The Twins Cup homework build
2021 Aprilia RS 660 — Build an RS 660 to the MotoAmerica Twins Cup recipe — not to race yet, but to learn what the class already knows.
Reference build assembled by the shop from community consensus — prices verified at publish.
- Arrow full titanium race systemThe value full system for the platform — same job as the exotic options for less. Track-only, needs the flash to fuel right.$1,850
- ECU flash with race mapMatched to the open exhaust; raises limiter behavior and cleans the bottom-end. Keep the stock map saved for resale day.$330
- K-Tech 25SSK RDS fork cartridgesSprung for my weight when ordered. Published Twins Cup settings for this bike exist — start there, not from zero.$1,100
- K-Tech Razor-R rear shockMatches the front. The stock shock is fine for the street and out of ideas on slicks.$1,000
- Race fiberglass bodywork kitUnpainted, with a race tail that fits over the stock subframe. Sell the pristine street plastics — they fund half of this.$900
- Bonamici rearsetsCommon in the class, GP-shift capable, replacement pegs and toe pieces available separately — the crash-economics matter more than the billet.$580
- Woodcraft clip-ons3-piece design: crash replacement is a $25 bar tube, not a $165 assembly.$165
- GB Racing engine cover setRequired for race groups by most orgs. The 660 cases sit wide — cover them before the first track day, not after.$350
- Vortex sprockets + EK race chain, gearing kitAlready a 520 chain from the factory — this is gearing and a lighter race chain, not a conversion. Carry a spare rear in a second size.$230
- Brembo Z04 front brake pads (pair)The class-standard pad upgrade. Cold bite is poor — track use only.$180
- Pirelli Diablo Superbike slick setSC1 front / SC1 rear to start; warmers stop being optional at this point. Budget tires as a per-weekend consumable, not a mod.$560
The story
The RS 660 became a race platform the day it launched — Twins Cup grids filled with them almost immediately, which means the setup homework is already done and floating around every paddock. This build copies the recipe instead of inventing one: full system, flash, real suspension front and rear, crash-proofing, race bodywork. When the class has already found the fast configuration, originality is just a slower lap time.
The money followed the grid. K-Tech front and rear because that's what a good share of the field runs and there's published settings data for this exact bike; the stock suspension is genuinely decent for the street and clearly soft once you're on slicks. Arrow full system over the flashier options because the class rewards a value pick that makes the same power — the weight drop is real and the midrange fills in with the flash. What I deliberately kept stock: the electronics and the quickshifter. Aprilia's package is legitimately good and the rulebook doesn't reward spending there.
The unglamorous truth is that the bodywork and case covers are the most important parts on the sheet. Race fiberglass and covers are what let you crash on Saturday morning and still ride Saturday afternoon — that's the whole difference between a track hobby and a very expensive single session. And the number plates don't make you fast: my first outing on slicks I was seconds off what this suspension is capable of. The bike is built to the recipe. The rider is still in development.
Shop this build
One retailer search per line. Most of these parts are model-specific, so confirm the exact part number for your year, generation and market before you buy.
| Part | Typical price | Where to buy |
|---|---|---|
| Arrow full titanium race systemExhaust | $1,850 | |
| ECU flash with race mapTune | $330 | |
| K-Tech 25SSK RDS fork cartridgesSuspension | $1,100 | |
| K-Tech Razor-R rear shockSuspension | $1,000 | |
| Race fiberglass bodywork kitBodywork | $900 | |
| Bonamici rearsetsLevers | $580 | |
| Woodcraft clip-onsLevers | $165 | |
| GB Racing engine cover setCrash Protection | $350 | |
| Vortex sprockets + EK race chain, gearing kitDrivetrain | $230 | |
| Brembo Z04 front brake pads (pair)Brakes | $180 | |
| Pirelli Diablo Superbike slick setTires & Wheels | $560 |
Some links may earn Unstocked a commission at no cost to you. Prices shown are typical street prices — always verify fitment and price at the retailer. Model-specific parts (exhausts, rearsets, bodywork) can run 2–3× between platforms — the sheet shows a typical figure.
Ride your version
Open this exact parts list in the composer. Swap what you’d change, then share your own link — the sheet lives in the URL.