Hi. I have a Giant Acapulco mountain bike from about 1995. I bent the large front chain ring, so the front derailleur was not performing well. So, I decided to change out the crankset. It also seemed like a great way to learn more about my bike and how to repair it. Now that I have done so, I am wondering whether I made a mistake along the way.

The chain line is supposed to be 50 mm; I'm at 51.5 mm. The gap between the crank arms and chain stays are unequal--20 mm on the left and 17.5 mm on the right. There's also 5 mm of spindle showing between the interior-most plane of the left crank arm and the frame. Finally, the front derailleur will not reach the outermost chain ring.

Here are the before an after specs on my project:

Before:
Crankset - Sakae Microdrive SR Powerflo-F
Bottom Bracket - Unknown (replaced because the spindle width was too wide for the new crankset)
Front Derailleur - Suntour FD-XR15

After:
Crankset - Shimano Alivio FC-M410
Bottom Bracket - Shimano BB-UN26 68x113 (not E or K)
Front Derailleur - Same/unchanged

Am I expecting too much precision in the measurements or is something really wrong? Is my front derailleur simply incompatible with the new parts? Any help would be greatly appreciated.

1.5mm is less than a sixteenth of an inch. You should be able to make it work. Have you adjusted the high limit on the FD?

I maxed out the high limit, and the derailleur won't reach the outer ring. The spacing between the outer and inner rings are the same with both the old and new cranksets--15 mm. However, it appears the chain line is wider for the new one than the old one; the rings are further from the bikes centerline than with the old cranks. I just want to make sure I did not mess something up.

Is the unequal gap between the crank arms and chain stays meaningful or significant or is some variation normal? (As you can probably tell, I am new at this.)

OK, rereading your post, I see you are putting this on a mountain bike. I am assuming your BB shell width is 73mm (MTB) not 68mm (road). If this is correct, installing a BB designed for a 68mm shell will push everything toward the drive side 2.5mm. You should be able to get an MTB BB with a 113mm spindle to take care of the problem.

Thanks for your reply. I measured the frame for the bottom bracket, and it is 68 mm. Maybe it's not a "real" mountain bike frame? It had a real low price point. Also, the BB-UN26 68 x 113 is the one Shimano recommends for my new Alivio crankset, and Alivio is a mountain bike component line. Is it possible I did something wrong in installing the bottom bracket? I used a torque wrench to get it right. And I don't see how I could have gotten it too far in on one side or not far enough in on the other. But is that possible? Also, if the same bottom bracket shifts to the left 1.5 mm, that would increase the inequality in spacing between the crank arms and chain stays by that much also--to 21.5 mm on the left and 16 mm on the right. Correct? Please let me know if anything I'm saying doesn't make sense.

Alivio is used in the MTB, hybrid and comfort lines which is why they give specs for 68mm and 73mm BBs. Installing the BB from the drive side you tighten it fully so there is no way to have it too far out unless you are using spacers. With the high limit backed out fully, have you physically tried to move the FD out to see how far it goes or only used the shifter? Also, as that is, I believe, a triple FD, is it possible a part of the inner plate is hitting and not allowing it to swing all the way out? Aside from those, I'd go to the Park Tools repair site and follow the steps from beginning to end on setting up the FD and see if that solves it.

As you mentioned, I installed the BB from the drive side and tightened fully using a torque wrench. Then I installed the opposite side. The FD does not touch the chain rings at all. I removed the cable to the FD, unscrewed the high limit stop screw, and pulled on the FD to see how far it would go. It reaches but does not clear the outer ring. Frankly, if the chain line were 50 mm rather than 51.5 mm, I'm not sure the extra 1.5 mm would get the FD to clear the large ring enough for the chain to operate at all or effectively. I'm wondering whether the Suntour FD-XR15 derailleur is for a 47.5 mm chain line? I could not find any specs on it.

I think I'd do the eyeball test.
Put the chain on the middle chain ring and cog (4th, assuming you have a 7 speed rear)
From the rear, sight along the chain and see if it angles to one side or the other.
IF it seems close, repeat, using 3 & 5 and see if they look equally bad.

BTW, could the chain rings be installed "flipped"?

older mtbs had 47.5 chainline

I performed the eyeball test. The chain appears to go away from the bike, to the right, as I looked toward the front. That would seem to be consistent with my finding that the chainline was outside the 50mm spec, but visually, the chain appears to veer to the right more than the 1.5 mm difference between the 50 mm spec and the 51.5 mm I'm finding when I measure the chainline from the front. Could it be that the both the front and rear derailleur are designed to operate with a 47.5 mm chainline (as reptilezs indicates)? That would certainly explain the result I just got; the chain's divergence is more consistent with a 4 mm difference between the front and back than a 1.5 mm difference. Am I misunderstanding something, and does any of this really matter? I could replace the front derailleur for about $15, and maybe that's the way to go. But then do I also need to replace the rear derailleur and other things? Would changing any of these parts make a difference at all and could I have done something wrong that I have not thought of?

Sounds like you just need to go to a shorter BB.

Have you thought of swapping the rings over from the new crankset to the old crankset? An additional plus is that old Sakae crankset almost certainly looks nicer than the new Alivio.

If you're set on using the new crankset, you don't need as much clearance as you have between the chainstays and crankarms, so you might be able to get away with a significantly shorter BB than specced (110 or even 107) as long as there's clearance between the BB shell and the crankarm. A modern 113 mm MTB crankset on a 68x113mm BB often has quite a gap there; does yours?

Old Suntour components are lovely, and it would be great if you could make this work without putting on a new FD.

I wouldn't worry too much about the small differences in chainstay clearance.

I can't remember how those BB's work, which is the fixed cup. Would putting spacers on the non-drive side move the spindle (and therefore the chain rings) toward that side, in other words closer into the frame?

If not, my advice would be to try to find a similar BB with a shorter spindle and maybe adjust with spacers on the drive side if needed.

Thanks everyone for your input. Re dave35, the old BB has a wider spindle width than the new one and made the new crankset stick out even further. As for those who suggested the shorter BB, that was something I thought as well. Shimano's BB-UN26 comes in a 68x110, which would bring the chainline down to 50 mm. Although I don't think that would help with the FD (or even the RD, as reflected by the "eyeball test"). I may have to go shorter. I'll certainly look into it, although it's a little frustrating since I used the one Shimano recommended. I even called them, and they confirmed I should be using the 68x113. Ugh! Well, I can certainly say I am learning a great deal (I'm just not sure what overall lesson to take away from the experience yet). Thanks again.

Be aware that the difference with a 110mm BB is more than 1.5 mm; if I remember correctly, 113mm BBs are slightly asymmetrical, and most of the extra length is on the drive side. This might be the case with going 110->107 as well; expect 4.5-5mm movement on the right side and just a little bit on the left (my best guess; I've never used a 107 myself).

Oh, that's good to know. I was considering the 107. I'll see if I can find specs on the 110 first and compare it to the specs on the one I have now to see how things might change. Thanks again.