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  1. #1
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    Mid Bottom bracket shell tolerance

    Bit of a strange one here and I realise I'm being lazy and weird but here goes:

    Here's the weird bit: I've got a jump bike with a Mid bottom bracket shell. It has crappy heavy steel cranks on it, I'd like to run something nice, basically 24mm shimano standard. I've found the dimensions of the Mid bearing (19.05x41.275x11.049t) and found that they have a 68mm wide shell. I realised that a bb30 adapter bottom bracket (specifically this one from Raceface Race Face) will work with a little machining on the press fit area.

    Here's the lazy bit: I won't be able to measure the ID of the BB shell until after the holidays but I'm at the workshop on Monday so I'd like to machine the Raceface BB then. So, does anyone know what tolerance mid shells are typically done to? They are nominally 41mm, so I'm thinking they will be something like 40.95 +/- 0.05. The instructions in the Raceface BB ask for a shell that is 41.96 +0.025 -0.0 and the cup measures 42.06 so that's a difference of 0.075-0.1mm.

    So I'm thinking machine to 41.1mm and expect it to be on the snug side, remachine if needed?

    Does anyone build with mid shells and know what they ream to?

    Thanks,
    Matt

    PS I hate bb standards but thhis seems to be working in my favour of making something work when it really shouldn't

  2. #2
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    a Quick update for reference:

    Mid Bottom bracket shell tolerance-img_1816.jpg
    Here's what I started with, heavy steel cranks on the right with bearings etc in place (seriously overkill bearigns, can't imagine people need to replace these very often) and the Raceface bb and Middleburn cranks to go on.

    Mid Bottom bracket shell tolerance-img_1818.jpg
    So I made a quick mandrel to hold the cup like so

    Mid Bottom bracket shell tolerance-img_1819.jpg
    I measured my shell at 41.3mm, so I didn't have to remove much. I machined the 42mm section down to 41.45(ish)mm

    Mid Bottom bracket shell tolerance-img_1820.jpg
    In the frame with cranks on

    Mid Bottom bracket shell tolerance-img_1821.jpg
    This completes my conversion from singlespeed with heavy cranks to 1 x 3 with nice cranks. I will be putting a narrow/wide ring on at some point.

    Matt

  3. #3
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    Excellent work, great to see a somewhat easy solution for upgrading what I would consider to be a useless bottom bracket standard!

  4. #4
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    Thanks for this, I have an All City frame that has a mid BB in it and I've been doing a bit of research to see how I could go about fitting X-type or GXP cranks rather than the standard BMX style cranks.

    BTW, those "crappy heavy steel cranks" look like Profiles, they're good stuff for their proper application.

  5. #5
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    "Mid" is a bmx standard that has been around for over a decade on chromoly dirt jump/street/park frames. it has all but replaced every other option on bmx frames. I am surprised to see it on a mtb, unless it is a dirtjump/park frame, basically a big bmx bike. if you are going to run a single-ring up front, there are dozens of crank options that do not involve modifying anything. Odyssey Twombolts or Lightning Bolts, Profile, Primo Hollowbites or Powerbites, plus tons more.

  6. #6
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    Trust me, they are not profiles, the bike is a 2013 Saracen Amplitude jump bike, the cranks are described as "4130 Cro-Mo 48t spline cranks 2-piece design with 25T alloy chainring". For my purpose they are overkill and heavy.

    There may be other crank options out there to run a single ring, but I already had the Middleburns, a set of SLX and deore hollowtech 2's so wanted to use a 24mm axle. Plus, this was pretty simple to do and if anyone else wants it doing I'm happy to. I'm not sure about the options for GXp as I had no need to look into it, if Sram/Truvativ do a similar adapter it should be equally easy.

    Matt

  7. #7
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    Quote Originally Posted by 18bikes View Post
    "4130 Cro-Mo 48t spline cranks 2-piece design with 25T alloy chainring". F
    aka Profile knockoffs. they are probably about the same quality and weight as Profiles. however, it sounds like your mod worked well, good job!

  8. #8
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    Quote Originally Posted by mack_turtle View Post
    aka Profile knockoffs. they are probably about the same quality and weight as Profiles. however, it sounds like your mod worked well, good job!
    Whoah! Don't drop Profile down to that level! Profile makes awesome products, American made and durable. Anybody can make a 48 spline knockoff but it won't be Profile quality. I love my Mid BB on my BMX bike. Its different in BMX than MTB or road. American BB's are worthless, Spanish BB's have the tendency to blow out easily and Mid is the happy balance. In BMX, the Mid BB standard is arguably the best solution on the market.
    My motorcycle runs on infant blood

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