roccobike

I've got a vintage Specialized Stumpjumper that I just finished putting back together. I found a set of Sun Rims laced to LX hubs in like new condition that I'd like to use on this bike. The only item is, the rear wheel is set up with a Shimano 8 speed cassette/freehub. The bike is set up with 7 speed LX.
OK, so I know I can put the 7 speed cassette on the 8 speed freehub and use spacers but I have two questions.
1. Where should I put the spacers? Should I put them on the freehub first, thereby pushing the cassette closer to the deraileur or should I put the cassette on the freehub, then put spacers on as needed, keeping the cassette closer to the center of the wheel?
2. I'm not sure what will work as spacer. I've got a bunch of those black plastic spacers found in 10 speed Shimano cassettes. Will they work, or do I need something more substantial.
Thanks for any input. Despite all the work I've done on bikes, this is one installation I've never performed.

Retro Grouch 

Put the spacer on first.

I don't think that you really have a choice because the small sprocket is position specific for securing the lockring.

Kontact 

You want to use aluminum or steel spacers, and they go one first, then the cassette. You need 4.5mm spacer.

hokiefyd 

Yes, I believe the spacer would go on first. You generally want to keep the highest (smallest in diameter) cog at or near the same position relative to the derailleur. This means spacing a 7-cog cassette over to the outside rather than sliding it as far inbound as possible. Any spacer of the appropriate thickness should work. Cassettes have used metal (usually aluminum) and plastic spacers for years. I would build it up with just enough spacer material (using one or more physical spacers) so that your smallest cog is just proud of the end of the freehub enough so the lock nut will tighten it and take up any slack.

In the end, it doesn't have to be super precise -- there should be enough adjustment with the derailleur to accommodate your setup.

Kontact 

I completely disagree - plastic spacers are fine between tall cogs, but not appropriate for the low lip of the freehub body. It will get crushed over time. No cassette uses plastic on the far left edge.

hokiefyd 

I'll yield to that. I didn't consider that short lip at the inbound side of the freehub. Thanks for pointing that out.

roccobike

OK, got it. Spacers first, use metal spacers.
Thanks, I was going to use plastic, but I'll find some metal spacers instead.
Thanks to all those responding.

Bill Kapaun

They make a specific 4.5mm cassette spacer. Wheels Mfg. is one maker.

The question I have is-
Do they both have the same OLD?
Your 8 speed wheel may be 135mm and the frame may only be 126-130mm.

Kontact 

Nope. Shimano uses the exact same freehub body on 130 road hubs as 135 mountain hubs. The only difference between some of the old Deore 130 hubs and the later 135 was the axle length on the left side.

This change was mainly made to for chainring clearance.

Bill Kapaun

You really need to pay attention.
THE FRAME MAY HAVE NARROWER SPACING!
"vintage Specialized Stumpjumper" & "7 speed"

The increase in spacing was to reduce dish when they went to 8 speed, to NOT reduce wheel strength as much. Being it was for a MOUNTAIN BIKE, it was assumed more strength was needed vs a road bike. Probably a good assumption.

Kontact 

What are you talking about? 8 speed XTR MTB components were 68mm BB and 130mm rear hub spacing until 1996.

7 speed MTBs were originally 126mm.

Bill Kapaun

Which appears to match the OP's bike.
AGAIN-
"I've got a vintage Specialized Stumpjumper"

Sticking in a 135mm OLD wheel may be a little tough without cold setting/
Not everyone wants to deal without.
Without the OP replying about what OLD the frame is, this discussion is double pointless from you. You're making assumptions which haven't been proven/disproven.

Spend your effort helping the OP instead of biting ankles that haven't been exposed.

Kontact 

I apologize - I completely misread your "do they have the same OLD?" as saying that they don't.

My second post was accurate - the move the 135 spacing was not to accommodate 8 speed, which predated 135 by several years.