Richard,
British, ISO, Italian, French and Swiss adjustable BB cups on the left side of the frame have RH threads.
Swiss, British and ISO BBs have LH threaded fixed cups (on the right side of the frame).
Italian and French BBs have RH threaded fixed cups (on the right side of the frame).
RH threaded pedals on the right side of the bike can work loose from rotation. That's why LH threads are used on right side pedals. Marten Gerritsen gives a good explanation.
Same thing with Swiss, British and ISO BBs. LH threaded fixed cups will not usually work loose.
Your other question as to why threads do or don't come loose; there are a number of causes but loose fitting cups are probably the biggest reason for them coming loose.
Fasteners and parts made with machine thread specifications (as opposed to wood screws, sheet metal screws and so on)are manufactured to tolerances appropriate for there application.
Standard thread sizes are nominal; for example a 1/2" bolt is not exactly 1/2" in diameter (same for metric sizes).
Most fasteners and parts that are designed to be disassembled are made slightly undersize and the mating parts oversize to allow some "slop" for easy of assembly. In manufacturing this is referred to as class of fit. Fasteners such as headless studs that are permanently screwed into parts are made oversize to provide an interference fit so they don't come
On production frames, BBs were usually machine tapped. If the taps were worn then they produced undersize threads. Any burrs left on the BB threads can make installing and removing cups difficult
Hand tapping BB threads can easily result in oversize or "bell mouthed" threads. When someone "chases" the threads with a set special BB taps to "clean up" the old threads they can multiply the problem.
There are some widely held misconceptions concerning how precisely bicycle components were manufactured.
I checked a lot of brand name cups with a micrometer and found that I couldn't trust the results if the size wasn't marked on them. I had to check them with thread gages to determine if they were metric or British threads. I've found a number of Stronglight and TA cups with mis-marked ring codes.
This spring I put together a frame made in the early 60s by a US builder. It has a British threaded BB. I wanted to use a Stronglight 49 crankset. I sorted through over a dozen Stronglight, TA and even Campy cups before I found a pair that didn't rattle around too badly. I used heavy duty Loctite to keep them from coming loose.
Chas. Colerich Oakland, CA USA
r cielec wrote:
> Ahoy !
> The recent discussion of Swiss threads has prompted me to expose another empty space in my head where knowledge should have been filed many years ago.
>
> Bottom bracket cups and threads:
> Any mechanical engineers out there who can explain why the cups remain tightened or why they loosen ( I mean with proper installation not because of error, cross-threading, neglect) ?
> Re: Threads are opposed RH and LH ? Does it matter which side is RH or LH ? Can LH on right with RH on left still keep the bb cups tight?
> Would a LH-LH loosen up?
> Why Italian RH-RH and only one lock ring?
>
> Thanks
>
> Richard Cielec
> Chicago, Illinois; U.S.A.