Posted by JRC Engineering on Oct 2nd 2020
Standard Bore Sizes for British Motorcycles | JRC Engineering Inc
Standard Bore Sizes for British Motorcycles
Triumph · BSA · Norton · Velocette — standard bore reference in millimetres with inch conversions
This is a standard bore size reference for classic British motorcycles — Triumph, BSA, Norton, and Velocette. Use it as a starting point when ordering pistons or planning a rebuild. The actual target bore for your engine must be calculated by your machine shop from a live piston measurement, not from this list alone. If you have questions, give us a call — we're happy to talk through what you need.
How to Use This Reference
Cylinders are bored to fit the actual piston — not the other way around. The bore size in the tables below is the nominal standard. Before any machine work, your shop must measure the piston you intend to run with a micrometer and set the bore target accordingly.
Where to Measure the Piston
Measure at the bottom of the skirt, perpendicular to the wrist pin, 1/2 inch up from the bottom edge on the thrust face. This is the piston's largest diametrical dimension and the correct reference point for bore sizing.
Pistons are intentionally oval and tapered near the crown. The extra material near the crown and wrist pin bosses expands under heat — this taper and ovality is by design, not a defect. Always measure at the specified point.
Converting MM to Inches
Divide the millimetre measurement by 25.4. Example: 76mm ÷ 25.4 = 2.992 inches.
Oversize Reference
| Oversize (inches) | Oversize (mm) |
|---|---|
| +.010" | +0.25mm |
| +.020" | +0.5mm |
| +.040" | +1.0mm |
| +.060" | +1.5mm |
| +.080" | +2.0mm |
Note: a .020" overbore only removes .010" of material per side. If bore walls have deep corrosion or pitting, you may need to jump two oversize steps to clean them up.
Triumph — Standard Bore Sizes
| Model | Displacement | Standard Bore (mm) | Standard Bore (inches) |
|---|---|---|---|
| 3T | 350cc | 55mm | 2.165" |
| 3TA | 350cc | 58.25mm | 2.293" |
| T15 | 150cc | 57mm | 2.244" |
| T20 | 200cc | 63mm | 2.480" |
| 5T (pre-unit) | 500cc | 63mm | 2.480" |
| T150 / T160 (Trident) | 750cc | 67mm | 2.638" |
| T100 / 5TA / T100C | 500cc | 69mm | 2.717" |
| 6T / T110 / TR6 / T120 (Bonneville) | 650cc | 71mm | 2.795" |
| T140 / TR7 (Bonneville 750) | 750cc | 76mm | 2.992" |
Note: Early T140 models (before serial T140V XH22019 / TR7V AH24044) may have a 75mm bore at 724cc displacement. Always verify with a live piston measurement.
BSA — Standard Bore Sizes
| Model | Displacement | Standard Bore (mm) | Standard Bore (inches) |
|---|---|---|---|
| M20 | 500cc | 82mm | 3.228" |
| A7 | 500cc | 62mm / 66mm | 2.441" / 2.598" |
| A10 | 650cc | 70mm | 2.756" |
| A50 | 500cc | 65.5mm | 2.579" |
| A65 | 650cc | 75mm | 2.953" |
| B25 | 250cc | 67mm | 2.638" |
| B40 | 350cc | 79mm | 3.110" |
| B44 | 440cc | 79mm | 3.110" |
| C15 | 250cc | 67mm | 2.638" |
| B50 | 500cc | 84mm | 3.307" |
| Rocket 3 | 750cc | 67mm | 2.638" |
Norton — Standard Bore Sizes
| Model | Displacement | Standard Bore (mm) | Standard Bore (inches) |
|---|---|---|---|
| 16H | 500cc | 79mm | 3.110" |
| Big 4 | 650cc | 82mm | 3.228" |
| Atlas 650 | 650cc | 68mm | 2.677" |
| Atlas 750 | 750cc | 73mm | 2.874" |
| Commando 750 | 750cc | 73mm | 2.874" |
| Commando 850 | 850cc | 77mm | 3.031" |
Velocette — Standard Bore Sizes
| Model | Displacement | Standard Bore (mm) | Standard Bore (inches) |
|---|---|---|---|
| Venom / Clubman / MSS | 500cc | 86mm | 3.386" |
| MAC | 350cc | 68mm | 2.677" |
| Viper / Viper Clubman | 350cc | 72mm | 2.835" |
Have Questions About Your Rebuild?
These figures are a planning reference. Standard sizes vary between manufacturers, piston brands, and production years — your machine shop will need to measure your actual piston to set the final bore target. If you have questions about bore sizing, oversize options, or what to expect before bringing your engine to a shop, give us a call. We're happy to share what we know. Contact JRC Engineering.
Frequently Asked Questions
What is the standard bore size for a Triumph T140?
The standard bore for a Triumph T140 / TR7 is 76mm (2.992 inches). Earlier T140 models may have a 75mm bore — verify with a piston measurement before ordering.
What is the standard bore size for a Norton Commando 850?
The Norton Commando 850 has a standard bore of 77mm (3.031 inches). The Commando 750 uses a 73mm (2.874") bore.
How do I convert motorcycle bore sizes from mm to inches?
Divide the millimetre measurement by 25.4. A 76mm bore equals 76 ÷ 25.4 = 2.992 inches.
How much material does a .020" overbore remove?
A .020" overbore removes only .010" per side. If bore walls have deep pitting, you may need +.040" or larger to clean up the damage. Your machine shop can advise after inspecting the cylinder.
Where exactly do I measure a piston to get the bore size?
Measure at the bottom of the skirt, 1/2 inch up from the bottom edge, perpendicular to the wrist pin, on the thrust face. This is the piston's largest diameter and the correct reference for calculating bore clearance.