The most technically complex thing invented since 1820 that could be reverse engineered in 1820?

[silenus]

@Babale

I was going to say an internal combustion engine, but you’re only saving 40 years or so.

If you wanted to be evil about it, take back an AK47. Take the bullets to some chemists – smokeless powder is only 20 years away, and your bullets could be the extra kick in the pants they need to develop it sooner. Producing that rifle in the 1800s would require the world’s best and brightest mettalurgists and a while lot of money, but if you find a monarch (or revolutionary, though sadly you just missed Napoleon) to back you, you could conquer the world!

I went to the same place, but not as complex. I would arrive with a Kar98 Mauser rifle and ammo. Bolt action still makes it light years ahead of the standard of the day, and much easier to manufacture and adapt.