Full-flow vs bypass filtration: which filter system does your machine need?

Every hydraulic circuit and every engine needs a full-flow filter. A bypass filter is an optional second filter that cleans a small side stream of the same oil to a much finer grade. It extends oil and component life, but it never replaces the full-flow filter. If you only fit one, fit the full-flow filter.

The question comes up because the word "bypass" means two different things in filtration: a separate side-stream filter, and the small relief valve built into almost every full-flow filter. Mixing them up leads to the most expensive mistake on this subject, which is taking out the main filter and relying on a bypass unit alone.

What a full-flow filter does

A full-flow filter sits in the main oil path, so every litre the pump moves goes through it on every pass. It is the filter that protects the pump, valves, cylinders and bearings, and it is the one your machine's service schedule refers to.

• It sees 100 per cent of the flow. That means its media has to be open enough to pass the full flow without starving the circuit.
• It is rated for that job. Across the 1,282 hydraulic filters in our catalogue, the most common ratings are 25, 12, 10, 7 and 5 micron. Across our 515 oil filters, 25, 10 and 12 micron are the most common.
• It has its own relief valve. When the element clogs or the oil is cold and thick, a spring-loaded valve opens and lets oil past the element so the circuit is not starved. On our oil filters that valve most often opens at 1.75 bar; on our hydraulic filters 3 bar is the most common setting.

That relief valve is the source of the confusion. It is called the bypass valve, but it is part of the full-flow filter. When it opens, unfiltered oil is going round the system. A filter running on its relief valve is protecting the pump from starvation, not from dirt. Our guide to what to do when the clogging indicator trips covers how to tell when that is happening.

A typical example is SPM-HYD-1169, a spin-on hydraulic filter with 25 micron microfibre media, a 2 inch thread and a relief valve that opens at 1.75 bar.

What a bypass filter does

A bypass filter, also called a side-stream or kidney-loop filter, is a separate housing that takes a small share of the oil from a pressure port or the reservoir, filters it slowly through very fine media, and returns it to the sump or tank.

• It only sees part of the flow. Because it is not in the main path, it can use media far finer than the circuit could tolerate in a full-flow position.
• It works on the whole oil volume over time. Each pass cleans a little more of the tank, so the overall cleanliness of the oil improves hour by hour.
• It removes what the full-flow filter lets through. Fine silt, soot and wear particles below the full-flow rating are its job.
• It cannot protect a component on its own. Oil reaching the pump or valves has not necessarily passed through it, which is why it is always a second stage.

Which system does your machine need

• Standard hydraulic circuit (excavator, loader, crane, aerial lift): a full-flow filter is required. A bypass filter is an optional upgrade that extends oil and component life.
• High-cycle or severe-duty hydraulics (long shifts, dusty sites, tight-tolerance servo valves): run both.
• Engine oil on standard service intervals: the full-flow filter the manufacturer specifies is enough. A bypass filter is optional.
• Engine oil on extended drain intervals or with high soot: run both, and use oil analysis to confirm the oil is staying in condition before you stretch the interval.
• Return-line filter only, no pressure filter: the return-line filter is your full-flow filter. Check its rating suits the components before adding anything else.
• Replacing the full-flow filter with a bypass filter: never. The main circuit loses its only protection.

Where each filter sits in the system

In a hydraulic system there are usually several filter positions, and only one of them is a bypass filter:

• Suction strainer: a coarse screen at the pump inlet that stops large debris. It is full-flow, but it is not your fine filter.
• Pressure filter: full-flow, straight after the pump, protecting the valves downstream.
• Return-line filter: full-flow, before the oil goes back to the tank, catching wear from cylinders and motors.
• Kidney-loop filter: the bypass filter, with its own small pump or tapped flow, cleaning the tank continuously.

On an engine, the full-flow spin-on or cartridge filter sits on the block in the position the manufacturer designed. A bypass filter is a separate unit, fed from a pressure port and draining back to the sump, often mounted remotely where there is room for it.

Whichever position you are servicing, the change itself is where most contamination gets in. Our guide on how to change a hydraulic filter element without contaminating the system goes through it step by step.

How to choose the right full-flow replacement

Because the full-flow filter does the essential work, get its replacement right before thinking about a bypass unit:

• Match the rating, not just the size. A filter that looks identical but is rated 25 micron instead of 10 micron fits perfectly and lets far more dirt through. Our beta ratio guide explains how to compare ratings properly.
• Match the relief valve setting. A valve that opens too early sends unfiltered oil round the system at every cold start. Use the setting your machine specifies.
• Match the thread and seal. Thread, gasket diameter and height all have to agree with the old filter.
• Change it on time. See when to change an oil filter for intervals by machine type.

Every product page in our hydraulic filter and oil filter ranges lists the rating, relief valve setting, thread and dimensions, plus the original part numbers each filter replaces. A spin-on oil filter such as SPM-OIL-1019 (25 micron, 3/4 inch thread) shows the full list of machines and part numbers it covers. If you only have the machine model, the compatibility finder will narrow it down, or send us the old part number through a quote request.

Frequently asked questions

Can a bypass filter replace my full-flow filter?
No. A bypass filter only treats part of the oil, so the pump and valves are left unprotected. Always keep the full-flow filter in place.

Is the bypass valve in my oil filter the same as a bypass filter?
No. The bypass valve is a relief valve inside the full-flow filter that opens when the element is clogged or the oil is cold. A bypass filter is a separate second filter.

Will a bypass filter let me extend my oil change interval?
It can, because it keeps the oil cleaner for longer. Only stretch the interval once oil analysis shows the oil is still in condition.

Do I need a finer full-flow filter instead of a bypass filter?
Only if the circuit is designed for it. A finer element in the full-flow position raises the pressure drop and can open the relief valve more often. Fit the rating your machine specifies.