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I am not sure if this is a stupid question or not. Finding compatible parts is not so easy for MTB, after quite a search I found my bottom bracket shell is not so standard, PF107.

I bought a Rotor raptor bundle, this includes a supposedly-30mm external-diameter spindle axle that should fit the Rotor bottom bracket 4130, which supposedly has 30mm internal-width. I did not buy the wrong parts, they are mentioned in the manufacturer website, even the BB says it is compatible with the axle.

However, there is no way I can put the axle through the bottom bracket, at least with my hands. They both have 30mm exactly, I would say either of them should have a slightly different measure, so that the axle can slide into the bottom bracket.

I am new to fitting MTB parts, but my road bike doesn't have this issue, the spindle goes through the bottom bracket effortlessly.

Hence the main question is: Should the axle fit by force into the bottom bracket? (I would need to apply quite some pressure, probably just like fitting the bottom bracket to the frame).

If not, would I have the same problem with other 30mm bottom brackets?

I am wondering if this is a design mistake on the bottom bracket or on the axle. If most 30mm bottom brackets have slightly more than 30mm internal diameter, then a 30mm diameter spindle can fit, and I can just return the BB-4130, go and buy another BB107 for 30mm and solve the problem.

Otherwise, if the axle is too wide (so, most 30mm spindles are slightly less than 30mm and this one is not), then it is pointless trying to fit this crankset and I should return all the products right away.

I do not have another 30mm crankset or BB to know what's the issue, my road bike uses 24mm.

Any help is much appreciated. Thanks!

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    Having a caliper to measure diameters of the spindle and the BB holes could help with understanding what is going on. Maybe one of the components is defective and is not truly round (measuring at several different positions is required). On the other side, I always use a wooden mallet to slightly whack the spindles of my Hollowtech II and GXP 24/22 cranksets to drive them in and out, as there is very slight interference fit between them and the BB cups. – Grigory Rechistov Oct 9 '20 at 18:38
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    @GrigoryRechistov Thanks for your comment. I did measure both with a caliper, they are exactly 30mm, I measured on several parts. If I measure for example the headset bottom cup and the headset bottm shell of the frame, they are exactly 56mm. This I expect because they fit by pressure. But shouldn't a MTB crankset go in like a road crankset? I feel I should be able to slide the spindle axle into the bottom bracket with my hands, not even applying grease (which I already have done) – Ricardo Garcia Oct 9 '20 at 18:57
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    They can sometimes be tight (road or MTB doesn’t matter). Tolerances, BB shell accuracy, etc all add up. Give it a healthy whack and see if it goes in. – MaplePanda Oct 9 '20 at 19:34
  • @MaplePanda thanks for your comment. What I do not want is to ruin the parts. Maybe the bb is not so expensive, but adding the axle to that makes it not so little money to throw into the garbage. Specially when I can return the parts. But if it is the same with all other spindles/BB, then I'd rather keep the parts I got and smash them in. What do you think? – Ricardo Garcia Oct 9 '20 at 19:48
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    @RicardoGarcia Just give it a light whack. Start with just using your palm, then move to a hammer and block of wood. Keep the force reasonable (no sledgehammer, no gorilla antics), and you will not break anything. With the hammer, I’d give it three whacks total and then give up. You could also try bringing it back to the shop along with the bike and letting them try installing the crank. – MaplePanda Oct 9 '20 at 21:40
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There is no difference in the fit expected based on road vs mountain.

I thought I would be able to provide tolerance numbers from the original BB30 technical documents put out by Cannondale. They used to be at www.bb30standard.com, which is defunct now but can be found on Wayback. However, turns out it seems to have only ever had shell dimensions. The spec seems to have left the specifics of spindle and bearing tolerances to manufacturers.

Generally speaking, it's a pretty tight slip fit, same as you're used to. I understand that you've measured with calipers and gotten 30mm exactly for both. With higher precision tools you could see more of the story, i.e. if something is out of round or if the parts are actually requiring a press fit or what. (Note that among commonly encountered vernier calipers, some have .05mm precision and some .02mm, which could be enough to make the difference in this case, which it usually doesn't with anything bike.)

I've encountered similar situations where it took some force to get it all together, but if it was a press fit I would be unwilling to do that unless Rotor told me it was how it was supposed to be, which I don't think it is.

Hammering in the spindle is not a good idea because side loading the bearings like that can damage them. It's the peak loads from hammering that can do it, and knowing how much is too much is a guessing game. Gentle taps are probably okay, especially if it works, but I still don't love it. I believe the notion is that radial bearings are supposed to be able to handle 10% of their allowable radial loads in the lateral direction. Usually when it's tight I try to walk the spindle into the first bearing by wiggling it up and down with force behind it. I deal with a lot of BB30 and do not find myself having to hammer cranks in basically ever.

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  • thanks for your answer. After a few responses it seems that it is not so easy to tell if they fit while being out of the frame. So, what do you think, should I take the risk of ruining them both by installing the BB and see if the spindle goes in? the parts are brand new and I just received them, so I can return them – Ricardo Garcia Oct 9 '20 at 20:06
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    I didn't understand before that the BB isn't installed yet. Yes, I would just go for it. Try walking/wiggling it in like I mentioned. Use a lot of grease. I personally think that hammering the spindle through is bad form because it's difficult to say whether the peak loads are within what the bearings can take without picking up damage. – Nathan Knutson Oct 9 '20 at 22:38
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    Although the real first thing I would do personally is try to get a micrometer or a caliper with 0.02mm precision on it to see what's really up. If my spindle numbers were coming out bigger than the bearing ID numbers, I'd go for the return. – Nathan Knutson Oct 9 '20 at 22:44
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The fit off the axle in the bearings needs to have no play between the race and axle. The axles are designed to be tighter than a 'slip' fit, but not a full 'interference' fit. This means the axle will go through the bearing inner race with a little force. Often, hand force is not enough.

You can use a soft mallet to carefully tap the crank through the bearings. It's a good idea to protect the drive side crank arm with a few layers or rag.

See this Park Tool Repair Help page: https://www.parktool.com/blog/repair-help/crank-removal-and-installation-two-piece-compression-slotted

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  • thanks for your answer and for the link. So, would you take the chance of installing the BB into the frame and trying to slip the axle at the risk of ruining them both? Now I can just return them – Ricardo Garcia Oct 9 '20 at 20:02
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    Agreed on the soft mallet - rubber coated ones are available for this purpose. The instructions for my Quarq cranks explicitly say to tap in with a mallet, whereas I don’t believe Shimano asks for this. – Weiwen Ng Oct 9 '20 at 20:30
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    @RicardoGarcia I’d say go ahead and install, the axle is supposed to be a little tight – Argenti Apparatus Oct 9 '20 at 23:51
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Update: I think you all were right. I have not installed the bottom bracket yet, I want to have the rest of the parts of the bike before (I am building it); But I used a bank-screw to hold one side of the bottom bracket and used a mallet to slide it in. It went through, and after the first section it could slide. It seems that the first part of the axle is wider by a margin my caliper cannot measure, but enought to make it tough to get in.

I must say that it was just as tough to get it out of the BB, but I guess that's not a big issue, as long as it does what it is supposed to do.

Thanks all for your help!

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