Swapping front sprockets

[dtpolk3]

I have a Mongoose XR-75 that is in very nice shape...except for the FRONT 3-sprocket set (chainwheels?). Some teeth are worn out on the two largest sprockets. Chain looks good. I have replacements (from another bike-same model), and I am ready to swap them out.

While I am a pretty good (auto) mechanic, I haven't done this particular job before, and I'm not exactly sure how to go about it. It appears that the sprockets are held on with hex screws. Right? Looks like a fairly easy process...but is it? Is there a tutorial here at the site that covers this bike?

Thanks for any suggestions someone could give!!

[Joe_W]

If the chain rings are already worn you will definitely need to replace the chain and the cassette, too. Never ride a chain so long that it "stretches" (well, the pins / plates, the joints wear and this "stretches the chain) this much.
Swapping out chain rings is easy, just the hex screws, as you expected. Most chain rings (the modern ones) need to be installed in the correct direction, there are ramps and pins and stuff on it to make shifting smoother, so check that before you swap them out. Tighten the screws bit by bit and ... dunno how to say that in English... criss-cross? So never two adjacent screws directly after another - like on cars (wheels, engine, whatnot). There is a special tool to hold the nut, basically a very wide bladed, angled screw driver with somthing to center it on the blade. It is hard to achieve the desired torque without it. It costs someting like 10 bucks or less.

[dtpolk3]

The chain and cassette (the whole rear tire and derailleur) were the first parts I swapped out. Just wasn't sure about doing the front sprockets. The parts already swapped were in good shape and I only rode the bike once since then (maybe 1/4 a mile), so I doubt the chain was damaged.

So I just remove the guard and then the hex screws. I suppose I could use a wide blade screwdriver on the nuts. What is the desired torque?

[Joe_W]

Ok. Sometimes people will not do that (actually, quite often). So: good work so far!
The torque depends on the crank set, the manufacturer has (sometimes) info on it, but Alex made a list with the most common parts:
http://bikeride.com/torque-specifications/
The screwdriver would need a blade about ... dunno, 2cm wide (or so) and fairly "sharp", just take a look at the frameward side of the assembly. If you can find some sheet metal lying around you can try making your own tool. I found that the card slot covers from old computer cases almost fit, I had to make the edge a bit narower. Basically bikes are easy to work on, most of the stuff can be understood by looking hard at it and using your experience - like how to tighten parts that have several screws, what to grease, etc. Ah, talking of: I like using one of the lighter types of Locktite on the chain ring bolts, some grease on almost all other screws.

[cny-man]

(12-19-2013, 11:14 PM)dtpolk3 Wrote:  Some teeth are worn out on the two largest sprockets.

It's unusual for "some" teeth on a chainring (front sprocket) to be worn. If the teeth you are referring to are shorter than others, or have ridges on the inside (and sometimes small "pins" nearby) those are actually design features to improve shifting. You may want to check the excellent sheldonbrown.com site for pic and explanation.
http://www.sheldonbrown.com/gloss_ra-e.html#ramp

[Joe_W]

I guess he wants to swap them out because he is having troubles after replacing chain and cassette, so he should likely do that. As all of it comes from the same donor bike it should work.

[cny-man]

It is extremely unusual for chainwheels to wear out. As he only said some teeth were worn, rather than he was having a specific problem, it's premature to say he should go ahead. Also, chainwheels from different cranksets cannot necessarily be interchanged, as the "bolt circle" may be a different diameter. and even the entire right crankarm may not be compatible, as different bikes can take different spindle lengths in the bottom bracket.

As for the torque question, that's just silly on chainwheel bolts - you tighten them firmly, like most nuts/bolts. The tool to hold onto the back portion is readily available and cheap, probably available at a local shop, but one can take an old screwdriver with a wide, thin blade and grind a recess in the center portion if the bolt sticks through the nut too far.

[Painkiller]

I would like to see pics, I did not know that low end of a bike even had replaceable chainrings. Any one I have had the misfortune to work on was steel arms, stamped steel riveted rings. What he needs is a crank puller and a good eye to align the rear derailluer when setting it all up

[cny-man]

We do need pics of what he thinks is wear, as well as an explanation of what problems, if any, he is having with the chainwheels. Then solutions can be determined. The rear derailleur would have nothing to do with replacing the chainwheels.

[Painkiller]

"The rear derailleur would have nothing to do with replacing the chainwheels."
Never said that at all.I was referring to " a good eye to align the rear derailluer when setting it all up"
It does have to do with making the bike shift as nice as it can.
here are a couple pics of a crank that came off a bike like that, but since they have been around awhile it may not be like the Op's
As riveted as they come, no teeth designed for shift points