00U800 / U800 Plunger Assembly – Compression Dead‑Volume Elimination For Instantaneous Rail Pressure Build‑up

Primary fitments: WEIFU pump heads designed for Euro VI‑E and China VI heavy‑duty engines (500–750 hp), as well as high‑speed marine gensets. Directly replaces OEM U800 in Bosch CP4.5, CP5.2, and Denso HP8 pump housings. Also retrofits into Scania DC16 and MAN D38 pump bases with no modification to the pushrod or tappet.

Hydraulic rigidity (defined as dP/dV, the pressure change per unit volume change) is inversely proportional to the dead volume. By cutting the clearance volume by 22 %, the 00U800 elevates the effective rigidity by nearly 30 %. This translates into a steep pressure‑rise slope – measured at 4.2 bar/μs during the first 50 μs of delivery, compared to 3.3 bar/μs for standard plungers. The steeper slope ensures that the injector needle reaches its opening pressure quicker, reducing the opening delay by 0.08 ms at typical rail setpoints. In real‑world terms, this improved "pressure immediacy" allows the ECU to use smaller pilot injection durations, cutting NOx by 2–3 % without sacrificing torque response, as verified on a 16‑L V8 engine under the WHTC cycle.

While many plungers rely on through‑hardening or carburising, the 00U800 employs a plasma nitriding process at 520 °C in a nitrogen‑hydrogen atmosphere. This creates a 15–18 µm compound layer (ε‑phase) with a hardness of 1,200 HV, backed by a diffusion zone that gradually decreases hardness into the core. The benefits:

Lower friction coefficient – the nitrided surface exhibits 0.07–0.08 in lubricated sliding, reducing frictional losses by 12 % over chrome‑plated variants.

Superior scuffing resistance – the hard layer withstands momentary boundary lubrication during cold starts, where fuel viscosity is high but flow is limited.

Dimensional stability – the low processing temperature avoids distortion, maintaining the barrel clearance within ±0.5 µm after treatment.

The convex crown is ground after nitriding to preserve the exact spherical contour, ensuring that the dead‑volume reduction is not compromised by coating build‑up.

Unlike conventional plungers where only the radial clearance is measured, the 00U800 demands verification of the axial clearance between the plunger crown and the barrel top face at TDC. Using the supplied feeler shim set (0.05 mm to 0.15 mm), proceed as follows:

Install the plunger without the delivery valve holder.

Rotate the pump camshaft to bring the plunger to its highest position.

Insert the feeler gauge through the delivery valve bore; the correct gap should be 0.08–0.12 mm.

If the gap is outside this range, adjust the shim under the barrel flange accordingly.

This step ensures that the convex crown does not contact the barrel top (which would cause seizure) while maintaining the minimum possible dead volume. The kit includes a simple go/no‑go gauge to expedite this check in field conditions.

Q1: Does the convex crown alter the plunger's stroke or effective pumping volume?
No – the stroke is unchanged. The crown only modifies the shape at the very top, reducing clearance volume without affecting the swept displacement. The delivery quantity per stroke remains identical to the OEM specification.

Q2: Can the 00U800 be used in pumps with mechanical governors (non‑ECU)?
Yes – but the faster pressure rise may cause the governor to overshoot slightly at idle. We recommend a small readjustment of the idle speed screw (increase by 10 rpm) to compensate; the engine will feel more responsive.

Q3: How does the plasma‑nitrided surface perform with high‑sulphur fuel (>500 ppm)?
The nitrided layer is highly corrosion‑resistant to sulphuric acids formed during combustion. However, we still advise using a fuel additive to neutralise acidity, as the base material beneath the layer may be vulnerable if the coating is compromised.

Q4: What is the maximum allowable top clearance before the plunger needs replacement?
If the axial clearance exceeds 0.18 mm (measured with feeler gauge), the dead volume has increased unacceptably, and the pressure‑rise benefit is lost. This typically occurs after 8,000–10,000 hours, depending on fuel quality.

Q5: Is there any special run‑in procedure for the nitrided plunger?
No – the nitrided surface is pre‑conditioned and does not require a bedding‑in period. However, avoid extended idle running (>30 minutes) during the first 10 hours to ensure proper lubrication of the new clearances.

Q6: How do I order the correct selective grade (diameter tolerance) for my pump?
The grade (A, B, or C) is stamped on the barrel flange. Match the grade number with your original plunger's marking. If uncertain, measure your pump's barrel bore with the provided calibration gauge – we also offer a free bore‑sizing service.