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I'm buying new freewheel and noticed that not every freewheel can be screwed onto my hub because they have different thread sizes.

For example: I've found the following thread size specifications:

M30x1
M35x1
"big" 1,37" about 34.8mm
1.375" x 24 tpi

I've read that some type can only use 16 or higher tooth so in case it is important, my current hub has a 16 or 18 tooth bmx freewheel screwed on.

Questions

How many are there and what are they called properly?
Will any work on some other type?
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tl;dr I just want to make sure I'll buy a freewheel which can be screwed on successfully. –  BMXftw Jun 5 at 17:35
    
Yes you have to match the thread size. You buy the size that fits your bike. How does it matter how many sizes don't fit your bike? The size or model should be stamped on the current freewheel. If there are no marking on freewheel and you don't know how to measure the size then just take to a bike store and tell them you need one that size. –  Blam Jun 5 at 18:09
    
@Blam According to online specifications it has "big" 1,37" about 34.8mm.Is this the same as M35x1 or something else? –  BMXftw Jun 5 at 18:24
    
@Blam "How does it matter how many sizes don't fit your bike?" I only care about which fit but if I know all that don't I can know all that do. –  BMXftw Jun 5 at 18:31
1  
1.375 x 24 is the standard size for BMX and easy to find. Why are you trying to match it to metric? Your hub has 16 or 18 tooth? Just count them. –  Blam Jun 5 at 20:15
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1 Answer 1

The ISO freewheel thread standard is 1.375" x 24tpi

Most freewheels I have encountered have been ISO.

Some hubs use "British" threading at 1.370" x 24tpi

If you use an ISO freewheel on a British thread, it will work, but you should check there is enough thread engagement to avoid stripping the hub. The trials-oriented retailer TartyBikes suggests 9 thread is enough (Trials bikes often run a freewheel at the crank using a 1.370" x 24tpi thread)

Italian freewheel thread is 1.378" x 24tpi: Fine on ISO, but even shallower thread engagement on British hubs than ISO on British. Still viable though.

French freewheel thread is 1.366" x 25.4 tpi or 34.7 x 1mm

If you tried mixing 24tpi with 25.4tpi at the same diameter, the threads would be strained and with enough engagement would eventually jam. As the French diameter is smaller, an Italian, ISO or British freewheel will just about thread on but will have very little thread contact and will slip under load, blunting the hub threads.

Metric BMX is M30 x 1[mm] or 1.181" x 25.4 tpi. This is a small enough diameter that all the above sized freewheels will be obviously sloppy on an M30 hub.

I've never seen an M35 x 1 freewheel, but it seems close enough to French that it should work.

Mini-freewheel (Which I've seen on some DMR Revolver hubs) is 1.18" x 24tpi

Mini-freewheel and Metric BMX are close enough to cause trouble. The diameters are within tolerance but there will be strain on the threads and the hub threads will be less snug if you later replace the freewheel with the correct item.

Praise be to Sheldon.

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