1997 Trek 820 Replacement Crank Problem

[wfcollins]

I am working with my 14 year old son to rebuild an old Trek 820. It had a Shimano FC-CT92 crank that had stripped gears so I bought a new FC-M131 to replace it. When I tried putting the new crank on, there are 4 tabs that are on the new crank that do not exist on the old one that are preventing the crank from turning once I tighten it up. I have read on this forum that the BB should be replaced with the crank. Is this needed to extend the crank out farther from the frame? I did not replace the BB as I was not sure which one to buy. I have been researching this for several days and I am not any closer. Any help would really be appreciated. I want to get my son interested in something besides his XBox. Thanks, Bill[/php]

[nfmisso]

Hi Bill;

First, verify that it has a 68mm BB shell width, best to measure or check Bikepedia - make sure you match the year, because sometimes it changes:
http://www.bikepedia.com/quickbike/BikeSpecs.aspx?Year=1997&Brand=Trek&Model=820&Type=bike#.Uvf_sMuPJy0

If the BB shell width is 68mm; get a Shimano UN55 in 68x118 or 68x122.5 size.
If the BB shell width is 73mm; get a Shimano UN55 in 73x118 or 73x122.5 size
http://www.amazon.com/Shimano-Square-Bottom-Bracket-68x118mm/dp/B005DTIDDY/
http://www.amazon.com/Shimano-BB-UN55-Square-Bracket-73x118-mm/dp/B00629OBB2/

My 1994 Trek 930 has a 73mm BB shell width.

Shimano recommends the 122.5, but I find that puts the chain rings too far from the frame for my liking.

You will also need a removal tool to install the new BB, and a torque wrench is very worthwhile. The 3/8" Harbor Freight one is what I use.
http://www.amazon.com/BIKEHAND-Bicycle-Shimano-Bracket-Removal/dp/B00811WQJS/
http://www.harborfreight.com/3-8-eighth-inch-drive-click-type-torque-wrench-807.html

I have FC-M131 on my GT, and we installed them on Alex's GT also:
http://forums.bikeride.com/thread-3167.html
http://forums.bikeride.com/thread-4930.html

If you are close, come on over, and I'll be happy to assist you.

[wfcollins]

Nigel,

Thanks so much for the fast response and all of the info. I will get the BB ordered and I may take you up on your offer if it is not too much trouble. I just live up the road in Menlo Park so not too far.

Bill

[nfmisso]

(02-09-2014, 10:36 PM)wfcollins Wrote:  Nigel,

Thanks so much for the fast response and all of the info. I will get the BB ordered and I may take you up on your offer if it is not too much trouble. I just live up the road in Menlo Park so not too far.

Bill

Come on over. Saturdays are usually our bike working day.