Front And Rear Housings, Valve Seats And Oil Boards: Hitting 5 mg With High-Pressure Cleaning

Overview

A transmission front housing, a valve seat body and an oil board have almost nothing in common geometrically, yet they often share a cleanliness target of five milligrams. That shared number hides three different cleaning problems, and treating them as one is why lines that work for housings fail for oil boards.

Big Bird Industrial (formerly Shimada Big Bird Industrial) has built high-pressure cleaning systems for powertrain components since 2001. This article compares the three part families, explains where the milligram budget actually goes, and describes how a staged high-pressure process is configured for each.

The common thread is that five milligrams is not a washing intensity. It is a budget that has to be allocated across surfaces, and most of it is spent on the parts of the part that are hardest to reach.

1. Where The Milligrams Come From

Residue on a cleaned part is not spread evenly. It concentrates in four places: blind holes and their bottoms, threaded holes, the interface between two machined surfaces, and any pocket where fluid can pool during drainage.

On a housing, the dominant contributors are the large internal cavities and the machined joint faces. On a valve seat body, they are the small bores and the seat faces. On an oil board, they are the drilled galleries and the cross-drill intersections.

That mapping matters because it tells you where to spend pressure, flow and time. A process that treats the whole part uniformly will over-clean the easy surfaces and under-clean the ones that carry the budget.

2. Housings: Volume And Joint Faces

A housing is mostly empty space with a large surface area. The cleaning task is to wet every surface and then remove the loosened contamination before it re-deposits. Turbulent flow does the wetting; the joint faces and the seal grooves need targeted jets because they are recessed and shielded.

Re-deposition is the hidden risk. Contamination removed from one wall can settle on another before the rinse, particularly in a large volume with slow flow. The countermeasure is to keep the flow moving until the rinse begins, and to design the drainage so that the part does not sit in its own dirty fluid.

Joint faces deserve separate attention because two flat surfaces clamped together create a capillary gap. Fluid enters during washing, stays during rinsing and emerges as residue after assembly. A process that does not separate the faces during cleaning will always carry some of that gap contamination to the end of the line.

3. Valve Seats: Small Bores And Faces

A valve seat body is the opposite case: small volumes, small features, high precision. The cleaning problem is entry. A jet cannot turn a corner, so a bore that opens sideways needs either a nozzle positioned to look into it or a plug-sealing approach that forces flow through it.

The seat face itself is a sealing surface, and it is where damage is least acceptable. High pressure applied directly to a lapped face can erode the edge, so the process has to clean the face without impinging on it at full energy. The usual compromise is a lower-pressure flood across the face combined with a targeted jet on the surrounding geometry.

Debris in the bore below the seat is the other contributor. It arrives from machining and from the deburring operation, and it is only removed if the flow passes through the bore rather than across its opening.

4. Oil Boards: Galleries And Intersections

An oil board is a manifold: a plate with a network of drilled passages that must be clean because they carry oil to a bearing. The passages are long relative to their diameter, which is exactly the geometry that defeats a spray.

Two techniques work. Flushing forces fluid through the gallery end to end, which requires sealing one end and introducing flow at the other. Plug-sealing does the same job with a compliant nozzle pressed against the opening. Both convert a surface-cleaning problem into a pipe-cleaning problem, and pipe cleaning is far more effective per litre.

Intersections are the residual risk. A cross-drilling leaves a burr on the far wall, and the pocket at the intersection traps debris that a straight flush passes over. High pressure at the intersection, applied after the flush, is the usual answer.

5. Staging The Process

The three families share a stage logic even though the details differ: bulk removal, main clean, targeted clean, rinse, dry. The difference is the emphasis within each stage.

6. Making 5 mg Repeatable

A target is only repeatable if the process has a control variable for each contributor. Bath concentration, temperature, flow rate, nozzle condition and rinse quality are the five that move the number, and each needs a defined check interval.

The most common drift is nozzle wear, which is invisible in operation and changes the impact pattern gradually. A monthly pattern check on a test plate is cheap and catches it before the residue number does.

7. Verifying 5 mg

The verification method has to be fixed before the first part is cleaned, because the method determines what the number means. A rinse extraction over the whole part gives one figure; a rinse over defined areas gives several; a wipe gives a different figure again.

For a mixed family of housings, valve seats and oil boards, the practical approach is per-area sampling with a stated extraction volume and a stated analysis method, aggregated to a per-part figure by area weighting. That allows comparison across part families without pretending they are the same surface.

The repeatability of the method itself should be established before it is used to judge the process. A method that varies by two milligrams cannot control a five-milligram target.

8. Where Rework Comes From

Most rework on these lines is not a cleaning failure; it is a drainage or drying failure. Fluid that stays in a pocket carries contamination to a surface that was clean when it left the rinse.

The second most common cause is re-deposition during a long rinse, when contamination lifted earlier settles back. Counter-flow rinse and a shorter dwell between cleaning and rinsing address both.

The third is handling: a part transferred with wet gloves or set down on a dirty surface picks up contamination after the process is complete. Handling belongs inside the controlled zone, not outside it.

Frequently Asked Questions

Is higher pressure always better for reaching 5 mg?

No. Pressure without flow cleans a stripe; flow without targeting misses recesses. The useful combination is high flow for coverage and high pressure only at the features that need impingement.

Can one line clean all three part families?

Often yes, with different fixtures and recipes. The machine can be shared; the nozzle arrangement usually cannot.

Why does a housing pass and an oil board fail at the same setting?

Because the oil board needs through-flow, not surface impingement. Its residue is inside the passages, where a spray never reaches.

How is the 5 mg measured?

By a defined extraction method on a defined surface set, then gravimetric analysis. A number without that definition cannot be compared between suppliers.

Conclusion

Five milligrams is a budget, and the budget is spent in different places on a housing, a valve seat and an oil board. The process that meets it reliably allocates flow, pressure and time to those places rather than treating the part as one surface.

Control the five variables that move the number, and the target becomes a process parameter. Ignore them, and it becomes a weekly argument with quality.