How to Choose the Right Oil Filter for a Compressor, Engine or Hydraulic System

Choosing an oil filter for a compressor, diesel engine or hydraulic power unit comes down to six things: the thread and seal, the dimensions, the filtration rating, the media, the bypass valve setting and the pressure ratings. Get all six right and an aftermarket filter will do the same job as the one that came off the machine. Get one wrong and you can starve a bearing or leak oil across the floor.

This guide works through each one in the order you should check them, using the specifications we hold for the 480 oil filter families in our catalogue.

Spin-on or element: know which one you have

There are two physical types of oil filter, and they are not interchangeable.

• Spin-on filters are a sealed steel can with the media inside and a threaded centre hole. The whole can is replaced. They are common on smaller screw compressors, diesel engines and mobile hydraulics.
• Element (cartridge) filters are just the media pack and end caps. They drop into a permanent housing, and only the element is replaced. Larger compressors and most industrial hydraulic systems use them.

Of the 480 oil filter families we list, 199 are spin-on (they carry a thread specification) and 281 are elements. If your old filter has a thread, you need a spin-on. If it slides over a centre post or locates in a bowl, you need an element.

1. Thread and seal (spin-on filters)

The thread is the first thing to confirm, because a spin-on with the wrong thread will not fit at all, and one with a near-miss thread can strip the filter head.

Thread standards on industrial spin-ons fall into two families:

• Unified threads (UN and UNF), quoted as a size plus threads per inch, such as 1 1/2 inch x 16 or 3/4 inch x 16. These dominate on filters for American and many European machines.
• GAS (BSP) threads, quoted as a pipe size, such as 3/4 inch or 1 1/4 inch. These are common on Italian compressors.

The most common threads across our spin-on range are UN 1 1/2 inch x 16 (35 families), GAS 3/4 inch (22), UNF 1 3/8 inch x 12 (20) and UNF 1 inch x 12 (17). Note that a 3/4 inch GAS thread and a 3/4 inch UNF thread are different threads. The nominal size alone does not tell you which you have. Check the threads per inch with a thread gauge or match it against the filter's listed specification.

Then check the sealing gasket. Its outside and inside diameter need to land on the filter head's sealing face. A gasket that is too small will seal on the wrong part of the head, and it will usually leak once the system reaches pressure.

2. Dimensions

Outer diameter and overall height decide whether the filter fits the space. On elements, the inner diameter at each end cap matters just as much, because that is where the element seals against the housing. Measure all three on the old filter in millimetres and compare them with the listed specification. Every product page on our site publishes these figures so you can check fit before you order.

3. Filtration rating

The filtration rating, quoted in microns (µm), tells you the particle size the media is designed to catch. Across our oil filters the most common ratings are:

• 25 µm: 125 families, typical of compressor and engine lube oil filters
• 10 µm and 12 µm: 156 families combined, typical of finer compressor and hydraulic duty
• 5 µm and 7 µm: 40 families, used where the system is sensitive to fine wear particles
• 60 µm and 90 µm: 49 families, mostly wire mesh suction or pre-filters

Match the rating of the filter you are replacing. A finer filter is not automatically better. A finer media loads up faster, and if it clogs before the next service, the bypass valve opens and unfiltered oil goes straight to the machine.

4. Media type

The media is what actually does the filtering, and it changes how the filter behaves over its life.

• Inorganic microfibre (glass fibre): 179 of our families. Holds more dirt for its size and keeps its efficiency as it loads. Standard on fine hydraulic and compressor filters.
• Impregnated paper (cellulose): 144 families. Cost effective and well suited to 25 µm lube oil duty.
• Wire mesh: 71 families. Cleanable or long-life coarse filtration, often on suction lines.

Replace like with like. Swapping a microfibre element for a paper one of the same micron rating will usually shorten the service interval.

5. Bypass valve opening pressure

Most oil filters have a bypass valve. When the media becomes too restricted, the valve opens so oil keeps flowing to the machine rather than starving it. The opening pressure is set by the machine designer, and you need to match it.

In our range, 138 of the 181 families with a stated bypass setting open at 1.75 bar, which is typical for compressor oil filters. Others open at 2.5 bar, 1 bar, or as low as 0.3 bar. If you fit a filter with a lower setting than the original, it can open in normal running and pass dirty oil. A higher setting can starve the machine on a cold start when the oil is thick.

6. Working, burst and collapse pressure

The filter has to survive the pressure in the circuit. Check three figures:

• Working pressure: the pressure the filter is rated to run at continuously. 12 bar is the most common rating in our spin-on range, which covers most screw compressor oil circuits.
• Burst pressure: the pressure at which the can fails. 21 bar is the most common figure in our range. Hydraulic spin-ons run much higher, with some rated at 70 bar.
• Element collapse pressure: the differential pressure the media can take before it folds. This matters on elements, especially in hydraulic circuits.

Never fit a filter rated below the system's working pressure, even as a temporary fix.

Using the part number to shortcut all of this

If the old filter's label is readable, the part number is the fastest route. The same filter is usually sold under many brand codes, and a good cross-reference table maps them all to one physical part. We explain why that happens in why the same filter has four part numbers. Our compatibility page lets you search by your machine or the code you have.

Even with a matching code, check the six points above against the listed specification. A cross-reference tells you two parts are intended to be interchangeable. The dimensions and ratings confirm it.

Quick checklist

• Spin-on or element?
• Thread size and threads per inch (spin-on), and gasket diameter
• Outer diameter, height and inner diameter in mm
• Filtration rating in µm, matched to the original
• Media type, matched to the original
• Bypass valve opening pressure, matched to the original
• Working and burst pressure at or above the system's rating

Browse our oil filters or hydraulic filters, each with the full specification published. If you cannot find your part, send us the code or the measurements through our quote request page.

Frequently asked questions

Can I use an automotive oil filter on my compressor if the thread fits?
No. Automotive filters are built for different pressures, bypass settings and oil types. A compressor oil filter typically runs at a higher working pressure, and a mismatched bypass setting can pass unfiltered oil to the airend.

Is a lower micron rating always better?
No. Finer media clogs faster. If it clogs before the next service, the bypass valve opens and the machine runs on unfiltered oil. Match the original rating.

What does the bypass valve pressure mean?
It is the pressure difference across the media at which the valve opens to keep oil flowing. Match it to the original filter. 1.75 bar is the most common setting on compressor oil filters in our range.

Are 3/4 inch GAS and 3/4 inch UNF threads the same?
No. They have different diameters and thread forms. A filter with one will not seal properly on a head made for the other.