Maintenance Q&A
Advice on bypass filters and calibrating torque wrenches

By Paul Abelson, senior technical editor

Q. I’m confused, and I hope you can clear things up. A friend of mine recommended a Gulf Coast oil filter. He said he doubled his oil drain interval. When I asked how it worked, he didn’t know but said it was a bypass filter.

How exactly does a bypass filter work? Common sense tells me it’s filtering the oil, but the term bypass is confusing.

A. Yes, the terminology is confusing. You are right that bypass can refer to a mechanism to pass around an obstruction in a full flow filter. The theory is that it is better to have a small amount of dirty oil in your engine than to have the engine run with no oil at all.

The most frequent cause of blocked flow is the oil thickening when it is very cold. When the oil warms up, the oil blocking the filter thins and things get back to normal. Most of the additional soot and contamination will soon be filtered, restoring flow through the filter.

Bypass has another meaning when it comes to filtration. In order for all engine oil to flow through the primary (full flow) filter, the pores of the filtering medium have to be at least 25 microns. If they were any smaller, soot and wear metals would clog the filter too often, and that clogging would not be reversible. Most full flow filters capture 25 to 40 micron particles.

A micron, the measurement used when rating filters, is one millionth of a meter, or 39-millionths of an inch (0.000039).

The problem is that many clearances between bearing surfaces or between piston rings and cylinder walls are far smaller, often just a few microns. A film of oil fills that space, and any contamination being carried by the oil can create wear in these tight clearances.

For a filter to clean oil down to 2 or 3 microns, it would have to be huge. To allow advanced filter media to work effectively in an engine compartment, filters were developed to direct a little bit of the oil to the ultra-fine filter and return it directly back to the oil sump, bypassing the full flow filter.

Even though only 4 to 5 percent of the oil bypasses the primary filter, within less than an hour virtually all of the oil has seen the secondary filter.

There are a few considerations you should know about before buying a bypass filter or extending your drain interval.

Full flow filters are constructed of special filter papers with uniform openings between their fibers. The paper is flat, so it is pleated to increase the amount of surface area exposed to the oil. Bypass filters are three dimensional.

Except for internal mechanisms, the entire container is filled with fibrous material. Media can range from compressed sawdust and wood chips to shredded newspaper to tightly wound special filter papers. Gulf Coast, Harvard and Como use special dense paper, making them among the most effective stand-alone bypass units.

Some of the most popular manufacturers of full flow filters also make combination filters that divert a small portion of the oil through a chamber usually containing compressed synthetic (fiberglass or polymer) media. The smaller the container, the less oil the bypass can clean, and the sooner the filter must be changed.

TMC created a Recommended Practice (RP334B) Guideline for Establishing Proper Engine Oil Drain Intervals for Heavy-Duty Diesels. Consideration should be given to operating environment (on- or off-road, chemical or sandy conditions, etc.), driver techniques, filter types and capacities and oil analysis. Of these, oil analysis is perhaps the most important.

Before significantly extending your drain interval, you need a baseline. Take samples by siphoning oil through the filler pipe. If the oil is good, you can continue using it. Start with a sample at your normal drain interval. If results come back that the oil is good to go, repeat the analysis at 50 percent more miles. For example, if you normally change at 20,000 miles, sample at 20,000 miles and again at 30,000 miles. If the oil is still good, sample again at 40,000.

I can’t advise extending beyond twice your old drain interval, or up to 50,000 miles, unless you have a fairly new truck or are using a pure synthetic oil. There may be other problems that develop. And technicians tend to do more thorough inspections during “wet” preventive maintenance, which is with an oil change, than during “dry” ones without the oil change.

Q. A few months ago you answered a question about wheel studs shearing off and how they were over-torqued. I am starting to do some of my own work, but not tires yet. I bought a torque wrench for general work. It has a 150 lb.-ft. capacity. How do I calibrate it?

A. Depending on how much use you will give it and how you care for it, you may not have to. Mechanics who use torque wrenches daily may calibrate them every six months to once a year, unless they are dropped or otherwise abused. Most good ones come in a lined wooden box with foam padding. Even inexpensive ones have their own boxes. They remind you that a torque wrench is a delicate instrument.

First, inspect the tool to make sure it functions. The cheapest way to ensure it’s in range is to hang weights. Measure a point one foot from the center point of the drive socket. Mount a socket on the wrench and place it onto a tire’s lug nut. This will serve as a fixed point for measurements.

Torque is force times distance. The distance is one foot, so the force will be the torque in lbs.-ft. Take a set of weights that are easy to handle, about 40 to 50 pounds. Tie them together with wire strong enough to hold them. Set the wrench to the weight you are using. Using the wire, loop it over the wrench at the one-foot mark. Very gently lower the weight, keeping the wire at the one-foot mark. The wrench should click as soon as the load is released from your hands and is fully held by the wire. If it doesn’t, use an Allen wrench to adjust the torque wrench until it clicks at the right moment.

There are calibration services that use sophisticated electronic measuring devices, but they may charge more than some new wrenches cost today. LL


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Send your question to Paul Abelson, senior technical editor, in care of Land Line Magazine, PO Box 1000, Grain Valley, MO 64029; email them to or fax questions to 630-983-7678. Please mark your message Attention: Maintenance Q&A.

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