Published by: Murray Yacht Sales
Date: July 22, 2026
Universal Marine has long been an iconic name in auxiliary sailboat propulsion, a pioneer of diesel power solutions, and the creator of the legendary Atomic-4 engine. The Universal B-Series marine diesel lineup continues that legacy with advanced engine design, cleaner emissions, lower noise levels, and high low-end torque.
Whether you are seeking a reliable replacement for an aging Atomic-4 or evaluating the propulsion system on a prospective boat, this guide covers model decoding, technical specifications, dimensional profiles, and routine maintenance references across the Universal B-Series engine family.
Decoding the Universal “B” Series Models
Universal’s naming conventions provide clear insight into cylinder count, relative horsepower, and engine generation:
- M: Standard designation for Universal Marine Diesel Engines.
- 3 (where present, e.g., M3-20B): Denotes a 3-cylinder configuration.
- 20 / 25 / 35 / 40: Represents the approximate horsepower class rating of the engine (ranging from 20 HP up to 37.5 HP).
- XP (where present, e.g., M-25XPB): Designates an enhanced performance profile within that horsepower bracket.
- B: Denotes the B-Series generation—featuring advancements in emissions control, quiet operation, and refined physical profiles.
Current “B” Series Lineup Overview
- M3-20B: 20 HP @ 3600 RPM | 3-Cylinder
- M-25XPB: 26 HP @ 3000 RPM | 3-Cylinder
- M-35B: 35 HP @ 3000 RPM | 4-Cylinder
- M-40B: 37.5 HP @ 3000 RPM | 4-Cylinder
Key Features & Design Highlights
All engines in the Universal B-Series share core engineering standard features designed for durability, ease of operation, and marine environment performance:
- Fresh Water Cooling System: Fresh water cooling integrated with an onboard heat exchanger.
- E-Z Bleed Fuel System: Simplifies fuel system servicing and bleeding procedures.
- Low Noise & Vibration: Features a tuned intake silencer and engine-matched four-point, fail-safe neoprene adjustable mounts.
- Electrical & Charging System: Pre-wired engine harness with plug-in connectors, protected by circuit breakers, and equipped with a 12-volt, 51-amp battery charging alternator.
- Self-Contained Servicing Aids: Includes a top-located oil fill point and a dedicated lube oil drain hose for clean fluid changes.
- Cold Starting: 12-volt sheathed glow plugs assist quick cold starts.
Atomic-4 Replacement Note:
The M3-20B, M-25XPB, M-35B, and M-40B share physical and dimensional traits designed to assist boaters replacing aging Atomic-4 gasoline engines. However, these are not direct drop-in replacements in all cases, and minor engine compartment reconfiguration may be required.
UnivB-Series Specifications & Technical Comparison
| Specification | M3-20B PDF | M-25XPB PDF | M-35B PDF | M-40B PDF |
| Cylinders | 3 Cylinder, in-line | 3 Cylinder, in-line | 4 Cylinder, in-line | 4 Cylinder, in-line |
| Displacement | 43.8 cu. in. (0.7177 L) | 61.2 cu. in. (1.0 L) | 81.47 cu. in. (1.335 L) | 91.41 cu. in. (1.498 L) |
| Horsepower Output | 20 HP @ 3600 RPM | 26 HP @ 3000 RPM | 35 HP @ 3000 RPM | 37.5 HP @ 3000 RPM |
| Max Torque | 33 Lbs-Ft @ 2500 RPM | 50 Lbs-Ft @ crankshaft | 62 Lbs-Ft @ 2480 RPM | 73 Lbs-Ft @ 2180 RPM |
| Bore x Stroke | 2.64″ x 2.68″ | 2.99″ x 2.90″ | 2.99″ x 2.90″ | 3.07″ x 3.09″ |
| Compression Ratio | 23:1 | 23:1 | 23:1 | 22:1 |
| Dry Weight | 241 lbs (110.0 kg) | 295 lbs (134.0 kg) | 352 lbs (159.0 kg) | 356 lbs (161.5 kg) |
| Length x Width x Height | 26.4″ x 18.8″ x 20.1″ | 28.4″ x 18.9″ x 22.1″ | 32.7″ x 18.9″ x 22.1″ | 33.2″ x 18.9″ x 22.1″ |
| Standard Gear Ratio | 2.05:1 | 2.05:1 | 1.79:1 | 1.88:1 |
Maintenance & Service Reference Guide
Keeping up with scheduled fluid and filter maintenance is key to long-term reliability. Below is a maintenance cross-reference and capacity guide for Universal B-Series models:
Capacity & Service Cross-Reference Chart
| Engine Model | Oil Capacity | Coolant Capacity | Fuel Filter | Lube Oil Filter | Electrical / Alternator |
| M3-20B | 4.0 qts (3.8 L) | 3.75 qts (3.5 L) | Full flow, spin-on | Full flow, spin-on | 12V DC / 51 Amp (Opt: 72A) |
| M-25XPB | 4.0 qts (3.8 L) | 4.0 qts (3.8 L) | Full flow, spin-on | Full flow, spin-on | 12V DC / 51 Amp (Opt: 72A) |
| M-35B | 5.44 qts (5.1 L) | 6.0 qts (5.6 L) | Full flow, spin-on | Full flow, spin-on | 12V DC / 51 Amp (Opt: 72A) |
| M-40B | 5.4 qts (5.1 L) | 4.5 qts (4.3 L) | Full flow, spin-on | Full flow, spin-on | 12V DC / 51 Amp (Opt: 72A) |
Common Maintenance & Accessories Parts Reference
- Raw Water Pump: Gear-driven sea water pump standard across all B-Series engines.
- Spare Parts Kits: Factory-assembled “A” and “B” On-Board Spare Parts Kits are offered per engine model.
- Gasket Kits: Model-tailored gasket sets are available for top-end and complete engine service.
- Instrument Panels:
- Captain Panel: Includes tachometer and warning lights.
- Admiral Panel: Dimensions 9.75″ L x 4.0″ D x 6.0″ H; includes tachometer, key switch, start/glow buttons, and full mechanical gauges.
Conclusion
The Universal B-Series combines lightweight, compact engineering with high low-end torque and low noise levels. Whether you are operating an M3-20B in a smaller cruiser or an M-40B in a larger vessel, proper maintenance with correct service parts will ensure long-term performance on the water.