Sunday, May 24, 2009

Rookie Mistake

This just in... mechanic defeated by his own hubris.

I'd pulled off a pretty slick repair on our tandem so it would be ready for the farmers' market yesterday. I was feeling pretty good about myself. There was even some mention of my candidacy for the coveted Greatest Living Bicycle Mechanic in Central Iowa title.

Not so fast, slick.

I've been chasing a creak around my main ride, eliminating possible causes through a slow diagnostic process. Today, I had enough time to pull the cranks and bottom bracket. The plan was to grease the shoulders of the bottom bracket cartridge where they sit in the cups and wrap the cup threads with Teflon plumber's tape. It's an old trick from my days as a Cannondale mechanic, since a dry BB install would invariable make a small creak that would resonate through those big aluminum tubes until it sounded like a door opening in a horror movie.

That was the plan, anyway. And it worked for a while. I got everything out, cleaned it all up, greased up the cartridge, wrapped up the cups, and got the right side cup back in the frame, straight out of Sutherlands.

So, I started the left cup. And it felt a little snug. Before you ask, yes, I did know which cup had left-handed threads. Straight out of Sutherlands, remember?

"No biggie," I figured. "The Teflon tape makes it tight." So I kept cranking.

And it got even more snug. "Man, I must have put too much tape on that thing," I figured. So I grabbed a bigger adjustable wrench, clamped that bad boy on my BB tool and kept cranking.

It got even more snug. I could barely budge the thing, and it was barely a third of the way in. Only then did Mr. Fixit think, "Huh, that doesn't seem right." So I started backing it out again.

Cross-threaded. The aluminum cup was absolutely shredded where I'd ham-fisted it sideways into the steel bottom bracket shell. This is a family blog, so let's take the "edited for TV version of Snakes on a Plane" approach and pretend that I said, "I'm such a monkey-fighting idiot, cross-threading this Monday-Friday bottom bracket."

Proof that someone or something up there is looking out for morons and klutzes, at least it was an aluminum cup that sacrificed its life for the steel bottom bracket shell. If I'd needed the BB threads chased late on the Sunday of Memorial Day weekend, I would have been stuck. And I made a decent save by pulling a cup out of my fixie that just happened to fit despite being an entirely different brand of cartridge bottom bracket. So, the fixie's down until I replace the cup, but at least my daily driver is going again.


Anonymous said...

Always good to see Samuel L. Jackson sneak into a bike blog. :-)

Was this quotation of Mr. Jackson the result of that same sick feeling produced by a bolt that suddenly gets *too* easy to turn? It's been a while since I've sheared off a bolt, and I haven't cross threaded a BB or BB cup (yet).

Any idea what caused the cross threading? Maybe the teflon tape was enough to mask the usual feeling of proper thread engagement? That would hint that perhaps one shouldn't put teflon tape over the first couple of threads.

Well, get the bike back up and running, mull it over, and let us know if there's a good way to avoid this.

Steve K.
Peoria, IL

Jason T. Nunemaker said...

That is *exactly* the feeling... like when a bolt gets tighter and tighter and tighter, and $&%*!!! It just spins.

I think the cup was already a little chewed -- it's always been hard to start. I'm guessing you're right, that the tape kept me from feeling the threads until it was much too late. For the second try with the borrowed cup, I used one layer of tape at the beginning of the threads and two layers further up the cup.

The offending creak's gone today, so I'm chalking this one up as a war that I won even though I lost a battle along the way.

Steve Fuller said...

Did you start said cup by hand, or with a wrench? I've learned (the hard way) that you want to start bolts and things like that by hand whenever possible. You don't want to know what I've yelled in that same spot in your garage as a steel bolt cross threaded into an aluminum engine block...