Measure your CDI Flexible Furler's stainless cup diameter and headstay length to identify your model — FF1 through FF9 — then order the exact drum, drup cup, extrusion, or small-parts kit for that model. MAURIPRO stocks genuine CDI replacement parts for every Flexible Furler size, so you can fix the right part instead of guessing.
This is written for the cruiser (~28-47 ft) staring at an unmarked CDI Flexible Furler at the bottom of the headstay, trying to order a replacement drum, cup, or extrusion without a model number to go on. CDI's Flexible Furler line — FF1, FF2, FF4, FF6, FF7, and FF9 — looks similar boat-to-boat, but the drum, cup, and foil extrusion are model-specific and do not cross-fit, so the fastest path to the right part is two quick measurements, not a guess from the boat's length alone.
CDI Flexible Furler models at a glance
| Model | Cup diameter | Max headstay length | Max headstay wire | Max turnbuckle pin | Example part |
|---|---|---|---|---|---|
| FF1 | 3.5 in | 21 ft | 1/8 in | 1/4 in | Drum Assembly, FF1 |
| FF2 | 4 in | 29 ft | 3/16 in | 5/16 in | Drum Assembly, FF2 |
| FF4 | 5.8 in | 33 ft | 1/4 in | 7/16 in | Drum Assembly, FF4/FF6 |
| FF6 | 5.8 in | 39 ft | 1/4 in | 7/16 in | Drum Assembly, FF4/FF6 |
| FF7 | 6.5 in | 47 ft | 1/4 in | 1/2 in | Drum Assembly, FF7 |
| FF9 | 6.5 in | 53 ft | 5/16 in | 1/2 in | Drum Assembly, FF9 |
Specifications per CDI's published Flexible Furler sizing guide. All six models use #6 luff tape.
How do you identify which CDI Flexible Furler model you have?
Measure two things: the diameter of the stainless steel cup at the base of the furler, and your forestay's maximum headstay length. The cup diameter narrows you to one or two candidate models — 3.5 in is always FF1, 4 in is always FF2, but 5.8 in matches both FF4 and FF6, and 6.5 in matches both FF7 and FF9. Measure across the widest point of the cup with a tape measure or calipers, not by eye, since the FF4/FF6 and FF7/FF9 pairs are close enough to misjudge.
To get headstay length, hoist a tape measure on the headsail halyard until it stops on its own, read that measurement, then step back and estimate the remaining distance from the top of the tape to the headstay pin at the masthead; add the two together. Compare the result against the table above — a shorter length within a shared cup-diameter pair points to FF4 or FF7, a longer length to FF6 or FF9.
What if two models share the same cup diameter?
Check headstay wire diameter and turnbuckle pin diameter next — both step up between the paired models even when the cup does not. Between FF4 and FF6, the cup and wire spec match, but FF6 is rated to a longer 39 ft headstay versus FF4's 33 ft, so headstay length is the deciding measurement. Between FF7 and FF9, the wire diameter itself changes — FF7 tops out at 1/4 in headstay wire, FF9 at 5/16 in — so if your forestay wire measures over 1/4 in at its thickest point with calipers, you have the FF9, regardless of what the headstay length alone suggests. When two measurements disagree, headstay wire diameter and turnbuckle pin diameter are the more reliable tie-breakers because they're a fixed, easily measured spec, not an estimate.
Which replacement parts does each CDI model use?
Four part families cover most Flexible Furler repairs, and none of them cross-fit between models: the drum assembly (the black anodized spool the furling line wraps around), the drup cup (the stainless cup at the base that you measured to identify the model), the extrusion assembly (the foil sections that carry the luff tape up the headstay), and a small parts kit (bearings, pins, and fasteners for a full rebuild). FF4 and FF6 share the same drum assembly and drup cup part numbers since both use the same base hardware, and FF7/FF9 share a drup cup for the same reason — but the extrusion length and small parts kit are still model-specific, so always confirm against the table above before ordering a shared-hardware part.
Why does the drup cup matter so much for identification?
The drup cup is the single fastest identifier because its diameter is fixed per model and it's the most visible, most accessible part on an installed furler — you don't need to disconnect the forestay to measure it. It also happens to be one of the more commonly replaced parts, since it takes direct load and UV exposure at the base of the furler and can corrode or crack over years of service. Order the wrong cup and it simply won't seat against the drum housing, which is the quickest way to confirm a measurement error before you're stuck at the dock with a part that doesn't fit.
Do the extrusion sections wear out, and can you replace just one?
Yes — CDI's Flexible Furler extrusion ships as multiple foil sections joined along the headstay, and a cracked, UV-degraded, or bent section can be a genuine failure point independent of the drum or cup. MAURIPRO sells the full extrusion assembly per model rather than single loose sections, since section lengths and the luff-tape groove profile are specific to each model's stay length rating — ordering the complete assembly for your model guarantees the sections you receive are matched to each other and to your headstay length.
Which should you choose: a single part, or a full rebuild kit?
Order the single part — drum, cup, or extrusion — when you've identified one specific point of failure, such as a cracked cup or a slipping drum, and the rest of the furler is working normally. Choose the small parts kit when you're servicing an older furler with unknown internal wear, since it bundles the bearings, pins, and fasteners that are cheap individually but tedious to source one at a time, and catches the small wear items that don't announce themselves the way a cracked cup does. Either way, confirm your model with the cup-diameter-and-headstay-length check above first — ordering by boat length alone is the single most common way sailors end up with the wrong part.
Expert notes
From our rigging bench: the FF4/FF6 mix-up is the one we see most often, because both share the same 5.8 in cup and the same drum and cup part numbers — sailors assume that means the extrusion and parts kit are interchangeable too, and they aren't. Always check headstay length before ordering the extrusion or small parts kit specifically.
On older boats, we also recommend measuring the actual forestay wire with calipers rather than trusting the original CDI unit's rated max — many boats are rigged with wire well under the furler's maximum, and knowing the real number helps if you're ever cross-shopping a replacement furler rather than just parts.
Common mistakes
- Ordering by boat length instead of measuring the furler. A 34-foot boat can carry an FF4, FF6, or even an oversized FF1 depending on how the boat was originally rigged — measure the installed hardware, don't assume from LOA.
- Assuming FF4 and FF6 parts are fully interchangeable. The drum and cup match; the extrusion assembly and small parts kit do not.
- Measuring the cup at its narrowest point instead of its widest. This under-measures the diameter and can point you to the wrong model in a shared-diameter pair.
- Skipping the wire-diameter check on FF7 vs. FF9. Headstay length alone can be ambiguous near the boundary; the wire diameter (1/4 in max on FF7, 5/16 in max on FF9) is the more reliable tie-breaker.
Frequently asked questions
How do I know if I have a CDI FF1, FF2, FF4, FF6, FF7, or FF9 furler?
Measure the stainless cup at the base of the furler (3.5 in for FF1 up to 6.5 in for FF7/FF9) and your maximum headstay length (21 ft for FF1 up to 53 ft for FF9), per CDI's published sizing specifications. Cross-check both measurements against the table above, since FF4/FF6 and FF7/FF9 share cup diameters and are distinguished by headstay length and wire diameter.
Are CDI FF4 and FF6 parts interchangeable?
The drum assembly and drup cup are shared between FF4 and FF6, but the extrusion assembly and small parts kit are not — always confirm your model's headstay length before ordering either of those two.
Where can I buy genuine CDI Flexible Furler replacement parts?
MAURIPRO stocks genuine CDI drum assemblies, drup cups, extrusion assemblies, and small parts kits for the full FF1 through FF9 range — see the collection link below to shop by model.
What's the difference between the drum assembly and the drup cup?
The drum assembly is the black anodized spool the furling line wraps around; the drup cup is the stainless steel base cup that seats against the drum housing and is the fastest way to visually confirm your model by its diameter.
Can I replace just one section of the furler's extrusion?
MAURIPRO sells the extrusion assembly as a complete, matched set of foil sections per model rather than single loose sections, since section length and the luff-tape groove are specific to each model's rated headstay length.
Recommended gear
CDI Drum Assembly for FF2 Furlers — the furling-line spool, for a worn or slipping drum.
CDI Drup Cup for FF2 Flexible Furlers — the stainless base cup, for cracking or corrosion at the tack fitting.
CDI Extrusion Assembly (1.4") for FF2 Furlers — matched foil sections, for a bent or UV-degraded luff groove.
CDI Small Parts Kit for FF4 / FF6 Furlers — bearings, pins, and fasteners for a full service on an older furler.
You may also be interested in
- How to Choose a Headsail Furler: A Selection Guide
- Can You Service a Facnor Headsail Furler?
- How Does a Roller Furling Jib Work?
Last updated: August 22, 2026.