Milwaukee-Eight 117 Big Bore and Stroker Math: Displacement and Compression
Big bore kits and stroker cranks are among the most common engine upgrades for Milwaukee-Eight Harley-Davidsons. Whatever the brand of kit, the arithmetic behind it is the same: more swept volume, and unless something else changes, more compression. This guide works through that arithmetic for the 117, the engine in every 2026 Softail cruiser, so you can check a kit’s claims and see why kits come with the parts they do. It does not recommend a kit, and the bore and stroke sizes below are round numbers for illustration.
Quick answer
- Stock 117: 103.5 × 114.3 mm (4.075 × 4.5 in), 1,923 cc, 10.3:1. Clearance volume works out at about 103.4 cc per cylinder.
- A 4.25 in bore on the stock stroke gives about 2,092 cc (127.7 cu in), and 11.1:1 if nothing else changes.
- A 4.75 in stroke on the stock bore gives about 2,030 cc (123.9 cu in) and 10.8:1.
- To hold 10.3:1 with the 4.25 in bore you need about 9 cc more clearance volume per cylinder, from the pistons, gasket or heads.
Step 1: swept volume
Swept volume per cylinder is the bore area times the stroke: π/4 × bore² × stroke. For the stock 117 that is π/4 × 103.5² × 114.3 = 961.6 cc per cylinder, 1,923 cc for two. The displacement calculator shows it in cubic inches too: 117.3 cu in.
Step 2: clearance volume from the compression ratio
Compression ratio is (swept + clearance) ÷ clearance, where clearance volume is everything above the piston at top dead centre: the head chamber, the gasket, the deck gap and the piston crown. Rearranged, clearance = swept ÷ (ratio − 1) = 961.6 ÷ 9.3 = 103.4 cc. That is a worked-out figure, not a measured one: the published 10.3:1 is rounded, and real engines vary slightly.
Step 3: the new engine
| Bore × stroke | Displacement | Increase | Compression, same clearance | Extra clearance for 10.3:1 |
|---|---|---|---|---|
| Stock 4.075 × 4.5 in | 1,923 cc (117.3 cu in) | – | 10.3:1 | – |
| 4.25 × 4.5 in (bore only) | 2,092 cc (127.7 cu in) | 8.8% | 11.1:1 | about 9 cc per cylinder |
| 4.075 × 4.75 in (stroke only) | 2,030 cc (123.9 cu in) | 5.6% | 10.8:1 | about 6 cc per cylinder |
| 4.25 × 4.75 in (both) | 2,208 cc (134.8 cu in) | 14.8% | 11.7:1 | about 15 cc per cylinder |
The big bore and stroker calculator opens with the first example. Change the new bore and stroke to your kit’s figures.
Why compression goes up, and why kits deal with it
Pushing more swept volume into the same clearance volume raises the ratio. A jump from 10.3:1 to 11.1:1 is significant on an air- and oil-cooled engine running on pump fuel: the risk of knock rises, and the engine management has to pull ignition timing to protect itself. That is why kit makers specify pistons with a particular dome or dish volume, a particular head gasket, or reworked heads, and why the kit’s quoted compression ratio is the number to trust over this simple estimate.
To see where the extra clearance volume can come from, the compression ratio calculator adds up chamber, gasket, deck and piston volumes. A head gasket’s volume is its bore area times its thickness: at a 108.5 mm gasket bore, each 0.1 mm of thickness is about 0.9 cc. Making up 9 cc with gasket thickness alone would need about 1 mm more, which would also change the squish clearance; that is one reason the volume usually comes from the pistons instead.
Cams and dynamic compression
The static ratio is only half the story. The cylinder only starts compressing when the intake valve closes, after bottom dead centre, so the effective or dynamic compression ratio is lower. A longer-duration cam closes the intake later and lowers it further, which is why big bore kits and cams are chosen together. The dynamic compression calculator takes bore, stroke, rod length, static ratio and the intake closing point from the cam card; the cam timing calculator turns a cam card into duration, overlap and lobe centres.
Before you build
- Use the kit maker’s figures for bore, stroke, piston volume and compression, and measure where you can.
- A bigger engine needs a matching fuel and ignition calibration.
- Check emissions rules where you ride: many engine modifications are not legal for road use in some US states and other countries.
- Expect warranty implications; check with your dealer.
FAQ
How many cubic inches is a 117 with a 4.25 inch bore?
About 127.7 cu in (2,092 cc) on the stock 4.5 in stroke: π/4 × 4.25² × 4.5 × 2.
What compression ratio will my big bore 117 have?
It depends on the pistons, gasket and heads in the kit. With nothing else changed, a 4.25 in bore takes a 10.3:1 engine to about 11.1:1. Use the kit maker’s figure.
What is the clearance volume of a stock Milwaukee-Eight 117?
Worked out from 10.3:1, about 103.4 cc per cylinder.
Is a stroker or a big bore better?
The arithmetic does not answer that. A longer stroke adds displacement and raises piston speed at a given rpm; a bigger bore adds displacement and room for valves. Builders choose based on the rest of the engine and how it will be used.
Sources
- Motorcycle.com, 2026 Breakout specs (117 bore, stroke, compression): motorcycle.com
- Total Motorcycle, 2026 Fat Boy (117 Custom figures): totalmotorcycle.com
- Heywood, J. B., Internal Combustion Engine Fundamentals (compression ratio, swept and clearance volume).
- Bell, A. G., Four-Stroke Performance Tuning (cam timing and dynamic compression).
