Enlarge Sprocket Center?

C

Cycofast

New Member
Hello all,

My engine kit is on a "garbage day" salvaged mountain bike. The original back wheel was a bit bent so I decided to buy a new coaster break wheel from a local bike shop. The problem is that the center hole of my drive sprocket is slightly smaller than the coaster brake hub. After I put the fiber gasket in between I think I am getting a tight fit, but I'm not sure. I read that maybe I should have my sprocket machined so that the sprocket fits flush to the hub. I'm not sure this will make much of a difference with the fiber gasket though. Any suggestions? Thanks for your help!

Best,
Cycofast
 
Mark out a circle on the sprocket that will clear the bike hub. Then either have it machined or grind out the excess. I bought a 6buck rasp for my drill and cut mine out in an hour or so. I can't find a machine shop to do it.

You will like the coaster brake wheel since it will get rid of one brake lever for you. The enlarging of the hole doesn't have to be perfect but you do want it centered. Also you want to make sure the sprocket fits as flat as possible to prevent the chain from jumping off on you. If it isn't reasonably flat you will have problems I know I have more chain problems than anyone here ever had before.

You might need to go to the larger bike chain as well. The rear sprocket on a coaster might be a little too thick for the mtb chain. the thinner chain ring shouldn't give you any trouble.
 
Last edited:
I had the same problem. I put mine on huffy beach cruiser with a coaster break. I had to take the sprocket to a machine shop and i drilled mine out to 1 5/8".

Also, the torque arm (the arm that bolts on to the frame for the breaks) did not have proper clearance over the new sprocket and bolts. I took this piece to my university's sheet metal shop and used a bending press to make it right. the only problem was that i had to bend very close to the hole on the torque arm, and so i also had to bend the washer to fit the bend. Works Great!!!

Make sure you will still have frame clearance when you bend, or else you can reposition the torque arm where to will not interfere and attached it to the frame with a bracket.
 
No need to pay a machine shop when you can just grind off the outer flange of the dust cap, AND one side of the washer if you have a washer clearance problem.
 
I'd rather keep the dust cap to keep the dust out of my coaster break. it's my only break, and i don't want to compromise it's ability to keep schmutz out.

Also, making the sprocket hole bigger made better clearance for the bend in the torque arm, I don't it wold have made it with out the increased inside diameter; steel can't be bent too sharply or it will break.
 
If you just grind off the outside flange of the dust cap, it will still work as a dust cap (especially if you pack the rear wheel with grease).

As for bending the brake arm- you don't need to bend it much and it has nothing to do with the hole in the sprocket....
 
Do not worry, I had same issue with nexus hub from an electra cruiser. Dont muck around with your hub. Use a micrometer to measure the hub, then add another 1/8th inch for proper clearance. Helpfull if its a freewheeler that spinns independent of spoke flange that sprocket will be attached too. Take it to machine shop. If they are brain dead, you will have to tell them how to do it. It will be done on laith, but sprocket teeth get in the way of most three tooth chucks used today. they need to put soft copper shimms between sprocket teeth and chuck. this will properly center the sprocket, giving the sprocket teeth an even surface. Most laith chuck teeth are concave, and dont hold an uneven round object very well. I hogged mine out to 2 inch. No problems.
 
actually, they didn't want tot put it in the lathe chuck, because it's a relatively thin flat piece. they bored it out on a drill press. same result, good clearance, and my ride is running sweet. i had em take it out to 1 5/8" on mine from 1 3/8" (ish)
 
Another cheaper solution is to hog it out with a file, I've done this several times. Get a large file with a rounded side, an old chunk of wood to protect your workbench and file while rolling the sprocket on the wood or carpet also works good too. By rolling it keeps the hole even and round, I don't make it any bigger than it needs to be to aid in centering the sprocket when mounting. This usually takes me 10-15 minutes and usually comes out better than the factory hole...Kelly
 
Did anyone else have trouble keeping the stupid rag joints and half moon on while trying to bolt them together? If you do get some Gorilla snot and glue them in place then bolt up. It's actualy emblem adhesive the guys use it for gaskets to hold them in place. It worked like a charm.
 
I used C-4 on mine... Na really a file and a couple of C-clamps and a lot of elbow action.
Clamped right up to my neighbors picket fence. (c)
 
You must log in or register to reply here.
Back
Top