I'm trying to do a Freehub body swap on an old 600 tri-color wheel. The original body was 7S Uniglide/Hyperglide. When I put the 8/9/10S body on (labeled Deore XT according to the LBS) and tighten the 10mm fixing bolt it gets way too tight and won't freewheel. It binds really bad. Am I missing a washer or something? The body freewheels fine when it's not attached to the hub.

Thanks.

Shimano freehubs come in a variety of types as do Shimano hubs. Cones, axles and dust caps are also different on many models. If you are trying to use a freehub from a 135mm spaced hub on 126mm road freehub, you probablt have all kinds of compatiblity issues.
You may not be able to do what you'd like. I was always under the impression that you could not convert 7 spd freehubs to 8/9/10 freehub bodies.
There is a freehub compatibilty chart I have seen that is made by Shimano. Here's a link for it.

https://bike.shimano.com/publish/cont...ngeability.pdf

I got it. Found a large black washer in the parts box along with the old 7S Freehub. Put it between the body and the hub and now all is well. I must not have seen it when I removed the old body.

I did have to use a different cone on the right side to get the spacing I wanted, but that's only because I have two wheels for the same bike and I didn't want to have to make RD adjustments when I switch wheels. I had to be quite picky with both wheels, but now I can swap them without adjustment.

I've never run into problems swapping removable freehub bodies... even when there are issues, there's usually a way around it. I have a Dura-Ace AX hub with a Hyperglide shell on the non-removable freehub body guts.