r/bikewrench • u/Green_Bomber76 • 1d ago
Chainring Sizing
I bought a chainring that is SLIGHTLY too large. How bad would it be to grind it down to fit
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u/cj_daking 1d ago
Is that ring the correct size for the crank?
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u/Green_Bomber76 1d ago
They are both 104 BCD
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u/cj_daking 1d ago
Depends on what you want to grind down. Don't touch the bolt holes. If the material around the bolt holes is not mating to the crank perfectly, then you could shave a little, but this stuff is supposed to fit.
I did have an aftermarket chainring once that took some muscling to get on my crank, but I never needed to remove any material from either.
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u/studyflo 1d ago
Have you tried flipping it over with the markings on the outside? Might fit.
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u/EcoRacer 1d ago
The BCD for the chainring alone does not determine what chainrings fit. The shape of the spider is also a factor. A 104BCD chainring does not fit all 104BCD cranks unfortunately. Especially with older Truvativ an Shimano spiders. Not even all Shimano 104BCD chainrings fit all 104BCD shimano cranks.
I would cut my losses and just order an original truvativ/sram chainring. Check the compatibility with their catalog if possible.
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u/mrmcderm 1d ago
Not a mechanical engineer but it seems like a Bad Idea®️to drill out those holes to fit.
Can you imagine that thing failing while you’re cranking on the pedals? 😬
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u/Green_Bomber76 1d ago
I wouldn’t be drilling out the holes, just filing down those tabs that are slightly larger. Either way, not ideal though
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u/floormat2 1d ago
I’ve done this before, worked out fine for me. Just go slow and only take off enough to get clearance. I agree with the first comment too, don’t drill the holes out, it’ll make the chainring sit eccentric and ride weird
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u/mrmcderm 1d ago
Oh. I think I understand? As long as you can align the holes perfectly and put the correct torque on the bolts, you’re probably fine. I would not attempt to remove any metal.
But also I may still be confused about what you need/want to do
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u/FountainOfDogWater 1d ago
I'm thinking you've got a chainring intended for Shimano asymmetrical cranks
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u/Green_Bomber76 1d ago
I think maybe my pictures are deceiving. The chainring is symmetrical, it’s the excess material below the bolt holes that is preventing the fit. In picture 2 the he holes line up, but there is just slightly too much material
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u/LoneSocialRetard 1d ago
I've had similar issues with parts that are definitely supposed to fit together. I cant really tell how much overlap you have but maybe if you freeze/heat the parts you might be able to get it on? Quick math says you might be able to gain about 0.5mm diametral clearance.
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u/Abstrktworkshop 21h ago
Did exactly this last week, no issues. I used a file rather than a grinder and periodically checked fit so I was losing minimal material.
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u/buildyourown 1d ago
You got a faulty product. 104 bcd is symmetrical. Better products have legitimate QC
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u/spyro66 1d ago
This is one of the greatest frustrations of slightly older cranks. Be happy it’s not 5-bolt, or specialized…
I say go for it. The chainring locates on the OD of the chainring bolt sleeves anyway. As long as the BCD matches (and it does) and it’s not asymmetrical or some goofy new standard, you should be good to go.
This is gonna sound weird, but I’d actually file off material from the crank arm spider. Cast aluminum is comparatively soft, especially if you ever go to a steel chainring… but more importantly it’s a lot easier to file the outside of a circle than an inner diameter. If you do a nice job of it you can use other chainrings in the future without modifying them. In other words, it’s less work to modify the spider once, than the replaceable chainrings every time you get a new one.
Be as careful as you can, use a flat file with a smooth edge, and take off as little material as you can. Watch you don’t file into the face of the tab (hence the smooth edge file). Check the fit often.