cpsqlrwn

How would I determine the freewheel threading on an Atom hub, vintage early 80's. These are not the helicomatic hubs. This is on a 1982 Trek 613 and seems to be original equipment based on the Trek catalog. Wouldn't think this would be French threading on a Trek. Would there be a marking somewhere or do I need to remove the existing 6-speed freewheel and inspect or measure? Wanting to upgrade to 7 speed and wondering if a Sachs freewheel will work. Sheldon Brown's freewheel threading chart below. French is a bit of an odd one. Thanks for any info!

Italian 1.378" x 24 tpi
ISO 1.375" x 24 tpi
British 1.370" x 24 tpi
French 1.366" x 25.4 tpi

100bikes

I believe that Atom/Normandy/Maillard freewheels were marked on the back side of the body if they were French threaded.

Though I have never seen an Italian threaded version, it would follow that it would be clearly marked as well.

British/ISO are definitely the most common.

rusty

steine13

Press a piece of aluminum foil or even simple paper against the threads

Put no force on it after. Measure the widest range you can, count threads, that'll give you TPI. Easier than eyeballing with calipers.

JohnDThompson 

Try to thread a known, English thread bottom bracket lockring onto the hub. If you can thread the ring completely on, without excessive force or jamming, the hub threads are English. If the ring jams within a thread or two, it is French thread.

dddd

I suspect that in many or most cases, the relatively short length of engagement of just the lockring might allow it to fully spin on w/o force.

But an English adjustble bb cup should not thread fully into any French freewheel.

Trek never used French-threaded anything, other than the Helicomatic hub's threading(?).
Even Schwinn before them, had French parts made using only BSA dimensions.

oneclick 

Ok, here's what I can tell you: In that bin over there are
two Atom Bte. SDGC, both unmarked, both metric;
two Atom 77, both marked, one english, one metric;
two Cyclo 72, both unmarked, both metric;
six Maillard, examples each of marked english, marked metric, unmarked english, unmarked metric.
I also remember that there is a metric Suntour (New?) Winner on a bike somewhere that's marked M, and there's another metric freewheel somewhere (else) but he's unlikely to have one of those.

The easy way to tell threading on a freewheel (and hence also the hub it came from) is to mount it upside-down on the removal tool in a vice and see if an english adjustable BB cup will thread in. If it does so easily for some turns and then binds quickly, it's metric. The sudden binding is a result of manufacturing tolerances not being enough to hide the increasing mismatch of the thread pitch - it's ok until there's no more room, but then it stops.

Don't do this without a cup-removal tool.

dddd

The only thing that I will add to that is to always lubricate threads of any parts that are to be test-fitted, to make the test and the removal process much easier.

Edit, another also, tolerances and clearances aren't the same thing at all. Tighter manufacturing tolerances of both mating parts does allow for the use of "tighter" clearance specifications though, so the two are related. But the difference is more than semantics.

cpsqlrwn

I have been out of touch with the forum for a few days, but I just wanted to thank all those who responded with some very good information and suggestions. I should be able to move forward with this and resolve my questions.