How to Identify Your Boat Trailer Axle and Order the Right Parts

Look on the tongue or the front of the frame for a sticker or plate. BoatUS explains that it should list the max capacity, GVWR (gross vehicle weight rating) and GAWR (gross axle weight rating) for a given tire size and inflation. GAWR times the number of axles equals GVWR. That GAWR is the capacity you need to match or beat with any replacement axle or hub. Photograph the plate and the trailer maker's name; the maker's customer service can often tell you exactly what axle they installed from the VIN.

Clean the axle tube with a wire brush and look for a stamp or sticker. Dexter says that on its axles made after April 2001 there's a nine-digit lot (serial) number near the middle on the rear side of the axle beam, right under the words DEXTER AXLE, and that rubbing chalk over it makes it easier to read. Other makers use their own tags or stickers. Lippert, for example, puts its label in the same general spot (center of the axle tube, back side of the beam) and its description string starts with the model designation, followed by the hub face, track and spring center dimensions and codes for the spring mounting, lubrication and hub or drum. Lippert's advice if the label is gone matches Dexter's: measure hub face, spring centers and brake size. With a lot number, the maker or a dealer can usually pull the original build sheet. Write the number down and photograph it.

Look at how the axle hangs from the frame:

• Leaf spring: a round or square tube on stacked steel springs, held by U-bolts. Note whether the axle sits under the springs (underslung) or on top (overslung). Dexter notes that switching between them changes the trailer's height and towing behavior.
• Torsion (rubber) axle, such as Torflex: a square tube bolted straight to the frame with short arms at each end and no springs. Dexter says you must not weld on a Torflex axle or move its brackets, because heat damages the rubber inside, and its start angle (ride height) can't be changed after it's built.

Dexter's measuring guide starts with the hub face: the distance from one wheel mounting surface to the other. Pull a wheel or measure across the outside faces of the hub flanges where the wheels sit. This sets how wide the axle is and where the tires sit under the fenders. Have a helper hold the other end of the tape, and measure twice.

Next is the distance between the centers of the suspension mounting points. On a leaf-spring axle, measure from the center of one spring seat to the center of the other. On a torsion axle, measure between the mounting brackets. This must match the trailer frame. Dexter also lists the distance from the spring center to the hub face as a key number for load and stress, and track width for setups with dual wheels.

Sight along the axle tube from the side. Dexter explains that the slight upward bend in the middle of many axles is "camber," built in on purpose so the axle straightens under load. That's normal. What's not normal: a bend near one end, a tube that sags in the middle with no load, or tires worn on one edge. Those are signs of a bent axle (see "When to call a pro").

With the hub off (see our trailer wheel bearings guide), wipe the grease off the inner and outer bearing cones and the races (cups) pressed into the hub. BoatUS's bearing article says each bearing and race has a stamped part number, so you don't have to guess the size. Write down all four numbers (inner cone, inner race, outer cone, outer race) and the numbers on the grease seal. Take the old parts with you to the store and match them side by side.

Look at the hub cap:

• Plain dust cap: standard grease-packed bearings.
• Cap with a grease fitting, or a spring-loaded cap: a "grease-through" spindle or a bearing protector. Dexter's version, E-Z Lube, has a drilled spindle that carries grease to the inner bearing; Dexter says it can't be added to an axle that wasn't built with it.
• Clear cap with oil: an oil-bath hub.
• Sealed cartridge (Dexter Nev-R-Lube): pre-adjusted, permanently lubricated bearings. Dexter's glossary says this system can't be used on axles that go under water, so you shouldn't see it on a boat trailer.

Look behind the wheel:

• No brakes: just a hub and spindle.
• Surge brakes: there's a master cylinder at the coupler and brake lines running to each wheel. Look for drums (a closed drum behind the wheel) or discs (a rotor with a caliper).
• Electric brakes: wires, not brake lines, go to each wheel, and the tow vehicle needs a brake controller. Electric brakes are less common on boat trailers because they get dunked. For trailers that go in the water, Dexter offers marine hydraulic brakes with E-coated, stainless or zinc-plated parts to slow down rust.

Measure the drum or rotor diameter, and note the brake maker if there's a sticker or stamp on the backing plate or caliper. BoatUS's brake article lists corroded lines, rusty master cylinder fluid and rust around the backing plate as things to look for while you're down there.

Count the lug studs and measure the bolt circle (or note it from the hub's part number). Copy the tire size from the sidewall and the rim size and load info from the wheel label. Dexter warns that wheels and tires must be matched and approved for each other, and that hub-piloted and stud-piloted wheels use different lug nuts; mixing them can let a wheel come off. If you switch to aluminum wheels, Dexter says to check stud length, since the thicker mounting area can leave too little thread for the nuts.

Write everything on one sheet (or a phone note) and keep it in the tow vehicle:

• Trailer maker, VIN, GVWR, GAWR
• Axle maker, lot number, suspension type
• Hub face, spring centers
• Bearing, race and seal numbers
• Brake type and size, hub type, bolt pattern
• Tire size and pressure from the plate

Now when a bearing fails on a trip, any parts store can match it.

Call the axle maker or an authorized distributor with your numbers. Dexter says small orders are usually cheaper through its distributors than direct. When the new parts arrive, compare them to the old ones before you install anything: bearing numbers, seal size, hub bolt pattern and brake size should all match. If anything doesn't, stop and call the seller before you start.