Can You Put a Shaft Collar Clamp Behind a Dripless Shaft Seal?

Yes, fitting a stainless steel split collar directly behind the rotor of a dripless face seal is an excellent safety upgrade. It acts as a physical backstop, ensuring that if the primary grub screws fail, the rotor cannot slide down the shaft and cause a catastrophic flood.

Why Dripless Seal Rotors Slip

Mechanical face seals like the ubiquitous PSS (Packless Sealing System) by PYI, or alternatives like Lasdrop, rely on a rubber bellows to compress a stationary carbon stator against a rotating stainless steel rotor. That stainless rotor is held onto the spinning propeller shaft purely by friction, generated by stainless steel grub screws biting into the shaft.

Over time, intense engine vibration, shaft whip or a slight engine misalignment can coax these screws loose. Furthermore, if a previous owner reused old grub screws, their cup-shaped points will have flattened out and lost their grip. If that rotor slides aft towards the gearbox, the bellows loses compression. The carbon and stainless faces separate, and seawater pours into the bilge. A backup collar entirely mitigates this risk.

This applies specifically to face seals. If you have a lip seal, such as a Volvo Penta rubber bung or a Tides Marine SureSeal, there is no compressed rotor to slip back. A shaft collar is not required for those systems.

Choosing the Right Shaft Collar

You cannot use a cheap mild steel or zinc-plated collar from a hardware shop. It will rust into a useless lump of iron oxide within a month in a damp bilge. You need a collar machined from 316L marine-grade stainless steel.

• Two-piece split collar: Always buy a double-split (two-piece) collar. This allows you to bolt it onto the shaft in situ. A single-split or solid collar requires you to uncouple the shaft from the gearbox flange to slide it on, turning a ten-minute job into a weekend project.
• Exact sizing: Measure your shaft accurately with digital calipers. A 25mm shaft is not the same as a 1-inch (25.4mm) shaft. A 25mm collar on a 1-inch shaft will leave a gap and clamp unevenly, while a 1-inch collar on a 25mm shaft will bottom out before it grips. Common sizes include 25mm, 30mm, 35mm, 1-inch, 1.25-inch and 1.5-inch.
• Avoid anodes: Do not use a standard zinc shaft anode as a backup collar inside the boat. Zinc is a soft metal that deforms under pressure. If the stainless rotor slips and strikes the anode, the zinc will simply slide down the shaft with it.

Expect to pay between £20 and £40 for a high-quality 316 stainless collar from brands like Ruland or a dedicated marine transmission supplier.

How to Install the Collar

Before fitting the collar, verify that your primary dripless seal is compressed to the manufacturer specification. For a standard 1-inch or 25mm PSS seal, the bellows should be compressed by exactly 19mm (3/4 of an inch) from its resting state.