Hey UG,
I have a Fender P Bass Jr and for the really short scale I really love it for practice and screwing around at home. The thing is, the bridge and tuning pegs are starting to show their age. I already have some tuning pegs that should work but I'm wondering would a replacement P Bass style bridge work? Or would it be too big?

Would a regular P Bass bridge work on a P Bass Jr?

It's doubtful that anyone has a P-Bass Jr. and a full size Precision bridge just laying around to superimpose over your current bridge. At this point you just need to take a ruler or tape measure in to your local music shop and measure a standard P-Bass bridge. Being more or less rectangular in profile you can then cut a thin piece of cardboard to the correct dimensions and see if it will be too long, eat into the pickguard, etc.

The bigger issue is whether you should modify your bass at all. Once you go down the mod path the typical outcome is that you will have have increased the money you have in your bass while simultaneously decreasing its value (ask me how I know this). Modding is only useful if you are trying to create a unique bass or you have a tinker's mindset that cannot be suppressed. It is almost always smarter, money-wise, to just buy a better bass than what you have and be done with it.

Another consideration is whether or not the P-Bass Jr. will ever become a collector's item. If it does then it will be worth the most if it is bone-stock original.

Unless you are irrationally wed to the sub-short scale on the P-Bass Jr., you should probably consider moving to a 30" shortscale bass and either sell the Jr. or just keep it as a backup. Although there are numerous shortscale basses out there it is hard to beat the Squier Jaguar SS Special. MSRP is $180 but it can occasionally be found on sale for $155. More and better string choices with the shortscale as well.
When you say, show their age, can you be a bit more descriptive? Some aging is considered part of the natural process of an instrument. Hell people pay tonnes of cash for relic-ed basses from Fender's customer shop.

If it a playability issue, check with Stewart MacDonald and they may be able to help you out in getting the right parts.
Thanks for the advice. I should of mentioned that I'm aware of the whole modding of instruments and I always keep my stock parts so they can be swapped back before I trade up or sell. As well, I do have a couple of other short scales basses in my arsenal. For it being a collector, again I'm not worried. The Jr is just for fun! I'll just have to do what you said and write down the measurements and go to a shop and compare. I'm just worried that when the parts do need to be replaced, I won't be able to find the exact OEM parts as its really uncommon. (It still has the original pick guard sticker where it says number 013 out of 4000 )

For the other post when I say they're showing their age; the screws for the bridge saddles are a bit loose (probably just needs a setup) and the tuning pegs are missing the protective covers. And in general just show a bit of wear like scratches, looseness and what not. Nothing major, I'm just want to get an idea before things really crap out as it is not only uncommon but also an older model. I'll definitely, give Stewart Mac Donald a call and see what they can do, if not just call Fender and see what they have to say.
As long as the string spacing on your Jr. is the same as on the full-sized Preceison, a bridge swap should not be a problem. I would take a look at the Babicz Full Contact bridge and the Leo Quan Badass II.

Most bass tuning pegs are solidly built, and as long as you make sure that all the screws and nuts are properly tightened, you probably won't have any problems. Now if there is too much lash in the tuners, or if they are constantly slipping, then you should look into getting replacements.
"Maybe this world is another planet's hell?" - Aldous Huxley