Note: Quite a bit of this information shared in this post is woefully outdated. Please see the revised version of this post here:
http://cobfoolery.blogspot.com/2017/04/cobfoolery-tip-revised-replacing-stem.html
When we started selling Missouri Meerschaum corn cob pipes online at
Aristocob.com back in 2007 one of the biggest internal debates I had was what to call the part of the pipe that people put into their mouth. I had always called this the "stem", but others called it a "mouthpiece". To make things even more complicated Missouri Meerschaum referred to this as a "bit". My understanding was that the "bit" was the far end of the stem, where the biting takes place. At any rate, we decided that for the sake of continuity that we would refer to them as bits, just as Missouri Meerschaum (MM) did, but to this day I feel that was a mistake, so from here on in this post I will refer to these as stems.
Anyway, one of the fastest ways of modifying a corn cob pipe is to swap the stem. You can do this between genuine MM bits, as long as you understand that there are two different tenon sizes. Oh yeah, the tenon is the part of the stem that is dowel-like and inserts into the wooden shank. All of the MM filtered bits have an O.D. of 8.7-8.8mm and an I.D. that will accommodate a 6mm filter, bit it's important to note that they can be smoked with or without a filter installed. The I.D. of the wooden shank is approximately 9mm and this hole is often ever-so-slightly tapered. And all of these "filtered" bits are interchangeable, so if you want to switch the stem from black to amber, or if you want to swap it for a thinner or wider stem, you can do this at will. My personal favorite is the Danish stem in either black or amber.
Amber Danish stem from Aristocob.com
Another range of genuine MM stems is the non-filtered "slim" stems. These are found on the smaller pipes, like the Pony Express, the Mizzou, the Patriot, the novelty miniature cobs, and on the other end of the size spectrum this stem is standard on the MacArthur pipes.
Lastly in the original MM range is the non-filtered vulcanized rubber stem that is found on the Freehand pipe. This stem is only available in a bent shape, and
though it is an unfiltered stem, the tenon is sized to fit into the filtered shanks only. That makes this stem an ideal upgrade for the bent Legend, Washington, Country Gentleman, the Diplomat, the General, the Kolonel (5th Avenue) the Ozark Mountain, the Missouri Pride, the Woodie's and the Spool. Find all of these stock replacement stems at
Aristocob.com.
Want to bend a stock MM pipe? Watch this video.
There are now several aftermarket options available if you want to move away from the stock range of stems. Here are a few of your choices:
Custom Made Stems:
Walker Briar Works: Dave Wolff has been expertly repairing and restoring pipes for years, but recently launched a line of Vulcanite and Lucite (acrylic) stems for MM pipes that he calls the "Forever Stem". I purchased one of the Lucite stems for one of my pipes and found it to be of high quality and a good value.
An example of Walker Briar Work's "Forever Stems"
CustomCornCobs
: This is an aftermarket stem maker who has been offering his wares on eBay for a short while. I have no personal experience with their product, and frankly am shocked at their high price is on stock MM stems, they have been mentioned on the Pipe Smoker's Forum as a budget alternative to Walker Briar Works.
If you are a woodworker or at least are handy with a file and sandpaper, then there is no reason you can't make your own stems.
Here are some DIY Stem options:
Pipe Makers Emporium: The grandaddy of suppliers to pipe makers in the USA has to be Pipe Maker's Emporium in Phoenix, AZ. Paul Hildebrand was an avid pipe collector and enthusiast who had an interest in making and repairing pipes, so in 2004 he purchased Pipe Makers Emporium (PME), now the largest supplier of pipe making materials. Paul has since retired, but the business is still going strong in the hands of General Manager, Carolyn Perea. I had the pleasure of getting a tour of their operations by Carolyn in the Fall of 2012, and was thoroughly impressed by her knowledge of pipes and supplies.
Pipe Makers Emporium carries over 2,000 items; from Briar, Vulcanite stems, Lucite stems, Ebonite rods, buffing compounds, dyes, bands, and specialized tools for pipe making. They supply both professional and novice pipe makers from around the world.
UPDATE: PME has announced that they are closing after 11 years in business. They may still have inventory, so check them out ASAP.
Pen makers will find that their acrylic offering is vast and very reasonably prices. One important fact to note: The stem blanks that they sell are rough, unfinished and feature an oversized tenon blank that will not fit a pipe shank. You must cut the tenon to size, remove rough casting lines and finish the surfaces yourself. That's your reward for going it alone, so if that sounds daunting you might just want to click one of those links listed above under "Custom Made Stems"
An example of a vulcanized rubber stem blank from Pipe Makers Emporium
Do you really want a challenge? I like your style. You can do like many professional pipe makers do and create your own stems from wood, bone, acrylic or hard rubber. While you can get some of these from suppliers who cater to pipe makers (PME, for example), many of these components are also sold by knife making suppliers and woodworking stores such as
Woodcraft and
Rockler. Search their sites for "Pen blank".
An example of an acrylic pen blank from Woodcraft
That about wraps it up for now. Don't forget that the Missouri Meerschaum stems are very inexpensive and therefore you can swap them for a new one for pennies! Check them out here:
Spare Pipe Stems/Bits
I "met" Basil on the Pipe Smoker's Forum, but many months later had the pleasure of enjoying a few bowls of tobacco across the table from him at the T.A.P.s pipe show in Raleigh NC. 
My son "Boy" and I had a ball talking with Basil, and the time flew by as we smoked and laughed. As we parted company we made sure to get one of his business cards and I made note of his Etsy page Reddog Pipes where he sells his modified cobs asthe "Bullitt Proof Cob", as well as some cleverly refinished estate briar pipes. We won't hold that against him, though. 
