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I'm drilling cheap steel cogs for disc brake mount fixies. I can get 15-18T / 20T cogs. 15T only gets 3 bolt holes. $10 covers cog, drilling, and shipping. I make next to nothing doing this, but the karma is good. PM if interested.
 

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15 t gets 3

I drilled 6 in my 15 tooth cog, yes the chain did touch the rotor mounting flange, not a big deal. really,

Use a No 7 drill bit, you'll a better fit (no slop) between the bolt and the hole to be drilled, However, all 6 holes will need to be perfectly indexed to each other.

Also, on some hubs, the cogs splines will need to be filed down a touch to pass onto the hub

Verndogger
 

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If anyone wants to try this here are some posts I made in an earlier topic:

I used a cheap pressed steel cog that came with a SS converter kit. These are quite easy to drill. Mine is an 18 tooth. You need a standard 6 bolt dics to mark it up - lay the disc on top of the cog and center it up - make sure of this - eye up the centre edges of the cog against the centre circle of the disc and measure the distance in three places to check it is in the middle. Hold tight and draw the 6 round circles where the bolts go. Centre punch these and then using good quality HSS bits drill the holes. 2mm first all the way through and then a 5.5mm to finish. De-burr with a 7mm or similar and that's it. Mine ended up spot on - just take some time and care - worth a try for a few £'s (or $'s!). Especially if it is not for you. The washers are just to perfect the chainline.



Honestly, honestly it is so easy - just measure carefully and take time, anyone can do it. As I say a pressed steel cog is easy to drill and at around £3 each if you screw it up it doesn't matter - try another. The cog I drilled sits more central than the Surly cog on my cassette hub.

Regarding chainline, the disc mount position is obviously fixed in the ISO dimension. It can be spaced out a little simply by fitting washers between the cog and hub. If mounted directly to the hub the chainline in mm will be around 52/53mm IIRC. This is easy to check on your bike if you have a disc hub - measure from the inside face of the drop-out to the centre of the mounted disc. Subtract this figure from 67.5mm and that is your rear chainline. Check this with your current front chainline and you can see where you stand (measure from the centre of the seat tube to the centre of the teeth). You can fine tune the front chainline by varying the BB/spindle length/chainring position etc.

 

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Sent you a PM.
That would be perfect to convert my Cannondale SS conversion to fixed.
Thanks!
 
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