Royal Enfield 550025 – Nut-Main Shaft (M12 X 1.25 Nylock) OEM for 500 & 350 UCE / Classic & Bullet Models

Ensure secure fastening of your crank/drive main shaft with the genuine Royal Enfield 550025 Nut-Main Shaft. This OEM nut is engineered to the exact factory thread spec (M12 x 1.25 Nylock “P” Type) to retain the main shaft securely under dynamic engine loads. Used across many Royal Enfield models — including Classic 500, Bullet 500, Classic 350, Bullet 350, Continental GT 535, and other UCE variants — it’s the same nut fitted at the factory for crankcase/main shaft assembly.

If your existing nut is worn, the Nylock collar is damaged, or the bolt loosens under vibration, replacing it with 550025 restores the correct clamping force, preventing crank play, oil leaks, or shaft looseness. Because the nut is vital to engine safety, always double-check the old nut’s size, thread pitch, and feature (lock-type). This genuine part ensures factory fit, durability, and compatibility with stock shafts and crankcases.

Key Features:

• Genuine Royal Enfield OEM 550025 Nut-Main Shaft per factory spec. 

• M12 x 1.25 thread, Nylock-type lock-nut “P” type (self-locking) for secure retention. 

• Fits many UCE models: Classic 500, Bullet 500, Classic 350, Bullet 350, Continental GT, etc. 

• Hardened steel finish to resist vibration loosening

• Low profile — designed for precise fit with stock main shaft washer & components

• Critical for maintaining crankcase integrity & smooth power transfer

Compatibility Table:

Variants:

• 550025 (standard) — M12 × 1.25 Nylock “P” Type

• Sometimes called 550025/A in parts list when suffix is given. 

• Supplied as single unit

• Check Part Number Before Purchase

FAQs:

1. How do I ensure 550025 is the correct nut for my main shaft?
   → Remove the old nut and check if it’s stamped “550025”, note thread pitch (M12 × 1.25), and if it’s a nylon locking style (“Nylock”). Match these before ordering.

2. What happens if I use a generic nut instead of OEM 550025?
   → Risks include loosening under vibration, oil leaks, main shaft play, or damage to crankcase threads. OEM ensures correct locking and material quality.

3. Is 550025 used on 350 cc models too?
   → Yes, many listings (The Spares Company etc.) show fitment across Classic/Bullet 350 and 500 UCE models. But verify your existing nut for thread and style. 

4. Do I need any extra washers or parts while replacing?
   → Usually the existing washer(s) under the nut, main shaft and crankcase are reused if undamaged. But inspect for wear; washers that are distorted should be replaced.

5. What torque spec is needed for this nut?
   → Use the service manual for your model to get correct torque — over-tightening may damage threads, under-tightening may cause loosening.