O.S. MAX-B21 Competition 3.5cc (.21) Off-Road Nitro Engine
About this part
Reliable Nitro Engine Performance
The O.S. MAX-B21 Competition 3.5cc (.21) Off-Road Nitro Engine is designed to deliver reliable performance for both entry-level and sports-level racers. Built with technology derived from the renowned O.S. SPEED series, this engine offers a strong balance of smooth power and responsive acceleration. With its focus on ease of use and cost performance, the MAX-B21 is an excellent option for drivers looking to step up their nitro game.
About the O.S. MAX-B21 Engine
Balanced Power & Smooth Performance
The MAX-B21 is engineered to provide a well-balanced blend of power and drivability, making it ideal for a wide range of racing environments. Drawing from the O.S. SPEED lineup, it delivers smooth throttle response and consistent output across the RPM range. This allows drivers to maintain better control on technical tracks while still having the power needed for high-speed sections. The result is a versatile engine that performs equally well on and off-road.
Lightweight Cooling Head & Improved Efficiency
Featuring a redesigned outer head based on the MAX-21XR-B VER.3, this engine benefits from reduced weight for improved overall performance. The revised combustion chamber in the inner head enhances combustion efficiency, resulting in better throttle response, especially at lower speeds. This improved design helps maximize power delivery while maintaining efficient fuel use. The lightweight construction also contributes to better handling and balance in your vehicle.
Stable Idling & Easy Tuning
One of the standout features of the MAX-B21 is its stable idle characteristics, which help reduce the risk of stalling during operation. This makes carburetor tuning more straightforward, especially for less experienced users. The improved idle stability allows for smoother transitions from low to high throttle. Overall, it provides a more user-friendly experience without sacrificing competitive performance.
Features:
- 3.5cc (.21) competition nitro engine
- Designed for 1/8 off-road and 1/8 GT
- Based on O.S. SPEED engine technology
- Smooth and linear power delivery
- Revised combustion chamber design
- Improved low-speed throttle response
- Lightweight cooling head design
- Stable idle characteristics
- Easy carburetor tuning
- Strong cost-to-performance ratio
Includes:
- (1) MAX-B21 Competition Nitro Engine
- (1) P3 Glow Plug
- (1) Reducer 6mm (Orange)
- (1) Reducer 7mm (Yellow)
- (1) Exhaust Seal Ring
- (1) Dust Cap Set (3mm, 16mm, 18mm)
Specifications:
| Displacement | .21 cu.in (3.5cc) |
|---|---|
| Recommended Application | 1/8 Off-Road |
| Bore | 0.641in (16.27mm) |
| Stroke | 0.661in (16.80mm) |
| Intake Ports | - |
| Exhaust Ports | - |
| Carburetor Type | - |
| Glow Plug | P3 |
| Recommended Fuel | - |
| Break-In Fuel | - |
| Weight | 12.73oz (361g) |
CA Residents Prop 65 Warning
WARNING: This product can expose you to chemicals including Di(2-ethylhexyl)phthalate (DEHP), which is known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov Details...Reviews5.0 · 2 reviews
I received this engine in June 2026, I've completed my bench heat cycles and break-in tanks 5-8 on the street. This engine is very close in specs as the type R and the Blok engines. But I feel this engine has more power with the 22E carburetor.. In summery I can say "you can buy this engine with confidence, no questions asked".
I did not buy this engine from Amain as they didn't carry it when it first came out but I wanted to leave a review for others. This engine might be lower end budget price but it screams! The motor is B2104 based. It broke in easy, has been one of the most consistant running motors tuning wise i own. I run a lot of 21j carbs and this is my only engine with a 22e and it's been great. Idles like a dream. Even thought this motor is fairly basic it has tons of power and runs cool (has a fairly big cooling head). This motor was one of the best buys I've ever made. No regrets whatsoever and still running strong in my NB48 2.1 with almost 4 gallons.