Spark Arrestor

What's the difference b/t a US Forest Service approved spark arrestor and using a SS kitchen strainer? I know that sounds stupid but I just don't know the difference. I have a Two Bros M7 and it seems easy to take the end cap off and just cut a kitchen strainer to fit.

Nothing as long as the mesh is the same size, dollar store sells them cheer`s

Technically, this is what the USFS rangers use to determine approval.

"Qualifications for OHV Spark Arresters
The qualification standard requires spark arresters to be permanently marked with the model number and manufacturer’s name or trademark. Whenever contact is made with an OHV, regardless of what type of machine is used, a spark arrester inspection should be made.

Many people think that a muffler/silencer is also a spark arrester. Mufflers and silencers are only designed for noise control. Only an inspection can determine whether this is a spark arrester or a muffler/silencer.

If the OHV has been operating, beware of very hot metal in and around the exhaust system. The arrester will be a chamber-like device located somewhere along the exhaust discharge. Find the manufacturer’s name or logo and the model number located on the spark arrester. It may be necessary to clean a portion of the arrester to reveal that information. It can usually be found on an attached metal plate. The words “USDA Forest Service Qualified,” “Spark Arrester,” or “Qualified” stamped on a piece of equipment does not guarantee that it is a tested and qualified arrester.

Spark Arrester Inspection
Check to see if the entire exhaust system is sound and the arrester has been maintained. Also make sure that the spark arrester is mounted securely in the qualified position. In OHV use, this is usually the horizontal position. Proceed with a thorough inspection of the entire exhaust system.

With the engine off, and using a penlight, look into the spark arrester to visually ensure that the interior has not been removed or altered. You may be able to see the interior section that deflects the exhaust. Use a narrow rod, such as a 8-in wooden dowel, to gently feel for the internal parts of the arrester. Check how far you are able to insert the wooden dowel internally and cross check this measurement with the outside of the spark arrester/muffler. This will indicate whether you are able to pass the dowel the length of the arrester/muffler. If you are able to pass the dowel the total length, it is not a qualified arrester.

All trap spark arresters have internal fins or louvers that deflect the exhaust. If you suspect that an arrester may be altered, have the owner take the arrester apart for further inspection. Although the arrester/muffler may have the approved model number and the manufacturer information stamped on the shell, the inside components of the spark arrester mechanism may have been removed. Only through a thorough inspection can this type of modification be noted.

Arresters can be modified to avoid routine maintenance or to give the false perception of improved engine performance. Some modifications that have been detected include complete or partial removal of the spark arrester component parts, perforation of an arrester part, and installation of foreign objects into a muffler silencer, “mocking” spark arrester parts when a wooden dowel probe is used. Examples of these foreign objects are washers, bottle caps, and steel wool.
Here's the actual spec:
Screen-Type Spark Arresters as defined by USDA Forest Service Standard 5100-lb fall into a special category. They are intended for use on small engine applications. All exhaust products must pass through the screen, which has openings of 0.023-inch or less. the effective exhaust area of the screen (total area of all screen openings) shall not be less than 200 percent of the engine exhaust port area at its smallest cross-section. Screen material shall be heat and corrosion resistant, and shall provide at least 100 hours of service life. These are originally qualified for all the positions described in the "Application Positions" section."


Basically, the rule is that the spark arrestor has to stop carbon particles smaller than .024" which is why the mesh must be .023 or less. Any larger mesh doesn't pass the SAE J350 spec that the stuff is based off of. It also has to be made out of material that will last 100 hours of use (by spec). As a caveat, if it's not a USFS approved spark arrestor, it's not legal. you can't just slap mesh into your aftermarket exhaust. It has to be approved mesh, and most of the muffler manufacturers sell spark arrestor inserts for their mufflers. Easiest way to pass muster at a trail side inspection is have the paperwork for the arrestor with you.

Having said that, I know that some people get away with throwing other stuff in their mufflers. If a ranger with a clear understanding of the rules stops those people, the fine is large and the riding day is over until you get the issue fixed. Given that more and more states as well as the feds are now suing the pants off of anyone that starts a fire on public land, the enforcement aspect isn't the only one you have to worry about. Usually when someone starts a fire in a remote area, 100's to 10's of thousands of acres burn. In the interests of not getting riding area taken away, I personally encourage using approved spark arrestors on your bikes. The inserts are cheap, and they don't hurt power hardly at all. If you think you need that extra .5HP, ride at the track.