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My friend just got a J. Mascis Jazzmaster, I set it up for him and holy fuck is it sweet for a cheap Squier. But still, it's a cheap Squier. So were going to replace the bridge, trem, and most of the electronics. Did some googling earlier and it looks like the AVRI trem is a good bet for this guitar but what about a better bridge that fits on import sized bridge posts? Doesn't need to have roller saddles like the Wilkinsons I've read about people using since I have the necessary files to cut new saddles for the strings. I did this with the stock bridge and nut so he'd have no tuning problems when using the trem, and he doesn't. Would just be happier with something sturdier than the ABR-1 style bridge on there that barely intonated. As for electronics since the pickups are really P-90s I was thinking two 500K CTS pots and a Sprague .022uf cap. And a jack that doesn't want to spit higher quality 1/4" plugs out of it. Thanks duders.

I actually like the Tune-O-Matic on the Mascis, but if you really want a replacement bridge, the Staytrem (make sure it's the one specially made for the Mascis) is the way to go: http://staytrem.com/Page-/-Category-3
Drop-in replacement, really well made, super easy to adjust (saddles are set for the 9.5" radius) and intonate.
And I think you're on the right track with the electronics. Keeping the pickups is a good idea (they're not like Jazzmaster pups, but they're really good). So yeah, replace the pots and caps and the jack, and replace the three-way switch while you're at it. They're junk and will give out sooner rather than later.

It's not a bad bridge, it's better than the bridges on the Classic Player Jag/JMs but it's still a little cheap. I looked up Straytrem just now, looks like exactly what we want.

Sweet, yeah the P90s are great sounding, he's had other JMs before but likes the P90s in the J. Mascis best. They're definitely my favorite pickup aside from a good AlNiCo humbucker.

There's a better switch included in the parts kit I have my eye on for his guitar so I'll be swapping that out for sure as well.

All that leaves is the trem itself. Any suggestions other than the AVRI version? Or how about one that screws or locks in? I'm not too privy about that but IIRC the CP Jag/JMs have a trem arm that screws in but I don't hold those guitars or their hardware in high regard.

^ John at Staytrem is great to deal with. If you want one, he'll get it to you ASAP. He makes great stuff, too. And yes, it'll be way better than the stock bridge.
As for the trem, again I think you're on the right track with the AVRI. That's what all the guys at OSG do with theirs.

I really, really love the Mastery bridge on a jazzmaster.
After using and abusing a guitar with one on it, I've never had any problems with it at all.
They're expensive but worth every penny in my book.

^ Mastery is great, but it needs a bit of futzing around to install one on a Mascis JM. Not a drop in replacement, unfortunately.

Turns out my buddy has a bad power tube in his YRM-1 so his cash is going to a new set of tubes before we get to replacing the bulk of the electronics and eventually the bridge on his guitar. The Mastery was my first thought but it doesn't really work for this guitar. Still going with a Staytrem when the time comes.

The trem unit, unless you really need the lock, there's no point in upgrading.

I bought a Mascis when they first came out assuming I'd need the lock.

it like the rest of the Squiers that followed I haven't felt the need to change a one.

Only Jaguar I use the lock on is my Classic Player with the guitar synth pickup on (and that's to keep me from using the trem a little since it gets harder to waggle) if I'm playing a synth instrument that doesn't have that note bend capacity.

FWIW, the screw in arm on the Classic Players work but I wouldn't run out and replace everything with them. Maybe if I was one of the guys who trashed collets on regular JM/Jag trems, but I'm not (and I know to tighten them if they get old and maligned, and don't take the arms off unless needed).