New bottom bracket for my old bike: UN51 to UN71

Shimano UN71 Bottom Bracket

Shimano UN71 Bottom Bracket

Insert text flowing to the horizon, star wars style:

Long, long ago, when the earth was still new, and I still lived in the big city and commuted to school on my bike… and used to walk to the video rental store holding hands with my girlfriend…

My much beloved Klein came off the roof of my car on I5 and was destroyed… visions of that bike being run over by a big white pickup (wow those wheels had a lot of travel…) I needed a new bike, using insurance I had enough to get a Kona Hei Hei, but not enough to actually pimp it out. I had a box of old parts, wheeled and dealed for the parts I needed, etc. One place I saved was to go for a Shimano UN-51 bottom bracket. It was quite cheap, I think the shop let me have a lightly used one for $10, and was notoriously reliable though heavy at an alleged 350 grams.

Here’s a link to bikepro description:

The better option would have been the UN-71, the XT bottom bracket. Supposedly the same reliability but with alloy body and cups it came in at 260grams. Unfortunately at the time it was $50-$60 and I just didn’t want to spend the extra cash on it.

Surely some time in the future I’d have the money and could upgrade. Was an obvious place to spend money, $50 to save a 1/4 pound.

So… time passes. Its now 20 years later. All this stuff is vintage and obsolete and not really available any more except via ebay at collector prices. I see one now for $89…

So imagine my surprise finding exactly the size I need in a bin at my favorite bike store. Awesome!

No matter that I barely use the bike any more, here I can achieve a goal from long ago. Accomplish something! I buy the thing, I think $2, and also the correct park tool, I think $10. Arriving home I take off the crank and attempt to disassemble. I use my largest wrench but the cups won’t budge. And it brings back some distant memories.

When I got the bike in the early 90s I was ecstatic, a new baby. But the next day the bottom bracket started to creak. Notorious ti-frame bottom bracket creak… Take it back and bike store lubes and tightens. Starts creaking again a week later. Bring it back. They take the thing out and spooge a bunch of loc-tite onto it… No more creaking… but dang its going to be difficult to remove I suppose. Hope it wasn’t Loctite Red.

I use a handy breaker bar and am actually afraid for the bike. It really is stuck, no motion. Shit.

So into a bin the bottom bracket goes. Every few months I come across it and frustration sets in. Sigh.

Recently I obtained a Milwaukee corded impact wrench, for working on my truck’s suspension but didn’t really think about that bottom bracket until I came across it while searching in my parts box last weekend.

Off come the cranks, on goes the tool, the impact wrench. Luckily I had the UN-71 by my side to check which way the threads turn… I hit it with the wrench and my ears are assaulted. No motion but I expect it will take some time. I fetch my ear protection and sit there with the wrench firing, 15 seconds to the left, 15 seconds to the right and finally I start to see motion. Start to back the drive size cup out, maybe 30 degrees a minute. Very slowly but sure enough it is coming out!

After a few minutes the area starts to smoke, the Loctite is burning and a few minutes later the whole thing spins free in a cloud of smoke. Awesome! Glad I’m wearing thick gloves.

The other cup comes lose in another 5 minutes of noise and smoke. The edges of the park tool are shiny, starting to wear from that ridiculous abuse.

Check the inside of the frame, looks like new.

I take the two bottom brackets into the kitchen to weigh. That UN51 is only ~60grams heavier. Hmm. I blame the marketing folks. Maybe UN51 measurement included the crank bolts?

UN51 is 325g, UN71 is 265.

Shimano UN-51 Bottom Bracket (after living in my bike for > 20 years)

Shimano UN-51 Bottom Bracket (after living in my bike for > 20 years)

Two differences: the cups and bottom bracket body are steel on the 51, alloy on the 71.

Steel cups from UN-51, 24g.

Steel cup from UN-51, 24g (48g for pair).

Alloy cup from UN-71, 12g (24g for pair), cups alone only save 24g compared to UN-51.

Alloy cup from UN-71, 12g (24g for pair), cups alone only save 24g compared to UN-51.

I lube up the new bottom bracket with high quality synthetic suspension grease and install. Give it a few extra seconds with the impact wrench after it stopped spinning, install the cranks and go for a ride.

Bike SEEMS lighter but that might just be a little more flex? I dunno, not really important. No creaking.

Fulfilling to achieve an important life goal on a nice Sunday morning.



About esc

I'm a programmer in the Seattle area. Mostly work on compilers but also on optimization. Currently focused on gpu stuff...
This entry was posted in kona, shimano, titanium, titanium frame and tagged , , . Bookmark the permalink.

2 Responses to New bottom bracket for my old bike: UN51 to UN71

  1. andrew says:

    Did you find that the crank bolts were the same for the UN-51 and UN-71? I am replacing my 51 on a 93 Stumpjumper with a 71 and a m739 XT crank but the 71 off of Feebay did not come with bolts. Is it safe to assume I can use the bolts from my LX crank and UN51 for the new XT/71 setup?

    • zakthor says:

      The same bolts were used for both. I actually use some nicer looking ones made by suntour (I believe the suntour bolts I have are made from a high nickel stainless).

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