Working as a guitar tech with Sonic Youth for a dozen or so years, and now working with Nels Cline, I've become pretty familiar with the Jazzmaster and how to keep it rock solid on the road. The following tips are for road guitars, not necessarily nice vintage Jazzmasters used for recording or collecting. I'm all for keeping guitars as original as possible, but if you're taking it out on the road, occasionally some compromises need to be made.
1. First things first: Change the bridge!
If you've got a stock old-style Jazzmaster bridge, you know how useless they are with any sort of passionate playing. Fender has improved the bridge design a bit lately, but at the very least I'd recommend swapping it out with a Mustang-style bridge or, even better, the Mastery Bridge made by John Woodland. We put them on all of Sonic Youth's Jazzmasters and it really was a game changer. Even with the Mustang and Tune-o-matic bridges we previously used as replacements, there was a considerable amount of filing the string slots so the strings wouldn't slip out during hard playing, but with the Mastery those days are over. It's a simple drop-in replacement, designed to last forever and highly recommended!
2. If you use the tremolo a lot, you must lube.
There are many choices out there and I can't say which is the best, preferably a petroleum/graphite combo. Make sure your strings slide freely within the nut before applying it. Here’s how to check: Next time you change strings, take the strings you've just removed and slide them through their respective nut slots. There shouldn’t be resistance. If there is, lube the nut of the wound strings and then run a string back and forth through the slot until it feels right. This seems to be less destructive than sandpaper or a file. With the plain string slots, I might use some very fine sandpaper or Mitchell's Abrasive Cord, with some lube, to get it nice and slippery. Do this with every couple string changes and gradually it will (hopefully) stay more in tune. Also, make sure to get the underside of the string trees, preferably with the abrasive cord, or run an old low-E string under them.
3. Combat floppy tremolo bar syndrome.
This is a bummer with almost no solution, though some of Fender's newer designs are addressing this. For the traditional style bars, one thing we did in Sonic Youth is use a threading die to thread the end of the tremolo arm and screw on a lock nut so the trem arm would stay in place. Depending on the tightness of the lock nut, you can adjust the tension of the arm. One problem is this is a little destructive to the originality of the instrument, so you need to make that decision. Also, you can't remove the arm at all, which for Sonic Youth was okay because they could go crazy and the thing would never pop out—that's why they started doing that in the first place. With this mod you can't remove the arm when casing up your guitar but we've found it's not really necessary. Even in a hard case, just find the angle (sort of over the jack) that the arm will bend the least when it's in the case. Believe me, those SY guitars traveled millions of miles and it was never an issue. Always pulling the arm out when casing contributes to it getting "floppy" as well. More recently, Nels' main guitar had a super floppy arm and I didn't want to modify it in any way or change the Trem plate. I found the tiniest hose clamp I could find and squeezed it around the sleeve that holds the arm in on the underside of the tremolo plate. Not too, tight but just enough. So far so good! I'm just checking it every few string changes to make sure it's holding.
4. Consider the electronics.
If you're really rocking and the rhythm circuit is not your cup of tea, you could try what we did in Sonic Youth and get in there and do some soldering and completely bypass it, so you don't hit that switch accidentally and cut off your sound. Some players like to keep it as a convenient kill switch, and some folks really like the added tonal options the rhythm circuit provides—to each his own.
5. Replace your pickguard.
If you've got a vintage Jazzmaster with the old-school celluloid pickguard, these things tend to shrink over time and you might find it impossible to adjust pickup height as the covers are being squeezed by the pickguard. There are only a few solutions to this. On Thurston Moore's guitars I've dremeled out the pickup holes a bit to make them fit, but ideally you can swap out the guard with a new one.
Important: if you remove a vintage celluloid pickguard, mount it (with the original screws if possible) to a piece of wood so it doesn't shrivel up too much, in case you ever decide to put it back on your Jazzmaster.
As far as new pickguards go, the real Fender tortoiseshell-style is pretty passable, or another cool solution is an anodized aluminum pickguard from tone-guard.com. Sure, you don't want to lose any of your axe's mojo by changing the guard, but the whole point is functionality, and you've got the old one properly preserved in case you ever want to go back.
Bonus tip: On old Jazzmasters, the foam under the pickups tends to rot out. I've found a great replacement. At first, I was getting this stuff from a guy on eBay who was calling it "NOS Jazzmaster P-Bass Pickup Foam," but I found that it’s basically an adhesive weather stripping you can get at any hardware store. You can use it for all kinds of things, like inside the battery compartments of effects pedals.
Eric Baechtis San Francisco-based guitar tech who's been on the road since 1998. Most of this time was spent with Sonic Youth, but he's also worked with Primus, Queens of the Stone Age, Faith No More, Tom Waits, WILCO, and more.