All the freewheels (plus all screw on cranks and rear hubs) on our site use the industry standard 1.37" x 24 tpi thread and are 3/32" width (which means they fit 3/16" and 1/8" chains also). Most are specifically designed for trials use (apart from the ACS Claws), so have a very high number of engagement points (from 60 to 108) ensuring excellent pedalling response.
By far the most common size for freewheels on trials bikes is 18T and all the models we sell are available in this size - some come in 16T and 17T also. For a 20" wheeled bike the standard gear ratio is 18T up front and 12T at the rear. On a 26" bike most riders would use a 15T or 16T sprocket at the rear with an 18T on the cranks.
It's also possible to run most of our freewheels (with the exception of the Tensile as it's lockring tends to come loose) on a screw-on rear hub providing your frame has enough clearance. The freewheels with 'inverted' removal tabs (like seen on the Try-All) offer the best clearance, but all the freewheels have their overall width listed in their description in case you need to check.
The only freewheels we sell with sealed bearing units are the Echo Sealed (not available in the UK) and White Industries ENO, all the others have standard bearing units but we very rarely see any problem with standard bearings and are more than happy to recommend them. We also carry a full range of spares to fit Echo Sealed and White Industries ENO freewheels.
TartyBikes Ltd, Units 3-4, Club Street, Bamber Bridge, Preston, PR5 6FN