Product Description
CUMMINS 3900808 OEM 9MM FUEL INJECTOR SEALING WASHER – 5.9L CUMMINS (1989–1991)
Genuine Cummins 3900808 9mm fuel injector sealing washer for early 5.9L 12-valve B-series engines. This OEM copper washer restores proper injector-to-cylinder-head sealing, prevents combustion leaks, and helps maintain a clean, consistent spray pattern in your 1989–1991 Dodge Ram 5.9L Cummins or other B-series applications using 9mm injector tips.
Key Features
- Genuine Cummins copper injector sealing washer (MPN 3900808).
- Correct 9mm injector tip size for early 1st-gen 5.9L Cummins trucks.
- Prevents combustion blow-by at the injector seat for stable performance.
- Helps protect injector nozzles and seats from erosion and leakage.
- Sold individually – order 6 washers per engine (one per injector).
Fitment Details
Primary automotive fitment (CPP focus):
- 1989–1991 Dodge Ram 250/350 with 5.9L 12-valve Cummins, VE rotary pump, equipped with 9mm injector tips.
Additional B-series & industrial usage (verify by ESN/tip size):
- Cummins 4B3.9 / 6B5.9 B-series engines and non-certified 5.9L B engines where Cummins lists 3900808 as the injector seal. Always confirm by engine serial number (ESN) or OEM documentation before ordering for non-automotive equipment.
Because Cummins used both 9mm and 7mm injector tips on early 5.9L applications, this washer fits only engines with 9mm injectors. If you are unsure which size you have, measure the injector tip or confirm by ESN before purchase.
Cross Reference (Part Numbers)
- Cummins: 3900808 (injector seal / washer)
- Bosch: 2430105026 (copper injector nose washer)
- Other aftermarket equivalents: 675477, 675477C1, FP-3900808, M-3900808 (listed by various diesel suppliers; use 3900808 as the OE reference).
Related Cummins Genuine Parts on CPP Diesel
Pair this 9mm injector washer with other OEM Cummins fuel system hardware for a complete, leak-free install:
- CUMMINS 3918188 OEM 8MM FUEL SYSTEM SEALING WASHER – RAM 5.9L (1989–1998)
- CUMMINS 4942278 OEM FUEL INJECTOR RETURN LINE BANJO SEAL (WASHER) – RAM 5.9L 12-VALVE (1994–1998)
- CUMMINS 3905307 OEM FUEL INJECTOR RETURN LINE BANJO BOLT – 5.9L 12-VALVE (1989–1998)
What’s Included
- (1) Genuine Cummins 9mm fuel injector sealing washer – part number 3900808.
Note: Order quantity 6 if you are servicing all injectors on a 6-cylinder engine.
Installation Notes
- Clean the injector bore and seat thoroughly before installation.
- Install a new washer every time an injector is removed – do not reuse old copper washers.
- Verify injector tip size is 9mm and that 3900808 supersedes any previous part number for your ESN.
- Torque injector hold-downs to the factory specification and recheck after an initial heat cycle.
Specifications for cummins 3900808
| Brand | Cummins Genuine Part |
|---|---|
| Cummins Part Number | 3900808 |
| Description | 9mm copper fuel injector sealing washer |
| Material | Copper |
| Injector Tip Size | 9mm (early 1st-gen 5.9L applications) |
| Primary Engine Application | 1989–1991 Dodge Ram 5.9L 12-valve Cummins (VE pump) with 9mm injectors; also used on various Cummins 4B3.9 / 6B5.9 B-series engines (verify by ESN). |
| Quantity Required per Engine | 6 (one per injector) – sold individually |
| Cross Reference | Cummins 3900808; Bosch 2430105026; aftermarket equivalents such as 675477, 675477C1, FP-3900808, M-3900808. |
| UPC | Not specified by manufacturer |
| GTIN | Not specified by manufacturer |
| Package Dimensions (L×W×H) | Approx. 0.53 in × 0.53 in × 0.03 in (single washer, based on CPP Diesel listing) |
| Shipping Weight | Very low – ships as small-parts parcel; weight calculated at checkout |
| Shipping Info | Ships from CPP Diesel (Pennsylvania, USA). Shipping calculated at checkout; typical carrier options include UPS/USPS for small parts. |
| E-Codes / Emissions Notes | Not emissions-calibrated; hardware component for injector sealing only. |
| Prop 65 | WARNING: This product can expose you to chemicals including lead and other substances which are known to the State of California to cause cancer and birth defects or other reproductive harm. For more information, visit www.P65Warnings.ca.gov. |
FAQ
Yes – Cummins 3900808 is the OEM 9mm injector sealing washer used on early 1st-gen Dodge Ram 5.9L 12-valve engines with 9mm injector tips. Always confirm injector tip size or engine serial number if you are unsure.
How many washers do I need for a full set?
A 6-cylinder 5.9L Cummins engine uses 6 washers – one under each injector. Order six 3900808 washers if you are servicing all injectors.
Is this washer for 9mm or 7mm injectors?
This part is specifically for 9mm injector tips. Later 1st-gen trucks and many other applications use 7mm injectors and require a completely different sealing washer. Do not mix 9mm and 7mm hardware.
Is this a genuine Cummins part or aftermarket?
This washer is sold as a genuine Cummins part through CPP Diesel, not a generic aftermarket copy. Using OEM hardware ensures the correct dimensions, material, and long-term sealing performance.
Can I reuse my old injector sealing washers?
Reusing old copper washers is not recommended. Once crushed, they may not reseal properly, which can lead to combustion leaks, injector seat damage, and poor performance. Always install new 3900808 washers when injectors are removed.
Will this work on industrial or power-gen B-series engines?
Cummins lists 3900808 as an injector seal for various non-certified 5.9L B-series and some industrial 4B/6B applications. However, industrial equipment can have different specifications, so always verify fitment using your engine serial number (ESN) and OEM documentation before ordering.
