Banjo Bolt Size Chart: Thread Sizes, Crush Washers & How They Seal
If you've ever traced a brake hose to the caliper, you've met a banjo bolt — the hollow bolt that clamps the round "banjo" fitting on the end of the hose to the caliper port. It's one of the smallest parts in the brake system and one of the most misunderstood: the threads come in a half-dozen sizes, the seal depends entirely on two copper washers most people try to reuse, and nothing about it works like the flared connections on the rest of your lines.
This guide covers how a banjo fitting actually seals, every common banjo bolt thread size in one chart, how to measure the bolt you have, and how to install one so it never weeps a drop.
What Is a Banjo Bolt (and How Does It Seal)?
A banjo fitting has two parts: the banjo — the ring-shaped end of the hose with a hollow eye — and the banjo bolt, which passes through that eye and threads into the caliper, master cylinder, or ABS port. The bolt is drilled hollow, with cross-holes through its shank: fluid flows up through the bolt, out the cross-holes, through the banjo eye, and into the hose.
Here's the part that trips people up: the threads don't seal anything. The seal is made by two soft copper crush washers — one under the bolt head, one between the banjo and the port face. When you torque the bolt, the annealed copper deforms into the microscopic imperfections of both surfaces and locks the joint tight. No flare, no tapered seat, no thread sealant — just clamping force and copper.
That's why a banjo connection is completely different from the flared tube-nut connections on your hard lines — and why the fixes for a leaking flare don't apply here.
Banjo Bolt Size Chart
Banjo bolts are sized by thread diameter × pitch, plus shank length. Metric threads dominate — even on many domestic vehicles — but SAE sizes are common on older American cars and aftermarket applications.
| Thread Size | Type | Where You'll Find It | Shop |
|---|---|---|---|
| M10x1.0 | Metric, fine | The most common brake caliper banjo thread across import and late-model domestic vehicles | M10x1.0 banjo bolts |
| M10x1.25 | Metric | Very common on Asian makes — always verify pitch against M10x1.0 before ordering | M10x1.25 banjo bolts |
| M10x1.5 | Metric, coarse | Some domestic calipers and clutch hydraulic lines | M10x1.5 x 20mm |
| M11x1.25 | Metric | Less common; found on select calipers — easy to mistake for M10 or M12, so measure | M11x1.25 x 23mm |
| M12x1.0 | Metric, fine | Common on European calipers and master cylinders | M12x1.0 x 21.3mm |
| M8x1.25 | Metric, small | Small calipers, clutch slave cylinders, some motorcycle and ATV brakes | M8x1.25 x 16mm |
| 3/8"-24 | SAE, fine | Classic domestic vehicles and many aftermarket brake hoses | 3/8"-24 x 1" |
| 1/2"-20 | SAE, fine | Larger domestic calipers and performance applications | 1/2"-20 x 1-1/4" |
Two cautions when you read this chart. First, M10x1.0 and M10x1.25 are not interchangeable — the wrong pitch will start by hand, bind after a few turns, and strip the caliper's aluminum threads. Second, length matters: a bolt that's too long bottoms out before clamping the washers; too short and it only catches a few threads. Match the old bolt's shank length, not just its diameter.
Browse the full banjo bolts collection for every size above, sold in bags of five so a stripped or rounded bolt never stalls the job.
How to Measure a Banjo Bolt
- Thread diameter: measure across the threads with a caliper. Roughly 10 mm means M10; 9.5 mm (0.375") means 3/8"; 12.7 mm (0.5") means 1/2".
- Thread pitch: use a thread pitch gauge, or thread the old bolt into a known nut. For metric, pitch is the distance between threads in mm; for SAE, count threads per inch.
- Length: measure the shank from under the head to the tip.
- No old bolt to measure? The caliper or component's port is the reference — take the vehicle's year/make/model to the service manual, or test-fit from an assortment.
Crush Washers: The Actual Seal
Every banjo connection uses two sealing washers — one on each side of the banjo eye. They're made of annealed (softened) copper that permanently deforms as you torque the bolt. That deformation is the seal, and it's also why they're strictly one-time-use: a washer that's already been crushed is work-hardened and won't conform again. Reused washers are the single most common cause of a banjo joint that seeps no matter how hard you tighten it.
Size a crush washer by two dimensions: the inner diameter (just larger than the bolt shank, so a 10 mm bolt takes a ~10 mm ID washer) and the outer diameter (large enough to cover the sealing face of the banjo and port). Common pairings:
| Washer Size (ID × OD) | Fits Bolt | Shop |
|---|---|---|
| 10mm × 16mm | M10 banjo bolts | 10mm copper washers |
| 3/8" × 5/8" | 3/8"-24 banjo bolts | 3/8" copper washers |
| 11.2mm × 18mm | M11 banjo bolts | 11.2mm copper washers |
| 12mm × 18mm | M12 banjo bolts | 12mm copper washers |
| 1/2" × 20mm | 1/2"-20 banjo bolts | 1/2" copper washers |
Doing more than one job? The banjo bolt copper washer assortment keeps the common sizes in one tray, and the full sealing washers collection covers everything else — including the odd sizes on older European vehicles.
Installing a Banjo Bolt That Won't Leak
- Clean both faces. Wipe the caliper port face and the banjo faces with brake cleaner. Old crushed copper or grit under a new washer will channel a leak.
- New washers, both sides. One under the bolt head, one between banjo and port — always both, always new.
- Position the hose before final torque. The banjo rotates until the bolt clamps it. Aim the hose so it can't touch the tire, rotor, or suspension through full travel and lock.
- Torque to spec. Follow the vehicle's torque specification — typically much lower than the bolt looks like it could take. Overtightening strips aluminum caliper threads and splits washers; undertightening never crushes the copper.
- Bleed and check. Bleed the caliper, then have a helper hold firm pedal pressure while you watch the joint. A good banjo connection is bone dry.
If the connection still weeps after a proper install, don't keep tightening — disassemble, inspect the faces for scoring, and start over with fresh washers. And since a banjo bolt job usually means you're replacing a hose, our brake hose replacement guide covers the whole job end to end, with direct-fit rubber brake hoses in domestic, European, and Japanese styles.
Frequently Asked Questions
What is a banjo bolt?
A banjo bolt is a hollow bolt with cross-drilled holes that connects a ring-shaped "banjo" hose fitting to a hydraulic port, most commonly where a brake hose meets the caliper. Fluid flows through the bolt's hollow center and out its cross-holes into the hose. It seals with copper crush washers, not threads.
What size banjo bolt do I need?
Measure the old bolt: thread diameter with a caliper, pitch with a thread gauge, and shank length under the head. M10x1.0 is the most common brake caliper size, but M10x1.25, M12x1.0, and 3/8"-24 are all widespread — and the wrong pitch will strip the caliper, so always verify.
Can you reuse banjo bolt crush washers?
No. Crush washers deform once to create the seal and work-harden in the process. Reusing them is the most common cause of a seeping banjo joint. Washers are inexpensive — replace both every time the joint is opened.
Do banjo bolts need a washer on both sides?
Yes — one copper washer under the bolt head and one between the banjo fitting and the port face. Both surfaces are sealing surfaces; skip one and the joint will leak.
Why does my banjo bolt leak after tightening?
The usual causes: reused crush washers, dirt or old copper on a sealing face, a scored port face, the wrong washer size, or a bolt that's bottoming out before it clamps. More torque won't fix any of these — disassemble, clean, and rebuild with new washers.
Are banjo bolt threads supposed to seal with Teflon tape?
No — never put thread sealant or tape on a banjo bolt. The threads only provide clamping force; the copper washers make the seal. Tape fragments can enter the brake fluid and damage ABS components.
Get the Right Bolt and Washers the First Time
A leak-free banjo connection comes down to three things: the right thread size and length, fresh copper washers on both sides, and proper torque. Match your size in the chart above, grab the washer assortment so you're never tempted to reuse one, and find every size in stock at www.4lifetimelines.com — so you can get the job done right the first time.