I have a fairly new Giant XTC 800 MTB. I use it for commuting and decided to change out the stock FC-M612 crank (40-32-24T) for a FC-T551 (48-36-26T) crankset. With the M612 I was spinning out in the highest gear on flats and wanted to increase the top end gearing. I researched quite bit before I settled on the T551. It appears to have identical specs as the M612 with the exception of the taller gears on the T551. (Chainline is the same (50mm), spline length, diameter, both are spec'ed for the same bottom bracket (BB51/BB52)

My problem is, when I went to install the T551, I wasn't able to thread the crank arm fixing bolt on the NDS. It's almost as if the spline on the T551 is too short. However, I measured and compared everything with the original crank and all the numbers match up. I didn't touch the bottom bracket. And I tried to thread the crank arm fixing bolt as directed before tightening the pinch bolts. Looking at the spline, it is about 2 or 3mm shorter than the outside of the crankarm. Just short enough for the crank arm fixing bolt not to "bite".

Looking at the exploded view drawings for both, the M612 calls for the standard Shimano configuration of two 2.5mm spacers on the DS bottom bracket and a single 2.5mm spacer on the NDS bottom bracket (which matches the current configuration on the bike). The only difference is the exploded view of the T551 shows a .7mm + 1.8mm + 2.5mm on the DS and a single 2.5mm on the NDS. Obviously, it's the same dimension, just more spacers. One other difference on the T551 is there is a 6.5mm spacer between the outside of the BB and the largest chainring. However, I measured from that to the end of the spline and it's the same length as the M612 spline.

I have whacked the crank on the DS with a rubber mallet and I am sure it is completely seated. Am I missing something here? Hopefully I've provided enough information that you get the idea of what I'm talking about. If you have any advice, I appreciate any help you can give me.

Your new crank is a Trekking crank, perhaps designed for 68mm bottom brackets. Are there any spacers you can remove from either bottom bracket cup? That "fixing bolt" is to provide bearing preload, it does not have to be very tight

Per this document there is a different arrangement of spacers for 73mm and 68mm BB shells: https://www.btwin.com/notices/wp-cont...9/99deravt.pdf

I'm thinking you probably have a 73mm BB shell in which case you would use only one spacer on the drive side, none on the non drive side.Except in the presence of an E type derailleur in which case, no spacers.

I agree with the last poster, it sounds like you have a 73 mm bottom bracket. The "Shimano standard configuration" the OP refers to (7.5 mm of spacers total) is for a 68 mm wide bottom bracket shell. A 73 mm wide bottom bracket uses only 2.5 mm of spacers.

Thanks for your feedback!

Sorry, I failed to mention that the BB shell is a 68mm shell (I measured it) and this is a MTB setup. The currently installed BB is a BB51 which is what the M612 calls for. The T551 is spec'ed for a BB52, which, by my understanding, is the same as the BB51, just upgraded and replaces the BB51 in the Shimano lineup.

I've thought about just removing the NDS-side BB spacer (2.5mm) since I have plenty of room between the crank arm and the chain stay on the left side. I wasn't sure if this was a good idea or not (changes the symmetry, but the chainline doesn't get changed). By the way, I took the T551 crankset off the bike and threaded the fixing bolt onto the spline to eliminate any doubts about whether it fits or the threads were messed up. No problem with that.

I knew this was a Trekking crankset (hence the "T" in T551), but when I looked at all the drawings and the current BB setup, I couldn't find any reason that it shouldn't work. I did have to change out the front derailleur to accommodate the change in the chainring tooth counts, but obviously that has no bearing on the installation of the crankset (with respect to the fixing bolt). I was hoping I was just forgetting to do something that was obvious. If you have any other ideas, I appreciate your help.

If your bottom bracket has spacers on both sides, you have to measure bottom bracket shell width to the outside of the spacers since they effectively widen your bottom bracket shell. A normal configuration if your old crank was made for a 73mm bottom bracket and was installed on a 68 mm bottom bracket would be 2 spacers on the drive side and one on the non drive side. If your new crank is made for 68 mm bottom brackets you have 2 remove 2 spacers to compensate for the shorter spindle on the new crank, usually one on each side