Balmar Pulley Kit, Westerbeke 18, 21A, 27A, 35B, 38B, 42B
Balmar Pulley Kit, Westerbeke 18, 21A, 27A, 35B, 38B, 42B — Product description
The Balmar 48-WSP-18 is an AltMount serpentine pulley conversion kit for Westerbeke 18, 21A, 27A, 35B, 38B, and 42B diesels. It swaps the stock single-groove drive for a ten-groove J10 system so your alternator finally gets the grip to put out full amps without slipping a belt.
If you've fought a slipping V-belt on a Westerbeke diesel while trying to run a real charging load, this Balmar AltMount kit is the fix. It's a serpentine pulley conversion that swaps the stock single-groove setup for a ten-groove J10 system, so your alternator finally gets the grip it needs to put out full amps without smoking a belt.
The 48-WSP-18 covers a wide range of Westerbeke engines: the 18, 21A, 27A, 35B, 38B, and 42B. It actually replaces five older single-model Balmar kits (the 48-WSP-21A, 27A, 35B, 38B, and 42B), so one part number now does the work of the whole family. In the box you get three ten-groove (J10) pulleys, two J10 40-inch serpentine belts, and the mounting hardware to bolt it all up. One of those belts is a spare, which is exactly what you want tucked in a locker on a cruising boat.
Why serpentine? A ten-rib belt wrapping a wider pulley face carries a lot more load than a narrow V-belt riding in a single groove. That means less slip, less belt dust, and cooler running when you've got a high-output alternator pulling hard at anchor. Balmar's own testing on the AltMount line shows a clear improvement in power transfer along with lower noise and less engine vibration. On a sailboat where you're charging a big house bank off the main engine, that's the difference between keeping up and falling behind.
This kit is the natural partner for a Balmar high-output alternator upgrade. If you're stepping up from a stock 55-amp unit to a 6-Series charging 100 amps or more, the stock belt will slip under that load. The serpentine setup handles it. Pair the alternator with the right Balmar external regulator so the whole system charges hard without cooking itself, and you've got a charging package that actually earns its keep on a long passage.
Who's this for? Mostly cruising sailors and trawler owners running one of these Westerbeke blocks as auxiliary power. If you've added a bigger battery bank, moved to lithium, or you're just tired of topping off with the engine for hours, upgrading the alternator drive is one of the highest-value moves you can make in the engine room. The pulley kit is the foundation the rest of that upgrade sits on. Skip it, and even the best alternator can't deliver what it's rated for.
A couple of install notes from the field. Clean the crankshaft pulley groove before you set the new pulleys so the belt seats right and wears evenly. Torque the pulley set screws down and hit them with thread locker, because a set screw that backs out at sea is a bad afternoon. Check your belt alignment across all three pulleys with a straightedge before you tension it. And one spec-sheet detail worth flagging: if you're specifying a Balmar AT-200A alternator, you may need a longer belt than the two 40-inch belts in this kit, so plan for that when you order.
Keep that spare belt aboard. Serpentine belts are reliable, but they're a wear item, and a marine store two towns over may not stock a J10 40-inch belt when you need it. Having the exact match already in your spares kit means a ten-minute swap instead of a ruined weekend.
This isn't a dress-up part. It's a working upgrade for boaters who ask their engine to charge like it means it. If your Westerbeke is on that list and you're tired of chasing belt slip, this is the kit that ends it. Give us a call if you're not sure it matches your engine, we'll help you confirm before you buy.
Key Features
- AltMount serpentine conversion replaces the stock single-groove drive with a ten-groove J10 system for far better power transfer and less belt slip
- Fits Westerbeke 18, 21A, 27A, 35B, 38B, and 42B diesel engines from one part number
- Includes three ten-groove (J10) pulleys machined for the serpentine belt path
- Ships with two J10 40-inch serpentine belts, one of them a spare to keep aboard
- Reduces engine noise and vibration compared to a slipping V-belt drive
- Handles the load of a Balmar high-output alternator upgrade without slipping
- Mounting hardware included for a complete bolt-on conversion
- Replaces five older single-model Balmar kits: 48-WSP-21A, 27A, 35B, 38B, and 42B
Why Buy from NVN Marine
- Authorized Balmar reseller, full manufacturer warranty
- NMEA member and ABYC certified, advice from real boat techs
- Same-day shipping before 3 PM ET on in-stock items
- NY headquarters and Fort Lauderdale flagship retail store
Technical specifications
| Title | Balmar Pulley Kit, Westerbeke 18, 21A, 27A, 35B, 38B, 42B |
|---|---|
| Brand | Balmar |
| Manufacturer Part Number | 48-WSP-18 |
| UPC | 676205510080 |
| System Type | AltMount Serpentine Pulley Conversion Kit |
| Engine Compatibility | Westerbeke 18, 21A, 27A, 35B, 38B, 42B |
| Pulleys Included | Three Ten-Groove (J10) Pulleys |
| Belts Included | Two J10 Serpentine Belts (one spare) |
| Belt Groove Profile | J10 (Ten-Groove) |
| Belt Length | 40 inch Circumference |
| Mounting Hardware | Included |
| Replaces | 48-WSP-21A, 48-WSP-27A, 48-WSP-35B, 48-WSP-38B, 48-WSP-42B |
| Application | Marine Diesel Engine Alternator Drive |
| Category | Electrical / Alternators |
Frequently asked questions
Which Westerbeke engines does this kit fit?
The 48-WSP-18 covers the Westerbeke 18, 21A, 27A, 35B, 38B, and 42B diesel engines. If you're not sure which block you have, give us a call with your engine model and serial and we'll confirm the fit before you order.
What comes in the box?
Three ten-groove (J10) pulleys, two J10 40-inch circumference serpentine belts (one is a spare), and the mounting hardware needed to complete the conversion.
Why convert to a serpentine belt at all?
A ten-rib serpentine belt on a wider pulley face grips far more than a narrow V-belt in a single groove. That means less slip, less belt dust, and cooler running when a high-output alternator is pulling hard. Balmar's AltMount line is built to improve power transfer while cutting noise and vibration.
Do I really get a spare belt?
Yes. Two 40-inch J10 belts ship in the kit and one is meant to live in your spares locker. A serpentine belt is a wear item, and the exact J10 40-inch size isn't something every marine store stocks, so having the match aboard turns a breakdown into a quick swap.
Can I run a high-output alternator with this kit?
That's exactly what it's for. When you step up from a stock 55-amp unit to a Balmar 6-Series charging 100 amps or more, the stock V-belt slips under load. The serpentine drive handles the extra grip. Pair it with the correct Balmar external regulator so the system charges hard without overheating.
What belt do I need if I install a Balmar AT-200A alternator?
Heads up on that one. If you specify an AT-200A alternator, you may need a longer belt than the two 40-inch belts included here. Plan for the different belt length when you order so you're not caught short during the install.
Does the kit include mounting hardware?
Yes, the mounting hardware for the conversion is in the box. A few field tips: clean the crankshaft pulley groove before setting the pulleys, use thread locker on the set screws, and check belt alignment across all three pulleys with a straightedge before tensioning.
What older part numbers does this replace?
The 48-WSP-18 supersedes five older single-model Balmar kits: the 48-WSP-21A, 48-WSP-27A, 48-WSP-35B, 48-WSP-38B, and 48-WSP-42B. One kit now covers the whole Westerbeke range those parts used to serve.
How is warranty and returns handled?
This item carries the full Balmar manufacturer warranty. Because of how it's stocked, warranty questions and any concerns are handled manufacturer-direct rather than returned to the point of purchase, and we're happy to help you get in touch if anything comes up.