[The_Bob]

Hi all
Running a Shimano XT (FC-8120) crank on my 2022 Trek Top Fuel. It has a 55mm chain line and I’m puzzling about the spacers that need to be installed. When I took the crank off to grease it there was one 3mm spacer on the drive side. I had a look at the dealer manual and it seems to indicate that two spacers are needed one on each side of the BB. I checked the Trek manual and it has no info, though the close up picture of the non drive side of the bike seems to show a spacer installed. So, my question is, do I need to track down another spacer, or move the spacer to the non drive side or leave it on the drive side?

 

[Slowanimalswin]

Not sure about your specific crankset, but I bet the answer is in the service manual here:

https://si.shimano.com/pdfs/dm/DM-MAFC002-05-ENG.pdf

 

[The_Bob]

I checked the manual and it seems to say that two spacers are needed (one on each side of the BB). My 2022 Top Fuel and a friend’s both have one installed on the drive side, so did the 2022 Scott Spark I had for a brief spell. So are all three bikes incorrectly assembled or is the manual incorrect or unclear?

 

[*OneSpeed*]

Hi all
Running a Shimano XT (FC-8120) crank on my 2022 Trek Top Fuel. It has a 55mm chain line and I’m puzzling about the spacers that need to be installed. When I took the crank off to grease it there was one 3mm spacer on the drive side. I had a look at the dealer manual and it seems to indicate that two spacers are needed one on each side of the BB. I checked the Trek manual and it has no info, though the close up picture of the non drive side of the bike seems to show a spacer installed. So, my question is, do I need to track down another spacer, or move the spacer to the non drive side or leave it on the drive side?

I'd put the spacer back where you found it, reassemble, then measure the chainline. I'm guessing it's either 52mm or 55mm, probably 55 with Shimano I think.

The correct chainline is really the goal, regardless of how you have to move spacers to accomplish it.

 

[J.B. Weld]

I checked the manual and it seems to say that two spacers are needed (one on each side of the BB). My 2022 Top Fuel and a friend’s both have one installed on the drive side, so did the 2022 Scott Spark I had for a brief spell. So are all three bikes incorrectly assembled or is the manual incorrect or unclear?

Does your bike have a 73mm threaded bb shell? If so there should be one 2.5mm spacer on the drive side (according to the provided tech doc)

 

[BrianU]

Hi all
Running a Shimano XT (FC-8120) crank on my 2022 Trek Top Fuel. It has a 55mm chain line and I’m puzzling about the spacers that need to be installed. When I took the crank off to grease it there was one 3mm spacer on the drive side. I had a look at the dealer manual and it seems to indicate that two spacers are needed one on each side of the BB. I checked the Trek manual and it has no info, though the close up picture of the non drive side of the bike seems to show a spacer installed. So, my question is, do I need to track down another spacer, or move the spacer to the non drive side or leave it on the drive side?

According to Trek's website your bike does have a 73mm threaded BB shell, so IAW page 10 of the Shimano document, one 2.5mm spacer is required on the right side of the BB shell.

BTW something to keep in mind as far as the photo in the manual that came with your bike, those manuals can tend to sometimes be generic and not model or version specific, let alone current. If Trek had used a Sram BB and crankset on a previous version or different model bike with a 73mm threaded BB shell, then 2 spacers could have been required.

 

[The_Bob]

Thanks everyone. I’ll check the chainline measurement and go from there

 

[GoldenPromise]

I have the 8120 cranks on my Yeti Arc, and it has the 3mm spacer on both sides. Since this bike has the PF92 BB, I can't speak to the BB spacers, but you should have one on each side as the spindle is longer on 8120 vs 8100 cranks (which only use one spacer).

 

[J.B. Weld]

Since this bike has the PF92 BB, I can't speak to the BB spacers, but you should have one on each side as the spindle is longer on 8120 vs 8100 cranks (which only use one spacer).

For a threaded 73mm bb shell there should only be one (2.5mm) spacer for both 8100 and 8120 cranksets. Drive side.

 

[Sparticus]

“Should.”
I admit I’m a tinkerer and therefore I’m typically looking for ways to make things better. Improving chainline — that is, reducing it as much as possible — is often an option. I bought some thin 24mm BB spacers from Wheels Mfg and never hesitate to use them to fine tune my bikes’ chainlines.
I shoot for 2mm between ring & chainstay.
=sParty

 

[J.B. Weld]

“Should.”
I admit I’m a tinkerer and therefore I’m typically looking for ways to make things better. Improving chainline — that is, reducing it as much as possible — is often an option. I bought some thin 24mm BB spacers from Wheels Mfg and never hesitate to use them to fine tune my bikes’ chainlines.
I shoot for 2mm between ring & chainstay.
=sParty

No problem as long as all the spacers on either side add up to 2.5 mm.

 

[BrianU]

For a threaded 73mm bb shell there should only be one (2.5mm) spacer for both 8100 and 8120 cranksets. Drive side.

Come on J.B., it surely can not be that simple.

 

[bayposter]

No problem as long as all the spacers on either side add up to 2.5 mm.

Spacers effectively widen the BB shell so it's the correct width for the crankset spindle. If 2.5mm of spacers are needed, then a 1.25mm spacer on each side would perfectly center the crankset. But two spacers are more of a headache and 1.25mm doesn't make a huge difference to your chainline, so many manufactures just provide a single 2.5mm spacer.

Putting the spacer on the drive side is preferred (and sometimes required) because it increases tire and chainring clearance. But if there's enough clearance, you can move the spacer to the non-drive side to tweak your chainline. I've done this before with no problems whatsoever.

By the way, I'm not a big fan of the 55mm chainline on the new Top Fuel. Unless you need it for more tire and/or chainring clearance, your drivetrain will run smoother and last longer with the normal boost chainline of 52mm. You can achieve a 52mm chainline by replacing the XT M8120 crankset with the XT M8100 (or the less expensive SLX M7100).

 

[The_Bob]

Turns out one of the bearings was seized, so a new FSA BB was installed. One Shimano spacer between the crank and the BB bearing, installing another one between the LHS crank arm and the BB bearing does not work. Mystery solved in spite of what the Shimano manual says

 

[BrianU]

Turns out one of the bearings was seized, so a new FSA BB was installed. One Shimano spacer between the crank and the BB bearing, installing another one between the LHS crank arm and the BB bearing does not work. Mystery solved in spite of what the Shimano manual says

Are you referring to the Trek manual that came with the bike or the Shimano document for your crankset that Slowanimalswin posted a link to?

 

[The_Bob]

Are you referring to the Trek manual that came with the bike or the Shimano document for your crankset that Slowanimalswin posted a link to?

Referring to the Shimano manual. Trek manual says nothing about how many shims are needed

 

[J.B. Weld]

Turns out one of the bearings was seized, so a new FSA BB was installed. One Shimano spacer between the crank and the BB bearing, installing another one between the LHS crank arm and the BB bearing does not work. Mystery solved in spite of what the Shimano manual says

Isn't that what the Shimano manual says? One spacer on drive side?

 

[The_Bob]

The manual calls for one spacer between the BB shell and the BB cup for a 73mm BB shell. It also calls for two spacers between the outside of the bearings and the crank arms.

 

[foggnm]

I have the 8120 cranks on my Yeti Arc, and it has the 3mm spacer on both sides. Since this bike has the PF92 BB, I can't speak to the BB spacers, but you should have one on each side as the spindle is longer on 8120 vs 8100 cranks (which only use one spacer).

My understanding is the ARC has a little different thing going on. The standard is a 2.5 mm spacer on the drive side bb cup on a 73mm threaded bb. For non xtr cranks there are also spindle spacers between the cranks and bb. This is all for boost 148. There are exceptions depending on bike manufacturers. For example I had an Evil v1 offering and the bb spacing was not standard. The best advice I ever got was to look at your chain line in the middle (6th cog) of the cassette and the chain line should be straight. It can be very hard to actually measure chain line based on all the different ways bikes are made... especially when talking about a few mms. But yeah always stick with the bike makers recommendations.

 

[foggnm]

By the way....I've been trying various 30mm cranks the past year: eewings, SixC, Atlas. My impression is that the Shimano XT 1x cranks are still the smartest buy available. A $28 bb, durable, light, lots of after market chainrings, and one of the simplest install procedures. They are, imo, the wise man's (or woman's) crankset!