Well, it's all pain staking trial and error.
Since I'm NOT out to "corner the market", read on.
First, since their all old, used, vintage pieces, you just have to buy fuel filters off ebay, or what ever source you have. Then you buy misc. shot glasses that you like the looks of. Then you mix & match shot glasses with filter tops until you find glasses that fit the filter tops correctly.
You will most likely have to modify the wire bracket that holds the shot glass (bowl) tight to the filter top.
(sometimes, like the two I've already done, it's a perfect match, no mod's required)
(sometimes, you will need to fab a "bushing" to match the glass to the filter top, or sand the diameter of the glass "lip" smaller, and/or ream the diameter of the filter top larger)
I've found it's about one in twenty-five that just fit as is. Also, it's about one in ten that fit at all!
So, it's kinda like summer bikini's (at least with the one's I wear). You buy tops, you buy bottoms, and you HOPE that one combo stuns the crowd.
Keep in mind, the smaller filters, combined with the smaller shot glasses, usually "fit" motorcycles better then the larger (normal) sized ones, that work best on cars/trucks. And YES, the smaller ones are the harder to find ones.
If ALL this sounds like too much bullshizit to mess with, you buy one from some idiot that spends the time to make them work!
