Bicycle Mechanics - Cassette/Freehub incompatibility.

I'm rebuilding my old road bike which has 126 mm O.L.D. spacing. I found a new RX100 Hub (model FH-A550) which has the spacing I need and I'm pretty sure it's a 7 speed cassette (I didn't think 8 speed came in 126 mm spacing). Anyway, I bought an inexpensive Shimano cassette f (model CS-HG50) thinking that the two would be compatible. The cassette slides on to the hub fine and the lock ring threads correctly, but there is about 1 mm of latteral play with the cassette on the hub. Do I need a spacer or something or am I not doing something right. Since my last rear wheel was stolen, I don't have anything to reference.

Check your cassette. Is the first cog an 11T or 12T? Remove the cassette from the hub, and just slide the small cog on. If the cog stays perched on top of the body, then your lockring secures the cog in place, without putting pressure on the rest of the cassette. I've seen this too many times, and yes, the remedy is to put a small spacer (like the splined one found in a cassette) behind the cogs...:)

The first cog is an 11T. That's exactly how it works too, the small cog sits on the end of the hub, leaving a little bit of space between it and the next cog. Thanks for your help! Next question now, where does one acquire such a spacer? Is this something that a LBS should have for cheap? Thanks again.

A shop should still have some laying around, for around a buck.

www.loosescrews.com

check item #sh-11f1228.1. This is a 12t cog w/spacer, they don't seem to list the spacer seperately.