I don't have a whole lot of knowledge on this, but here goes. I have an old Murray Ultra mountain bike (prob. 15 yrs.+). It is a 15 speed bike, with a 5 speed freewheel. The whole Shimano system is the 515 which includes front and rear derailleurs.I really don't want to spend a lot of money on an upgrade, but an up grade of some sort is necessary because of simple wear and tear. All i'm asking is there a set,or equivalent where i can replace - at least the shifters to a more modern ones,and front derailleur without having to break the bank. At the very least the front derailleur and shifters.
Having restored a number of old cheap bikes, I can tell you that the biggest problem is almost never the derailleurs/shifters. Usually these bikes have not been ridden much and have spent most of their time in a garage exposed to the elements. The biggest problems are rusted parts, poor lubrication, and horrifically bad tuning. Frequently, on a bike like this you're likely to find that it was never properly put together.
Here's a quick checklist of things to go over that can be fixed economically:
If you have to replace the shifters or derailleurs, just try to find cheap replacements or used parts that are similar. You'll want to stick with 5-speed stuff, you aren't going to be upgrading to a 10 speed freewheel (they don't exist!).
Sorry for the answer to an old post, but I've done this exact upgrade recently on my rear.
The bike is a 60cm frame, so it was worth saving. Originally it had a 5 speed freewheel and 3 front chainrings. Wheel rims were alloy. The rear spokes were made of spaghetti so I was replacing one or two a month.
I scored a rear wheel with a freehub and QR skewer in it, which just fit inside the frame with some minor encouragement.
I bought a 7 speed cassette for $45 NZ, and had to fit 3.5mm of spacers because it was actually an 8 speed freehub. Spacers cost $5.
The old 5 speed indexed shifter was useless so I fitted a Shimano one with a lever that clicks to go up, and a push button to drop a gear. Works well and cost about $25 NZ I also fitted a new gear inner wire because I could, cost $6 NZ.
I soldered the end of the gear cable rather than using a crimp-on cap.
I reused all the old outers, except I trimmed 10mm off where some of them were a bit sharp exiting the frame mounts. All outers were lubed with Wolf's oil (a cheap and lightweight chinese oil.)
Definitely worth doing, and I have not yet broken a spoke in 3 months of riding.
You should find your rear derailereuyueyrueyruer will work fine over 7 or 8 gears, but do take the change to clean and lube it.
I did not do anything with the front chainrings other than a clean and paint, and tweak up the limit screws. They're still on a friction shifter, which is fine for my needs.