Product Description
CUMMINS 3900687 OEM REAR CAM EXPANSION PLUG – 5.9L 12V/24V (1989–2002)
The genuine Cummins 3900687 rear cam expansion plug is the OE press-fit plug that seals the rear camshaft bore on 5.9L B-series engines, stopping those annoying oil leaks at the very back of the block. It’s the same rear cam plug used on 1989–2002 Dodge Ram 2500/3500 5.9L 12V and 24V VP44 trucks, plus a wide range of B-series industrial engines, so you know the dimensions, metallurgy, and fit are correct for long-term reliability.
Key Features & Benefits
- Genuine Cummins rear cam expansion plug for 5.9L B-series engines (1989–2002 Ram 2500/3500 12V & 24V).*
- Correct OE press-fit design and dish profile for a positive seal at the rear cam bore.
- Designed to eliminate rear engine oil leaks caused by worn or improperly installed plugs.
- Supersedes older Cummins part number 3900686 for updated service coverage.
- Compatible with O-ring 3926047 where the block has the machined groove behind the plug (sold separately).
- Ideal to replace whenever the engine or transmission is out for clutch, rear main seal, or rebuild work.
Verified Vehicle Fitment – Dodge Ram
This plug is used in the rear camshaft bore on the following Dodge Ram applications*:
- 1989–1993 Ram 2500/3500 – 5.9L 12V 6BT (VE pump)
- 1994–1998 Ram 2500/3500 – 5.9L 12V 6BT (P7100 pump)
- 1998.5–2002 Ram 2500/3500 – 5.9L 24V ISB (VP44 pump)
Industrial / Off-Highway: 3900687 is also widely used across B-series 4B/6B engines in equipment, generators, and industrial builds. Always confirm by engine serial number (ESN) before ordering for non-automotive applications.
Location: Rear camshaft bore, hidden behind the bellhousing / adapter plate at the very back of the engine.
Cross-Reference & Related Cummins Parts (CPP Diesel)
Build a complete, leak-free block with these related genuine Cummins components on cppdiesel.com:
- CUMMINS 3922072 OEM FRONT/SIDE EXPANSION PLUG – 5.9L (1989–2002) – matching OE cup plugs for the front/side of the block.
- CUMMINS 3900955 OEM EXPANSION PLUG (FREEZE PLUG) – DIPSTICK LOCATION – RAM 5.9L (1989–2002) – side-of-block frost plug at the dipstick area.
- CUMMINS 3914035 OEM EXPANSION PLUG (SIDE OF BLOCK) – RAM 5.9L 12V/24V (1989–2002) – additional core plugs along the block side.
- CUMMINS 3920706 OEM EXPANSION PLUG (TURBO DRAIN) – RAM 5.9L/6.7L (1989–2018) – expansion plug at the factory turbo oil drain port.
- CUMMINS 3906698 OEM WATER PUMP RECTANGULAR RING SEAL – 5.9L/6.7L (1989–2018) – OE coolant seal to finish out a front-of-engine cooling refresh.
What’s Included
- (1) Genuine Cummins rear cam expansion plug – part number 3900687
Note: Rear plug O-ring 3926047 and any additional gaskets/seals are sold separately.
Installation Notes
- Remove the transmission or engine as needed to access the adapter plate and rear cam plug.
- Thoroughly clean the rear cam bore in the block; remove corrosion, old sealant, and debris.
- Inspect for a machined groove behind the plug; if present, install O-ring 3926047 before the plug.
- Apply a thin film of OEM-approved sealant to the outside diameter of the plug.
- Use a flat driver and drive the plug in evenly until fully seated without deforming the dish.
- Refill oil/coolant as needed and verify the rear of the block is dry after warm-up.
FAQ
WILL THIS FIT MY 1989–2002 5.9L CUMMINS RAM?Yes. 3900687 is the OE rear cam expansion plug for 5.9L 12V and 24V VP44 Dodge Ram 2500/3500 trucks built from 1989–2002, along with many B-series industrial engines. Always confirm with your VIN or ESN if you’re unsure.
DOES THIS PLUG INCLUDE THE O-RING?
No. The rear plug O-ring (Cummins 3926047) is a separate part and is only used on blocks that have a machined groove behind the plug. If your block does not have the groove, you will not use the O-ring.
SHOULD I USE SEALANT ON THE PLUG?
Yes. A light coat of the correct sealant on the plug’s outer diameter helps prevent seepage and makes installation more forgiving. Follow Cummins-approved sealant recommendations and clean all mating surfaces first.
WHERE IS THIS PLUG LOCATED ON THE ENGINE?
The plug sits in the rear camshaft bore, behind the bellhousing adapter plate at the very back of the engine. Access normally requires removing the transmission or pulling the engine.
IS THIS A GENUINE CUMMINS PART OR AFTERMARKET?
This listing is for a genuine Cummins rear cam expansion plug. Part number 3900687 is the OEM plug and supersedes earlier Cummins number 3900686.
WHEN SHOULD I REPLACE THE REAR CAM EXPANSION PLUG?
Replace it any time the engine is out of the truck, during a full rebuild, after a rear main seal or adapter plate repair, or if you see oil leaking from the back of the block around the bellhousing area.
Specifications for cummins 3900687
| Attribute | Value |
|---|---|
| Brand | Cummins (Genuine OEM) |
| Part Number | 3900687 (supersedes 3900686) |
| Engine Family | Cummins B-Series 5.9L (6BT/ISB) – 12V & 24V; common on 1989–2002 Ram 2500/3500 |
| Component Type | Rear camshaft expansion (core/freeze) plug – rear cam bore |
| Material | Steel expansion plug, OE dish-style |
| Approx. Plug Diameter | ~2 3/8 in (rear cam bore size; verify by ESN if uncertain) |
| Quantity Required | 1 per engine (sold individually) |
| UPC | 722267701260 |
| GTIN | 00722267701260 |
| Shipping Weight (typical) | Approx. 0.1 lb (0.05 kg) |
| Package Dimensions (typical) | Approx. 2.4 in (L) x 1.6 in (W) x 2.7 in (H) |
| Shipping Info | Ships from CPP Diesel; shipping method and cost calculated at checkout |
| Cross Reference | Cummins 3900686 (superseded), plus common aftermarket equivalents such as 3900687CUM, M-3900687, and similar cross-reference numbers (verify per manufacturer catalog). |
| E-Codes / Emissions | Not an emissions-controlled component; no specific E-codes or CARB EO required. |
| Prop 65 Warning | WARNING: Cancer and reproductive harm – see www.P65Warnings.ca.gov. |
| Typical Applications | 1989–2002 Dodge Ram 2500/3500 5.9L Cummins; various B-series 4B/6B industrial and off-highway engines. |
