RC Boat Flex Shaft Greasing: How Often, With What, and What Else Actually Matters

For a flex shaft, the trade-off is:

• Stiffer, more water-resistant grease (marine wheel-bearing type, typically around NLGI 2 with a water-resistant thickener) — better seal, longer protection, more drag and more heat at speed.

• Lighter grease or a dedicated cable lubricant — less drag, cooler running, but it washes out or flings off sooner, so it needs more frequent attention.

For a sport boat running a few packs a session, water-resistant marine grease packed properly is the sensible default. For a high-rpm setup chasing speed, a lighter cable grease and more frequent regreasing is the better trade.

What not to use: WD-40 or any penetrant as a substitute — it is a solvent with a light oil in it, it will not stay in a stuffing tube, and it strips what is there. Silicone spray in a flex tube is the same story. Vegetable oils go sticky and rancid. And do not pack a tube with grease meant for a car suspension joint and expect it to stay put at cable speed.

How Much Grease, and How Do You Pack It?

Enough to fill the tube and coat the cable, and no more.

• Clean the old grease out first — a rag through the tube, and wipe the cable down. New grease over old grit is how a shaft starts to wear.

• Coat the cable along its length as you slide it back in, so the grease is distributed rather than pushed to one end.

• Fill the tube, not the hull. Excess grease gets thrown off at the coupler, into the hull, and onto everything else. A little hydraulic resistance at the coupler end is normal; grease spraying out of the tube is not.

• Wipe the coupler and the first centimetre of cable clean so the collet grips metal, not lubricant. A greasy collet slips — and a slipping collet chews the cable end.

What Else Should You Do After Every Run?

The replies point at the right things; here they are as a routine that takes five minutes:

1. Dry the hull inside and out, and leave hatches or hatches-tape off overnight so it can breathe.

1. Check whether there was water in the hull — and where it collected. Water at the transom is usually spray; water around the shaft log is a seal problem.

1. Rinse with fresh water after salt or dirty water, then dry. Rinsing and not drying is how you trade salt for standing water.

1. Check the coupler and its set screws. Vibration loosens them; a loose coupler is the most common "my boat lost drive" failure.

1. Spin the shaft by hand and feel for grit or a tight spot. Grit means the tube needs cleaning, not more grease.

1. Regrease if any of the conditions above applied.

What About Storage?

This is where the thread’s best line sits, and it deserves emphasising: "Always do it before long periods of inactivity."

Corrosion is a time-and-water problem, not a running problem. A boat that runs weekly in fresh water stays remarkably clean; the same boat put away wet in October and opened in April will have rust on every steel part that water reached. So:

• Grease the shaft before storage, not after.

• Dry the hull completely, and store with the hatch open if the boat lives indoors.

• Take the battery out. Store it at storage voltage, somewhere dry.

• If the shaft comes out for the winter, store it straight, not coiled.

Do Motor Bearings Need Oil?

The thread offers "motor bearings might need some oil after 100h or so", which is a reasonable owner’s rule of thumb — and it carries an important condition the reply does not mention: it only applies to open or shielded bearings, not sealed ones.

• Sealed bearings have a lip seal and a factory fill. Adding oil does nothing useful; the only service is replacement.

• Open or shielded bearings can take a drop of light oil, sparingly. More is not better — excess oil attracts dust and grit, which is exactly what kills a bearing.

• A gritty or noisy bearing is replaced, not oiled. Once there is play or a rumble, the race is already damaged.

And the same rule as the rest of this article: keep water out of the motor. A brushless motor that has been submerged and not dried properly will rust its bearings and, in the worst case, corrode the stator laminations.

What Rusts That People Forget?

External steel is the part everyone sees and nobody treats: screws, turn fins, trim tabs, linkages, motor mounts, and the driveshaft itself. A light film of the same marine grease — wiped on, not packed on — is enough on most of it. Stainless fasteners are not rust-proof, only rust-resistant; in salt water they stain and eventually pit, especially where two different metals touch.

The cheap version of this routine: after the last run of the day, rinse, dry, and put a thin smear on anything that shines.

Is This the Same Problem as Oiling a Sewing Machine?

It is, and that is why this article lives here rather than on a boating site. Strip away the water and the four rules are identical:

Rule	RC boat flex shaft	Sewing machine head
Speed sets the grade	High cable speed wants a lighter grease	High-speed lockstitch wants a lighter oil — ISO VG 7–10 rather than 15
The lubricant is also a seal	Grease keeps water out of the tube	The oil film keeps lint and grit out of the bearing surfaces
Storage is when it degrades	Corrosion during months of inactivity	Dry bearings and gummed oil during idle weeks
More is not better	Excess grease is thrown into the hull and slips the collet	Excess oil becomes oil-stain defects on finished garments

And the grading confusion is the same one: a sewing machine oil is specified by ISO VG — kinematic viscosity in cSt at 40 °C (Juki New Defrix No. 1 is 7.3 cSt, No. 2 is 31.5 cSt, with ISO VG 7 / 10 / 15 the usual substitutes), because "sewing machine oil" is a use case, not a grade. A grease is specified by NLGI consistency plus the base oil’s ISO VG. If a supplier cannot tell you either number, they are selling you a name.

One habit transfers directly: clean before you lubricate. Grease over grit, or oil over packed lint, is how a bearing that would have lasted years dies in months.

If you want this written out as a routine for machines that run a shift rather than a few packs on a Sunday, see our sewing machine maintenance schedule: daily, weekly and monthly tasks — same four rules, expanded into a checklist with an owner for each task.

The Five-Line Version

1. Regrease after any run that put water in the hull, and always before storage.

1. Pack the tube, coat the cable, wipe the coupler — a slipping collet ruins the cable end.

1. Water-resistant marine grease by default; lighter grease if you are chasing speed and will regrease more often.

1. Rinse, dry, check the set screws, spin the shaft by hand.

1. Never a penetrant, never over-filled, never stored wet.

Where This Fits

This article is part of the maintenance library on this site. We build industrial sewing machines — lockstitch, overlock and specialised units for garment production — so most of what we know about lubrication, wear and corrosion comes from machines that run eight hours a day rather than a few packs on a Sunday. The same rules apply to both.

If you maintain machines for a living — or you are setting up a production line and want the lubrication schedule, needle systems and service intervals written down before you buy — that is what we publish here, and that is the conversation we are happy to have.

📩 Email: sales6@chinadoit.cn

🌐 Website: doitsewingmachine.com

References

• r/rcboats — “Flex shaft — longest can leave to grease? oil other?” (thread source; comments quoted verbatim from a client-supplied screenshot; one top comment appears with its author line cut off and is marked as unattributed). //www.reddit.com/r/rcboats/comments/1xiicw5/flex_shaft_longest_can_leave_to_grease_oil_other/

• Lubricant grading: grease consistency by NLGI number versus base-oil viscosity by ISO VG (cSt at 40 °C); grease as base oil plus thickener.

• Series-verified sewing-machine figures used in the comparison: ISO VG as the only meaningful oil specification; Juki New Defrix No. 1 = 7.3 cSt, No. 2 = 31.5 cSt; ISO VG 7 / 10 / 15 substitutes; faster machine takes lighter oil; over-oiling causes oil-stain defects; standard lockstitch head has no replaceable oil filter; clean before lubricating.