My SM-BB52 is reaching its end of life. It is third time I am replacing it over last decade. Generally cannot complain as new fitted part usually lasts me 2-3 years and replacement procedure is rather painless. For a part that costs ~£15 this is better than average.

I was thinking of replacing it with a higher spec part this time around. Because why not?

BB-MT800 from XT range appears to be the next thing from Shimano.

Does anyone have any first hand experience in upgrading from SM-BB51/52 to BB-MT800.

Some minimal searching on the interwebz only shows that there is a difference in BB tool interface: BB-MT800 needs a smaller tool and there is a plastic adapter usually included in the package. But what about performance? Are they better built? Do they last longer? Any advice appreciated. Will also consider advice for other parts as long as the price does not break the bank.


  • A higher quality item is always a better choice when it comes to parts such as bearings. The BB being in a quite exposed position, higher quality also means better bearings and seals.
    – Carel
    Aug 21, 2020 at 19:55
  • 2
    Note that "higher quality" does not necessarily correlate with "higher price". As far as engineering principles go, for example, I would trust the larger bearings in a Tiagra RS500 BB to have a longer life than the smaller bearings in an Ultegra RS60 BB. Sadly I can't comment on how this fares in real life, but it is something to consider.
    – jayded-bee
    Aug 2, 2022 at 15:00

5 Answers 5


I have a Shimano Deore FC-M591-S (silver) crankset on my touring bike. It came with SM-BB52. I also bought a BB-MT800 into my stock of spare parts. I have not installed the BB-MT800 because the SM-BB52 has not yet failed.

I can't say anything about the relative quality between SM-BB52 and BB-MT800, as I have not installed the BB-MT800 yet (and thus it hasn't failed and therefore I do not have its longevity figures), and because I have not caused a failure of the SM-BB52 yet so I don't have longevity figures for it either.

I picked the BB-MT800 into my stock of spare parts based on its price point. It in my opinion doesn't seem sensible to buy a cheaper part than BB-MT800 because to install the part it requires some amount of labor (I have to remove and install the crankset, and not only that -- my touring bike has a chainguard and that also needs to be removed and installed). Because labor has a high value, it makes sense to use that labor to install a high-grade part rather than a low-grade part.

The FC-M591-S came with a cheap bottom bracket, but if this wasn't the case, I probably would have bought a more expensive BB than the SM-BB52 in the first place.

The plastic adapter TL-FC25 seems to be included in the box. I have TL-FC32 but the plastic adapter can also be used with the TL-FC36.


As per the product sheet: the BB52 has "better sealing" for bearings, the MT800 has sealed bearings. But that doesn't say much...

I don't have first hand experience, but second-hand experience. My bike's brother came with BB52, and he replaced it with MT800. The MT800 has already outlasted the BB52 (the bike is a flat handlebar gravel, used on- and off-road).

  • First part of your answer is incorrect. BB52 has sealed bearings. I've taken one apart... Aug 2, 2022 at 7:55
  • 1
    @ArtGertner thanks for the feedback. Answer updated.
    – Rеnаud
    Aug 2, 2022 at 8:10

I ordered a BB-MT800 as a replacement for the BB-52 that came on my Trek Slash 9.8 XT, but I'm not sure it is compatible. The crank spindle/axle barely fit through the center of the BB. I had to hammer it in. And I definitely have increased resistance to crank rotation (pedaling). It won't swing around even one revolution when given a shove. Anyone know what I might've done wrong? I'm pretty sure I didn't overtighten the crank pretension.

  • 1
    You should post this as a separate question. Crank spindle is supposed to "barely fit" through bb, Otherwise it would be loose. You are not supposed to hammer it in, by doing so you might have damaged bearings which would explain increased resistance. Instead you should apply grease to the spindle. Aug 3, 2022 at 7:04

I have two mountain bikes, one has the (silver) Deore SM-BB52 and the other has the (black) XT BB-MT800, they are both good bottom brackets. I have had many of the XT bottom brackets over the years, and they seem to last well. The only annoyance is that they use a different tool and that the plastic adapter is fiddly to use. It was this that prompted me to try the Deore bottom bracket, which in my experience lasts almost as long as the XT, however, the silver colour would not look good on my other bike, so I forked out £20 for the tool to use the black XT bottom bracket on that. Now I have to tool, I will continue to use the XT on both.

TLDR: If you are happy with the Deore BB52, I would just continue to use that.


In my experience, and for some unknown reason (I guess the internal diameter of the bearings is not the same) BB-52 and BB-MT800 are not interchangeable. I also thought they would be, since they are both intended for cranksets with a 24 mm axle. I tried a BB-MT800 with a Deore 6000 crankset and the axle would barely fit through the BB, with a huge resistance to crank rotation. An alternative to BB-52 which does work is BB-MT501. It's basically the same BB but in black, which in my opinion looks better if you have a black crankset.

  • They are meant to be interchangeable, I have used both on the same bike, with the same crankset.
    – LC1983
    Mar 15, 2023 at 16:35

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