So, I like ridiculous things. Things that "normal" people question. Like keeping snakes and tarantulas as pets. So consequently, as I've gotten into working on bikes over the last 2 years, ideas are floating through my head of a Frankenstein like bicycle that nobody else can claim to have. That being said, I have a spare front fork from a mountain bike with 26" wheels. So I've been contemplating throwing it onto something like a 20" BMX bike and then slanting the seat forward to create almost kind of a chopper effect. I was just wondering if this would be possible without any modification to the frame of the bike itself. This bike would be solely for cruising around town and turning heads so I don't much care about the strength of the frame as long as it can hold up to some light riding on the street. Thanks for any advice.

You would definitely need to find a frame and fork with compatible steerer tube diameters and lengths. I'm NOT a BMX expert, but I think they, at one time, had smaller steerer tubes than typical. If that is the case, it may be difficult to find a 26" fork. I was working with a 1987 Ross mountain bike earlier this year, and the stem was something like 3/4" (not the typical 7/8"). Someone mentioned that it was the size of a BMX stem, and that's all I know about that. Even if that once was the case, maybe newer BMX frames and stems are conventionally-sized.

Good luck. Sounds interesting.

Be sure the larger tire will clear the downtube. Going from 20" to 26" is a big diameter change.

Thanks for this bit of info. I'll be sure to check these dimensions as soon as I find a BMX bike to use.

You might want to check in with the Alt-Bike Culture subforum, as it is full of kludged-together stuff like this.

Can you do some custom fabrication?

There are quite a few unique bikes here too.
https://www.ratrodbikes.com/

As far as the tire clearing the downtube. With a suspension fork, or a reasonably tall fork, that probably won't be an issue. However, you may get a lot of toe overlap.

You need to go to the forum at https://www.ratrodbikes.com/

It's a very active forum, filled with friendly, knowledgeable, helpful and skilled members. They have frequent build contests and it's exactly the crazy custom stuff you are dreaming about. It's a great place.

Efit: Cliff beat me to it.

I may have given you some bad info. It looks like the stem CLAMP diameter (to clamp the handlebar) is commonly 22.2mm on a BMX bike (vs. 25.4mm for mountain bikes). I couldn't find anything on the steerer tube diameter. The suggestion for the other forum is a great one. Good luck!

Thanks to everyone who provided advice. I'll check out the Rat Rod Bike forum. And to answer CliffordK, I can't really do the fabrication stuff. But I like to think I'm pretty crafty with my little toolset.

Yes and no. BMX bikes have evolved over the years. "Old" BMX bikes used the Schwinn standard headset: 32.6mm OD on the headset cups, 1" OD, 0.833" ID steerer. Newer bikes use 1 1/8" threadless while the newest are using "integrated" headsets.

Kids these days...

Just remember, a journey of a thousand miles begins with a single step.

Don't over think it. Just do it. And remember , failure is part of the learning curve.

Thanks to all who provided advice but this was the advice I took. It was a success. Pictures coming soon.