Cummins 3903857 — Genuine Vibration Damper / Flywheel Mounting Bolt for 5.9L (6BT/ISB)

MPN: 3903857  |  Also Known As: hex flange head cap screw, harmonic balancer bolt, damper bolt, flywheel bolt

Genuine Cummins hex-flange bolt engineered for critical crankshaft interfaces. Commonly used to secure the harmonic balancer (vibration damper) and, in certain applications, the flywheel/flexplate on 5.9L B-Series engines. Direct-fit for 1989–2002 5.9L (12V & 24V VP44). Sold individually.

Features & Benefits

• OE-grade fastener with correct flange head and shank engagement for high clamp load.
• Designed for B-Series crankshaft damper/flywheel interfaces to maintain alignment and prevent loosening.
• Heat-treated for strength and fatigue resistance under high torsional loads.
• Exact fit—no guesswork on length or thread pitch versus generic hardware.
• Backed by Cummins for proven reliability.

Verified Fitment (Automotive)

Industrial/Off-Highway: Used primarily on BS3 5.9L B-Series; may appear on select 4B/6B applications. Verify by ESN.

Specifications

• Type: Hex flange head cap screw (damper/flywheel service)
• OE Part Number: 3903857
• Thread Size: M12 x 1.25
• Under-head Length (approx.): ~36 mm (1.425 in)
• Quantity Required: Sold individually (order set as needed)

What’s In The Box

• (1) Genuine Cummins Hex Flange Head Bolt (3903857)

Installation Tips

• Clean threads and mating surfaces; use new bolts during damper or flywheel service.
• Apply thread locker only if specified by the service manual.
• Torque in a star pattern to factory spec; recheck after initial heat cycle if procedure calls for it.

FAQ

WILL THIS BOLT FIT MY 1989–2002 5.9L CUMMINS RAM?

Yes—3903857 is an OE hex-flange bolt used on BS3 5.9L B-Series applications, including 1989–2002 12V & 24V trucks. Always confirm by VIN/ESN.

HOW MANY BOLTS DO I NEED FOR THE HARMONIC BALANCER?

Typically 4 bolts secure the vibration damper on 5.9L B-Series engines. Order the correct quantity for your service.

IS THIS ALSO A FLYWHEEL/FLEXPLATE BOLT?

3903857 is referenced for damper and some flywheel/flexplate uses on B-Series. Transmission and year may change hardware—verify by VIN/ESN or service manual.

WHAT IS THE THREAD SIZE?

M12 x 1.25. Length is approximately 36 mm (1.425 in) under the head.

IS THIS A GENUINE CUMMINS PART?

Yes—OE Cummins 3903857.