axial distance from the head gasket face to the squish surface. A recessed band adds clearance and is positive. A projecting band or spigot that reduces the axial gap is negative. Confirm the reference surfaces directly.
Why can nominal and solder clearances differ?
Possible causes include incomplete gasket compression, piston crown shape, piston rock, bearing play, deck or head distortion, carbon, burrs, sealant, measurement location, or omitted steps. Direct measurement of the assembled engine should control the investigation.
Is dynamic closure predicted by this calculator?
No. The entered closure is only an allowance supplied by the user. Actual minimum running clearance depends on RPM, temperature, rod and bearing behavior, piston motion, deposits, component stiffness, assembly tolerances, and operating conditions.
Does a positive running margin prove the engine is safe?
No. The target itself must be valid for the exact engine, components, fuel, RPM, temperature, and intended use. The result does not check piston-to-head contact, ring location, detonation, chamber shape, sealing width, or structural strength.
Can the candidate gasket value be used as an ordering size?
Not without verification. It is a compressed-thickness calculation and assumes all correction is made at the gasket. Available gasket nominal thickness, installed compression, sealant, torque, material, port alignment, and cylinder height must be checked.
Is this a head-machining instruction?
No. Never machine the head or cylinder from this result alone. Verify true TDC, all cylinders, minimum clearance at several crank positions, gasket sealing, head geometry, compression ratio, plug reach, cooling, and material thickness with a qualified engine builder.