How to Choose the Right Disc Brake Kit for Your Trailer
The Four Numbers You Need
- Axle weight rating — 3,500 lbs / 6,000 lbs / 7,000 lbs (sometimes called 3.5K, 6K, 7K)
- Lug count — how many studs hold each wheel on (usually 5, 6, or 8)
- Stud diameter — 1/2", 9/16", or 5/8"
- Bolt pattern — lug count × diameter (e.g. 5 on 4.5", 6 on 5.5", 8 on 6.5")
Find all four, and your kit choice is locked in. Send these four numbers to our team if you'd rather have a tech confirm the match for you.
Step 1 — Find Your Axle Weight Rating
The axle weight rating is the single most important number. It tells you how much load each axle is built to carry — and it dictates which brake kit will physically fit and hold up. Heavier axles use larger rotors, bigger bearings, and beefier calipers.
Where to look:
- Trailer VIN sticker — usually on the tongue or front-left frame rail. Look for GAWR (Gross Axle Weight Rating).
- The axle itself — most manufacturers stamp a tag on the axle tube. Common markings: 3.5K, 5.2K, 6K, 7K, 8K.
- Manufacturer documentation — original spec sheet from Dexter, Lippert, UFP, or Alko.
Can't find a tag? Take the trailer's gross vehicle weight rating (GVWR) and divide by the number of axles. That's a close approximation. A 14,000-lb GVWR tandem axle trailer is most likely sitting on two 7K axles.
| Axle rating | Typical use | Typical lug count |
|---|---|---|
| 3,500 lbs (3.5K) | Boat trailers, utility, light cargo, small campers | 5 lug |
| 5,200–6,000 lbs (6K) | Mid-size travel trailers, equipment haulers, larger boats | 6 lug |
| 7,000–8,000 lbs (7K/8K) | 5th wheel, large RV, heavy car haulers, gooseneck | 8 lug |
Step 2 — Count Your Lugs
This is the easiest one. Look at one of your wheels and count the number of lug nuts (the bolts holding it on). The vast majority of trailers are 5, 6, or 8 lug.
If you have an aluminum wheel and can't see all the lugs clearly, count the recessed lug holes around the center hub. Lug count almost always tracks with axle weight rating — but always verify, because some custom builds break the pattern.
Step 3 — Measure Your Stud Diameter
Stud diameter is the thickness of each lug stud. It's stamped on the kit name (e.g. "(1/2 in.)" at the end of every product title) because the hubs and bearings are spec'd to match the stud.
If you don't have calipers, you can usually tell by feel and lug-nut size: a 1/2" stud takes a wrench socket around 13/16" or 21mm; a 9/16" stud takes 7/8" or 22mm; a 5/8" stud takes 15/16" or 24mm. The easiest way is to remove one lug nut and measure the stud at the base.
| Common pairings | Stud diameter | Typical lug-nut wrench |
|---|---|---|
| 3.5K and 6K axles | 1/2" | 13/16" / 21mm |
| 7K axles (standard) | 1/2" | 13/16" / 21mm |
| 7K axles (heavy spec) | 9/16" | 7/8" / 22mm |
| 8K axles | 5/8" | 15/16" / 24mm |
Step 4 — Identify Your Bolt Pattern
The bolt pattern is the circle of studs around the hub, expressed as (number of lugs) on (diameter of the circle they form). Common patterns:
5 on 4.5"
Standard 3.5K axle pattern. Used on the vast majority of boat trailers and utility trailers.
6 on 5.5"
Common 6K axle pattern. Also used on some 3/4-ton truck rims, which makes it easy to find replacement wheels.
8 on 6.5"
Standard 7K/8K pattern. Found on heavy 5th wheel trailers and 1-ton truck rims.
To measure: take any two studs that are directly across the hub (not the closest two). The distance between the centers of those two studs is your "on" value. For odd-lug-count patterns (like 5-lug), measure from the center of one stud to the back edge of the stud two positions over.
Slip-Over vs Integral — What's the Difference?
HydraStar offers most of our kits in two flavors. They both stop your trailer equally well — the difference is how they attach.
Match Your Specs to a HydraStar Kit
| Your axle | Single axle (slip-over) | Single axle (integral) | Tandem / Triple |
|---|---|---|---|
| 3.5K · 5-lug · 1/2" | 3.5K Slip Over | 3.5K Integral | Bundle two single kits |
| 6K · 6-lug · 1/2" | 6K Slip Over | Available on request | 6K Tandem Integral |
| 7K · 8-lug · 1/2" | 7K Slip Over | Available on request | 7K Tandem 1/2" |
| 7K · 8-lug · 9/16" | Available on request | Available on request | 7K Tandem 9/16" |
| 7K · 8-lug · 5/8" · Triple | — | — | 7K Triple Integral |
| Marine 7K · Triple | — | — | Marine 7K Triple |
"Available on request" means we keep the parts in stock individually — call our team and we'll bundle the right combination for you.
C-Series vs E-Series — What's Included?
You'll see two suffixes on our multi-axle kits. The brake hardware is identical — the difference is whether the kit includes the electrical accessories needed to wire up an EOH actuator.
C-Series (Complete)
Everything you need to wire and install:
- Disc brake hardware (rotors, calipers, brackets, pads)
- 7-way trailer plug connector
- Breakaway kit
- Battery kit
- Junction box
E-Series (Essential)
Brake components only:
- Disc brake hardware (rotors, calipers, brackets, pads)
- Use your existing wiring, breakaway, and battery
Save money if you're upgrading an existing setup that already has the wiring in place.
Replacing Another Brand?
If you already have disc brakes from another manufacturer and you're upgrading or replacing parts, two cross-compatibility notes:
Kodiak
Our 225 calipers are 100% interchangeable with Kodiak 225 calipers. The rotors, pads, and brackets are all spec-compatible. You can mix and match.
Tie Down Engineering
The caliper is compatible, but you'll also need our caliper bracket — the Tie Down bracket has a different offset. Easiest path is buying the caliper + bracket together as a pair.
Quick FAQ
Do I need a new actuator if I'm converting from drum to disc?+
Yes, almost always. Drum brake actuators top out at 1,000–1,200 PSI; disc calipers need 1,600 PSI to engage fully. We have a full breakdown in our PSI guide.
Does the kit include mounting hardware?+
Stud bolts, caliper bolts, and caliper bracket bolts are included. Wheel lug nuts, mounting hardware for the actuator (if you're buying one), and brake line tubing/fittings are not — those depend on your trailer's existing setup.
My trailer is unusual — mixed axles or non-standard pattern. Do you make custom kits?+
We don't manufacture custom kits, but we keep the component parts (rotors, calipers, brackets, hubs) in stock individually and can bundle a non-standard combination. Call us with your specs and we'll put it together.
Standard vs stainless — which should I get?+
Stainless caliper bolts and Dacromet-coated rotors are the right choice if your trailer ever sees salt — saltwater launches, road salt in winter, coastal storage. Standard hardware works fine in dry-climate, freshwater, or covered-storage trailers. Stainless costs a bit more upfront and saves rust headaches later.
How long does a typical install take?+
For a competent DIYer with the trailer on jack stands: 1–2 hours per axle for a slip-over kit, 2–3 hours per axle for integral (because you're also re-packing or replacing bearings). Allow extra time for bleeding the system if you're also installing a new actuator.
Send Us Your Four Numbers
Axle rating + lug count + stud diameter + bolt pattern. We'll match you to the exact kit — and if you have an unusual setup, we'll bundle the parts. Free, every business day.
Talk to a HydraStar Techor call (812) 655-4544 · Mon–Fri, 8am–4:30pm ET