If I were you, I'd just stop now and save your money for a better bike. The Schwinn Varsity weighs 42 lbs, or about 12-18lbs over what even a generic chromoly framed 80s road bike would weigh, and given that you can pick those up pretty cheaply on craigslist, I'd trade it in and go that route.

But assuming you're already sentimentally attached to this one, despite the fact that it weighs about two full gallon milk jugs more than a bike needs to, you're going to need to start with a 1 piece to 3 piece adapter, like this American to Euro BMX adapter.

Then add a generic road double crankset in your desired length and the appropriately sized bottom bracket, and a fresh chain cut to the right size.

Other than that, it's basically a question of taking the old parts off and putting the new parts on and transferring the pedals over.

u/[deleted] · 2 points r/bikewrench

There are different diameters. Most cartridge BBs are 68mm, with some 73mm.

I think this is your only option.

u/802bikeguy_com · 1 point r/bicycling

Truvativ makes a one piece (ashtabula) to English cartridge bottom bracket adapter. You'll need that, a bottom bracket of the appropriate length (usually defined by crank manufacturer) and your new single speed crank set.

u/randomusername3000 · 1 point r/bikewrench

Hmm you are right, that picture does show a 1 piece crankset, though this one on the Fuji site shows a 3 piece: http://www.fujibikes.com/usa/bikes/cruiser-comfort/beach-cruiser/cape-may

If you have a one piece, you can use this adapter to convert to three piece https://www.amazon.com/Truvativ-00-6415-027-000-American-To-Euro-Bmx-Adapter/dp/B000VT550K

Most chain tensioners are not designed for wide ranging chain slack, but a rear derailleur is designed for that. I would recommend using one even if you don't run gears on the back

if the chain is getting slack every two weeks then maybe the wheel is moving in the dropout? it shouldn't stretch that much that fast

u/neurocellulose · 1 point r/bikewrench

If it's the 28 TPI American size one piece, this is the adapter you're talking about. Going to need to remove the pressed in cups and then press in the adapter. After that OP can put any standard 68mm BSA BB in as far as I know.

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