Product Description
Cummins 3900687 — Genuine Rear Camshaft Expansion Plug for 5.9L (6BT/ISB)
MPN: 3900687 | Also Known As: rear cam plug, camshaft expansion plug, cam plug | Replaces: 3900686 | Common Companion Seal: 3926047 O-ring (if block has groove)
Genuine Cummins rear camshaft expansion plug that seals the rear cam bore on 5.9L B-Series engines to stop oil leaks. It’s the OE press-fit plug used on 1989–2002 5.9L trucks and many B-Series industrial engines. Located behind the bellhousing adapter plate. Sold individually (1 per engine).
Features & Benefits
- OE press-fit design provides a positive seal at the rear cam bore.
- Correct concave profile & metallurgy for long service life.
- Stops rear engine oil leaks common on higher-mileage 5.9L engines.
- Direct replacement for factory plug; no dimensional guessing.
- Pairs with 3926047 O-ring when your block has the machined groove.
Verified Fitment (Automotive)
| Year Range | Engine | Chassis |
|---|---|---|
| 1989–1993 | 5.9L 12V (6BT) | Ram 2500/3500 |
| 1994–1998 | 5.9L 12V (6BT) | Ram 2500/3500 |
| 1998.5–2002 | 5.9L 24V ISB (VP44) | Ram 2500/3500 |
Industrial/Off-Highway: Used widely across B-Series (4B/6B) applications; verify by ESN for equipment builds.
Compatibility Notes:
- Designed for pre-common-rail 5.9L (1989–2002). Common-rail engines (2003+) use different parts/locations.
- Some blocks use an O-ring behind the plug (P/N 3926047). Check for a machined groove in your block.
- Plug sits behind the bellhousing adapter; access usually requires trans or engine removal.
Specifications
- Type: Camshaft expansion (freeze) plug, rear cam bore
- OE Part Number: 3900687 (replaces 3900686)
- Common Companion Seal: 3926047 O-ring (if groove present)
- Approx. Diameter: 2-3/8 in (rear cam plug size)
- Quantity Required: 1 per engine (sold individually)
Often Paired With
- 3926047 O-ring (blocks with groove)
- Rear main seal & adapter plate gasket/seal (service-adjacent)
- Engine oil & filter after repair if contamination was present
What’s In The Box
- (1) Genuine Cummins Rear Cam Expansion Plug (3900687)
- O-ring and other seals sold separately.
Installation Tips
- Clean the bore thoroughly; remove old sealant and debris.
- Use an appropriate OEM-approved sealant on the plug perimeter.
- Orient and drive the plug evenly until properly seated; avoid deforming the face.
- Inspect for the O-ring groove and install 3926047 if equipped.
- Verify no oil seepage after warm-up.
FAQ
WILL THIS FIT MY 1989–2002 5.9L CUMMINS RAM?
Yes—this is the OE rear cam plug for 5.9L 12V and 24V VP44 engines (1989–2002). Confirm by VIN/ESN.
DOES THIS PLUG INCLUDE THE O-RING?
No—the O-ring (3926047) is sold separately and is only used on blocks that have a machined groove.
DO I NEED SEALANT ON THE PLUG?
Yes—use an appropriate sealant on the plug OD to prevent seepage and aid installation.
WHERE IS THIS PLUG LOCATED?
In the rear camshaft bore behind the bellhousing adapter plate at the back of the engine.
IS THIS AN OEM CUMMINS PART?
Yes—3900687 is a genuine Cummins plug; it supersedes 3900686.
