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Common Tread

How to repack a motorcycle muffler (or silencer)

Apr 18, 2018

Packing mufflers has to rank right up there with checking valve clearances and lubricating swingarm bearings as one of the most neglected motorcycle maintenance tasks.

It’s only a slightly inconvenient task. But the slow, steady slide from a properly packed muffler into a blown-out tin can is incremental, so it’s hard to notice when your silencer sounds like an echo chamber. Someone may tell you your bike sounds loud, or you may notice it yourself. You also may see some of the packing blowing out, if it’s severe. And you might notice the body of the muffler or silencer is getting way hotter than it used to — maybe you even found a melted body panel or number plate on your bike.

You should take time to repack your muffler. It’s a recommended maintenance item on both factory and aftermarket exhausts. In addition to the racket they make, blown-out exhaust packing robs you of low-end horsepower. And this isn’t just a dirtbike thing; many street bike exhausts need to be repacked, too.

My anecdotal research has shown that older dirt riders are usually good about repacking. I think it’s because many of them raced back when two-stroke bikes ruled in the dirt, and those have a habit of blowing excess oil into the exhaust. (You may have heard the term “spooge” tossed around which refers almost exclusively to the dribbling slobber of oil that makes its way down the pipe and out of the silencer.) The packing, which is almost exclusively woven fiberglass, gets matted down in that junk, and often compresses to the point where exhaust gases slip right by it, rather than traveling through the packing material.

Four-stroke motorcycles need a repack, too, though for different reasons. 4T exhaust temps are lots hotter than their 2T brethren, and the packing in those mufflers burns and vibrates. Strands get broken until they’re small enough that they eventually work their way past the core of the muffler and are blown out of the pipe with your exhaust gas. Off-road racers generally are aware of this and repack often. Less performance-oriented street riders, though, are often unaware that their exhaust packing is a consumable item.

How to do it

The process for packing either a silencer (2T) or a muffler (4T) is exactly the same.

Here's all the items I'll be using. RevZilla photo.

You need to perform muffler surgery, but before you remove anything, take a look at your muffler. Many are held together with rivets. Logically, if you are removing rivets, (a one-time use fastener) you’re gonna need some rivets and a rivet tool, so if you don’t own a good selection of rivets and a blind rivet tool, obtain 'em.

Not everything uses rivets, though. The silencer I am repacking in the photos belongs to one of our dirt riders here at RevZilla. I think it’s from a two-smoker Yamaha 125 of some form or flavor. Note that the silencer is actually held together with a pair of bolts to make repacking easy. FMF does this on many of their mufflers, as well.

Disassemble the muffler/silencer. Some have an end cap that removes from the outlet, and some are from the inlet or stinger end. Most units will require one cap to be removed, but not both. RevZilla photo.

Bolts make this job a little easier. Cobra does something similar on some of its fiberglass-packed cruiser exhausts: the baffles are held in with a set screw.

First, you want to put on gloves. Trust me on this. RevZilla photo.

Next, put on gloves. Latex, work gloves, whatever. Fiberglass literally contains microscopic pieces of glass. Anyone who has done this job before (or insulated a room) can back me up on this: you will get a nasty rash from fiberglass that’s irritating and itchy. Gloves are your friend. Change them early and often.

After that, disassemble this baby. Yank your spark arrestor or dB killer if you have either. Unbolt the baffle or end cap if you have bolts, or remove the rivet heads if the muffler is riveted. (The best way to do this, in my opinion, is to drill the center of the rivet, and then carefully shear the exposed head with a hammer and cold chisel. If your exhaust is a beat-up woods pipe, just place the chisel on there and peel ‘em off. If, however, your pipe is pretty and you want to keep your exhaust looking brand-spankin’ new, you are going to have to work pretty carefully. Stepping up the drill size progressively can help get the rivets loose enough to work them free without damaging the exterior of the muffler.

Work smart, not hard: You probably won’t need to remove all the rivets to unpack the muffler; you can pack most from just one end. The next step is separating the core from the body. They fit together like two concentric tubes. Regardless of what end of your can you’re removing, odds are excellent that the cap and can have been sealed with silicone.

Break the seal of silicone around the end cap. RevZilla photo.

Gentle probing with an appropriate tool to break the seal (picks and razor blades come in handy here) is a great starting point. Once that’s complete, persuasively — but gently — use a rubber mallet to separate body from core. If yours is super-stuck, you can skip to the next step, remove the packing, and then slide a piece of PVC pipe that fits over the core but inside the body, and beat on it (gently).

I placed the body in the vice (using soft jaws) and got out a plastic mallet. RevZilla photo.

Tapping the whole kit ‘n’ kaboodle against the ground, a curb or your workbench may well also assist in loosening up that which carbon or silicone has locked into place.

Pull it out
Wrestle the core out of the body. RevZilla photo.

If you’re working on a 2T silencer, you’ll probably pull out a gooey, oily mess. If you’re working on a 4T muffler, you’ll probably be looking at little or no packing left, perhaps with some scorch marks on the little bit still in there to remind you how that packing met its demise.

Greasy trash. Also, you can see well used muffler packing. RevZilla photo.

Start cleaning up. Get rid of the remaining packing.

This shot of the end of the packing material shows how saturated it had become. Had this been a four-stroke muffler, the packing would likely be far more blown out and burnt. Interestingly, when I handed this back to the owner, he said, "It's pounds lighter!" There was so much oil in that packing that the weight difference was noticeable! RevZilla photo.

Degrease and wipe down your 2T silencer, or use a wire brush to knock off the dry carbon and any other accumulation on a 4T muffler.

Wire brush
A couple quick swipes with a wire brush helps remove the residual silicone used to seal the inlet cap used in the last repack. RevZilla photo.

Once the core is good and clean, give it a good inspection to make sure any welds are still in good shape.

As long as it's out, cleaning oil residue from the core is a good idea. There's no point saturating new fiberglass prematurely. I cleaned up the body, too. Heck, if you want to get really crazy and polish it or something, this is probably the most convenient time to do it. RevZilla photo.

Once that’s done, it’s time to repack. Packing comes in a few varieties: sheet, loose/bulk, pillow, and probably some other formats I haven’t yet come across. They all work fine. Installation is the difference, really. I’ve never used the pillows; my experience has been limited to bulk and sheet. When I was a kid, my first repack job was using the bulk stuff. I bought it because it was cheap, and found that packing a muffler took approximately forever. I didn’t even know how long it took until the next time I had to pack a pipe and bought the fancy expensive sheet-stuff, which saved me eleventy-four hours of labor.

Measure the packing length relative to the core. RevZilla photo.

So my first tip is this: Use sheet packing instead of loose packing if you can get it. My second tip: If you have to use the bulk stuff, a broomstick or other dowel is helpful for ramming that fiberglass into the can. If you pack this way, pack tighter at the inlet and looser at the outlet, because the exhaust is hottest and moving fastest there.

Trim it. RevZilla photo.

I usually roll the core up in the sheet, trimming it if it overhangs egregiously. I roll it pretty damn tight, then hold it in place using a few turns of masking tape.

Roll your own
Roll it up. You're looking for the sweet spot between "don't split the burrito tortilla" and "gotta fit this sleeping bag into the stuff sack." RevZilla photo.

It’s possible to be too tight or too loose, but there’s a pretty wide envelope before you get into either arena.

Tape it all down to ease the reinstallation. Masking tape is usually what I use because it burns off quickly. RevZilla photo.

The tape will burn off quickly, and the packing will expand to fill voids.

Stuff it
Stuff it in there! RevZilla photo.

At this point, it’s time to reinstall. This is usually a little tricky, but not miserable. You'll want to tuck in the packing with your fingers as you are inserting the core (or use that dowel to compress bulk packing as you are shoving more in. This process is a bit different depending on the exhaust, but if you are packing a can, there are two things you’ll likely want to do.

Run a bead of high-temp silicone around the end cap right before you're ready to send the core back home. I like to use my (gloved) finger to smush it out a little so all the excess doesn't wind up on the joint, muffler body, and me! RevZilla photo.

The first occurs right before you get the end cap slipped in place. Give the mating surfaces a little shot of high-temperature RTV silicone. The second is to install the end cap carefully so the holes, be they threaded or drilled, line up just right so you aren’t fighting the cap as you refasten it.

Get the cap on
This is the tricky part. At this point, you need to tuck the packing in, not get silicone everywhere, and at the other end, make sure the core fits into its receptacle. Have fun, and I hope you have three hands. In all seriousness, it's not too hard. RevZilla photo.

One tricky part here is getting the end of the core slipped over the corresponding nipple on the muffler outlet. Shoving your finger up there and helping guide the core home is greatly beneficial here. Don't be tempted to hammer the thing in place; if it doesn't slip right into place, it's not lined up. Beating on it will just deform the core, which you'll have to repair or replace once you give up and remove it again.

Reinstall the fasteners, be they bolts or rivets, wipe it down with soapy water and a rag, and go pop it back onto your bike! RevZilla photo.

That’s the end of the job! Reinstall your exhaust (use fresh gaskets if necessary), wipe everything down, then take it for a spin and enjoy the purr (or ring-a-ding-ding) your bike makes once more.