Twin Cam 103 vs 107 big bore — is it worth it?
within 1% of peak across that whole range
Simulated rear‑wheel horsepower and torque — measured after the primary drive, clutch and transmission, the same way a chassis dyno reads a bike. These are not flywheel numbers, and they are not directly comparable to a manufacturer’s crank rating. Rev ceiling for this combination: 5900 rpm, limited by cam.
Where each one wins
| RPM | 103 | 107 | diff |
|---|---|---|---|
| 2000 | 99 | 101 | +2 |
| 2500 | 109 | 112 | +3 |
| 3000 | 117 | 120 | +3 |
| 3500 | 120 | 123 | +3 |
| 4000 | 120 | 123 | +3 |
| 4500 | 116 | 119 | +3 |
| 5000 | 110 | 113 | +3 |
| 5500 | 102 | 105 | +3 |
| 6000 | 69 | 71 | +2 |
Torque, lb-ft, at the rear wheel. Everything except the part named is identical between the two runs.
Four cubic inches, everything else held identical. This is a smaller step than 96 to 103 and it is fair to ask whether it earns the machining bill.
The delta table is the answer, and it is an honest one: the gain is real and it is spread across the range, but it is not transformative on its own. A 107 with a stock cam is not a fast motorcycle.
Where the 107 earns its keep is as headroom. It lets a bigger cam and better heads work without running out of engine underneath them — which is why it usually appears as part of a package rather than on its own.
The build
| Engine | TC103 (2010 CVO / 2012+) 3.875 × 4.375 |
|---|---|
| Crank / balancer | A motor — non-balanced (Dyna / Touring / CVO) |
| Pistons | Forged flat-top (11cc combustion gain) |
| Head gasket | Cometic .030" — neutral baseline |
| Cylinder heads | STOCK -06 + S&S CNC port service · 1.9/1.575 |
| Porting | Stage 2 CNC port (full) |
| Camshaft | S&S 570 · 240/255 dur · 0.570" lift |
| Valve springs | S&S 106-5909 Sidewinder — .650" max lift — 7mm stem, Twin Cam 2005-2018 |
| Rocker arms | Stock rocker arms — 1.625:1 |
| Pushrods | Stock pushrods |
| Lifters | Short-travel hydraulic (Feuling/Andrews) |
| Induction | HPI / SE 55mm throttle body |
| Exhaust | 2-into-1 stepped (D&D FatCat / E-Series) |
| Air cleaner | Screamin' Eagle / Stage I high-flow |
| Tune | Dyno-tuned (matched to build) |
| Power adder | None (naturally aspirated) |
The other one
| Engine | TC107 big bore (96/103 + 3.937) |
|---|---|
| Crank / balancer | A motor — non-balanced (Dyna / Touring / CVO) |
| Pistons | Forged flat-top (11cc combustion gain) |
| Head gasket | Cometic .030" — neutral baseline |
| Cylinder heads | STOCK -06 + S&S CNC port service · 1.9/1.575 |
| Porting | Stage 2 CNC port (full) |
| Camshaft | S&S 570 · 240/255 dur · 0.570" lift |
| Valve springs | S&S 106-5909 Sidewinder — .650" max lift — 7mm stem, Twin Cam 2005-2018 |
| Rocker arms | Stock rocker arms — 1.625:1 |
| Pushrods | Stock pushrods |
| Lifters | Short-travel hydraulic (Feuling/Andrews) |
| Induction | HPI / SE 55mm throttle body |
| Exhaust | 2-into-1 stepped (D&D FatCat / E-Series) |
| Air cleaner | Screamin' Eagle / Stage I high-flow |
| Tune | Dyno-tuned (matched to build) |
| Power adder | None (naturally aspirated) |