Which washer fits my instrument?

Measure the valve stem and the cup recess. A 7/32" stem with a wider cup takes the A85; a 3/16" stem with a shallow cup takes the A80. The outside diameter matters as much as the hole.

Does thickness affect valve action?

Yes. The 3/32" A85 gives more cushion and can feel softer under the finger; the 1/16" A80 is firmer and seats in shallow cups. Too thick a washer can bind, so match the original.

Are they the same felt quality?

Yes. Both are 100% wool felt made in Italy and Germany, in the same four colors. The only real difference is size.

The thicker A85 has more material to wear through, but fit matters more than thickness. A washer that is too tight or too loose wears out faster either way.

Can I use one size in place of the other?

Only if the stem and cup allow it. A 5/8" washer will not sit in a cup cut for 1/2", and a 3/16" hole will not grip a 7/32" stem.

Why are they sold only in dozens or hundreds?

They are repair-shop supplies, so they come in bulk counts. That suits techs who service many instruments, not a one-off repair.

A85 Felt Valve Washers vs Felt Valve Washers

These are the same 100% wool felt washer in two sizes, so the choice comes down to fit, not quality. The A85 is the larger, thicker washer at 5/8" outside, 7/32" inside and 3/32" thick; the A80 is smaller and thinner at 1/2" outside, 3/16" inside and 1/16" thick. Both come from Italy and Germany in white, black, green and red, and both sell only in dozens or hundreds, so buy the size that matches the valve stem and cup you are working on.

Choose the A85 if your valve stem and cup take a 7/32" hole and you need the thicker 3/32" felt.
Choose the A80 if you need the smaller 3/16" hole and a thinner 1/16" felt for a tighter fit.

Prices

A85 Felt Valve Washers: $8.00 at Ferree's Tools Inc (last seen Nov 10)
Felt Valve Washers: $8.00 at Ferree's Tools Inc (last seen Nov 10)
Difference: both cost the same at the latest prices (Nov 10)