title: O-Rings description: The Slocum hull and component O-rings: G2 single-seal vs G3 double-seal part numbers and sizes, the full spares kit, approved lubricants (Parker O-Lube vs Molykote and the community debate), inspection and replacement, sealing-surface scratches and the sanding controversy, lubricant dry-out, vacuum loss, and assembly tips.

O-Rings¶

O-rings are the glider's primary defense against flooding. Every hull joint, plug, and through-hull fitting relies on a correctly sized, clean, lightly lubricated O-ring seated in an undamaged groove against an undamaged sealing surface. Most Slocum leaks are not main-hull O-ring failures — they're bulkhead connectors, instrument seals installed with the wrong O-ring, or loose through-hull fittings — but the main-hull O-rings are what you inspect and replace most often, so getting their part numbers, lubricant, and handling right is foundational.

Single seal (G2) vs. double seal (G3)¶

A defining hardware difference between the platforms:

• The double seal lengthened the carbon-fiber hulls by 0.5" versus the G2, raising standard-config G3 displacement to ~57.7 L and lengthening the internal tie rod — G2 hulls will not fit a G3.
• Early G3s shipped with the G2-size ring (2-265) and were notoriously hard to open and close; TWR later issued the correct smaller 2-264 double-seal ring. If a G3 is a "fight for your life" to assemble, check you're on the right O-ring first.
• The G2 main hull O-ring (2-265/N674-70) is nitrile, 70 durometer, ID 7.734", width 0.139". The vacuum/evacuation boss plug O-ring is also nitrile 70-durometer: 3-904 (G-030), ID 0.351", width 0.072".

Part numbers — full spares kit¶

The complete O-ring spares kit is ASSY 4424 (a smaller kit, ASSY 4423, covers just the two most-used seals). Contents (part numbers are stable; prices are illustrative, ~2012):

Lubricant: what's approved, what the field actually uses¶

The official lubricant for main-hull O-rings and sealing surfaces is Parker O-Lube 884-4 (3135-LUBE / 3M-LUBE, "Parker Fibrous O-Lube 884-4", a petroleum naphthenic oil + barium soap), available from the TWR forum or by request from glider support. The maintenance manual procedure is simply: inspect, lubricate with Parker O-Lube, install, seat properly.

For electrical connectors the rules differ — don't cross them:

Inspection & replacement¶

• Inspect every O-ring for cleanliness, nicks, slices, dents, and cracks before each deployment; inspect the sealing surfaces for scratches and defects under good light (a flashlight raking across the surface reveals scratches a fingertip misses).
• Replace as needed before every deployment. Many teams running single-use primary batteries replace all hull O-rings every time they open the glider (i.e., every mission). With rechargeable G3s that can recharge/download without opening, a common plan is to open once a year to inspect, replace hull O-rings, and test batteries.
• Cleanliness is everything. Foreign particles in the gland cause leaks and shorten O-ring life. Be "paranoid": gloves, hair retained, clean bench. The vast majority of well-handled hull O-rings simply don't leak.
• Assembly (from the Parker handbook): keep installed ID stretch under 5%, don't exceed ~25–50% elongation reaching the groove, and never twist the O-ring into place. Seat it fully before drawing the joint closed.

Sealing-surface scratches — the sanding debate¶

Carbon-fiber hull sealing surfaces do pick up fine scratches (from sharp edges around ballast bottles, sliding batteries in/out, corner bumpers on the mainboard). What to do about a scratch that crosses the O-ring band is genuinely contested in the community — present both views and decide deliberately:

Vacuum, leaks & testing¶

The internal vacuum is your standing leak indicator — always monitor it before launch (less vacuum than expected = a leak; positive pressure can mean dangerous gas accumulation). It fluctuates with temperature, so log temperature alongside vacuum.

• Never power a shallow glider without a vacuum.
• A reasonable "sealed" bar: vacuum stable for 1–2 weeks (recording temperature). Deploy a glider sealed only ~24 h only in an emergency.
• Slow loss over months is common even on a well-sealed glider on the shelf — not necessarily a real leak. But re-verify, and inspect/replace O-rings before redeploying after long storage. A brand-new factory-sealed, helium-tested G3 can still slowly bleed down on the bench; common real culprits found include loose through-hull fittings (e.g., a nose altimeter cable pass-through that needed a ¼ turn) as much as the hull O-rings themselves.
• Don't over-pull the vacuum to mask a marginal seal — excessively high vacuum can pull bubbles out of the ballast-engine oil.
• Leak testing is hard: vacuum-and-wait can take months to reveal a slow leak, and gliders that hold vacuum for months on the shelf sometimes leak as soon as they're in the water. Teams use sectional pressure monitors, pressure chambers (commercial or DIY McMaster-fitting builds), or helium leak detectors (accurate but expensive). No method is foolproof — cleanliness and inspection remain the best defense.

Assembling the double-seal G3¶

A practical pain point worth its own note — closing a G3 against two O-rings:

• Make sure you're on the correct 2-264 G3 ring, not the larger G2 2-265.
• Manually push the hull sections fully over the O-ring seals before relying on the tie rod to draw them together — tie-rod lengths vary (especially with stack-on bays like an AD2CP), and short ones won't "catch" until the sections are nearly home.
• Pulling a light vacuum during assembly helps seat the joint the last bit (many teams do this).
• Keep good PEEK hull-opening tools (they outlast the soft black ones but still round off); some teams make thin plastic wedges to crack a stubborn, long-stored hull — wedge away from the O-ring sealing surface, never into it.

Quick reference¶

See also¶

• Tie Rod — what draws the O-ring joints closed, and why G3s can be hard to assemble.
• Pumps — Bellofram/oil-bladder rubber care and storage (related lubricant rules).
• Maintenance Schedule — where O-ring inspection fits in the service schedule.