Those dang stripped threads
Dear Lennard,
What if anything can be done when the threads of a bottom bracket shellon an aluminum insert are stripped? I have not seen the bike firsthandbut I am trying to help someone out. They say that there is not enoughmaterial to cut new threads in. I assume that a tap has been runthrough already with no success. Is there any hope? I don't knowwhich side is stripped but if it is the non-drive side I can assume itis less critical. Is there a bottom bracket that will work better in thiscase?This is an old Look carbon time trial frame.
JasonDear Jason,
You could have it tapped it out Italian thread, which is slightly bigger,and put in an Italian-threaded bottom bracket. English thread is 1.37 inchX 24 tpi, which is 34.8mm X 24tpi, while Italian is 36mm X 24tpi. Sincean Italian bottom bracket shell is 70mm wide, rather than 68mm wide, youmay have to use a thin bottom bracket spacer between the drive side cupand the shell, to take care of the discrepancy.

If a frame already is Italian threaded, the only hope I know off isan old Mavic bottom bracket, later taken over by Stronglight. It is squaretaper and very hard to find these days, but it requires no bottom bracketthreads. Ideally, you cut a bevel in the faces on either end for the lockring;Mavic used to sell this beveling tool, but I know some shops still haveit.
LennardA drain hole?
Dear Lennard,
Thanks for the entertaining and informative articles. You have probablyaddressed this question before: Should I drill a small drain holesin the bottom bracket shell to allow rainwater an easy exit? TodayI replaced my bottom bracket (worked fine, different brand of cranks),only to find about 3 oz of water drain from the bottom bracket shell.The last time I rode in the rain was Christmas day (10 days ago).It rains here all winter through the end of June, therefore, I am boundto get wet again. I imagine that water and dissimilar alloys willlead to corrosion over time. Also, why bother with lightweight bikesand parts when you unknowingly carry 75 grams of water up a hill?What would you suggest? Will I find water in the forks as well?
RobDear Rob,
Yes, drill the drain hole in the underside of your bottom bracket shell.You won’t find water in a carbon fork, but in a welded fork you often will.
LennardWhat size is it?
Dear Lennard,
I want to upgrade my crankset so where do I find the bottom bracketsize? I looked underneath the bottom bracket housing and didn’t see anything.I have a Trek 5000.
ChrisDear Chris,
A Trek always has English thread. It will not say so on the frame,but it will say 1.37 inch X 24 tpi (English) or 36mm X 24tpi (Italian)pm the bottom bracket bearing cup. In general, it is always English threadon any frame later than 1980 or so, unless it is an Italian frame brand.
LennardRegarding my recent column on outboard bottom brackets, many of you had this same response, but here it is from the source:
Dear Lennard,
We have designed a tool that allows shop guys to pull apart Shimano,Race face, and FSA outboard bottom bracket s and replace the bearings withour higher quality bearings. The result has been a dramatic increase inthe somewhat short lifespan most of these bottom bracket s offer. Here'sa look at the tool and the instructions on how to use it.
Thanks,
Brent
Phil Wood & Co.Regarding New Group, Bad Shift:
Dear Lennard,
I experienced a similarly frustrating problem with my shifting on afull suspension bike last season. Your writer should also check thelengths of their housing. If the housing sections are too short thederailleur will move under suspension load. This unintentional derailleurmovement will be especially pronounced if the housing under the bottombracket is too short. Try longer housing under the bottom bracketif it is a FS bike. My problems were with SRAM XO shifter and derailleurby the way.
David
Arlberg Sports’

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Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides " Zinn & the Art of Mountain Bike Maintenance" and "Zinn & the Art of Road Bike Maintenance."Zinn's regular column is devoted to addressing readers' technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.