This is my first four stroke engine (os 52) and I'm wondering where the nipple on the bottom of the engine goes to/ gets connected to, the instructions don't really cover it.

This is the breather for the crankcase. You don't have to connect it to anything but there will be a good bit of oil spit out this connection while running the engine. Most usually attach a short length of tubing to it and run it clear of the cowl to get the oil out into the slipstream. This is also the place you insert the after run oil if you use after run oil (IMHO a good idea to do so) as you can not get after run oil to the main bearings by putting it into the venturie or glow plug hole like you can on a two stroke. Not just a drop or two either, if you use it put in about a teaspoon full and the rotate the prop a few times with the breather outlet at the highest elevation (so you don't sqirt all the oil right back out the breather tube.). Now, some of the engine manufacturers now have a second connection on the intake where they recycle this oil back into the intake but; if your instructions don't cover this your engine doesn't have this capability. I've always thought that it was a bad idea anyway to recycle the used oil back thru the intake as it does have some contaminates in it by that time due to engine wear, especially during breakin periods.

That's a lot of oil in a .52. How does one go about getting that much oil into the crankcase via the breather tube when there is nowhere for the displaced air to exit? The crankcase will only draw a vacuum when the piston is traveling toward the head. As the piston starts it's downward stroke, air is pushed out of the breather along with any oil in the tube. Maybe during the off season or prolonged storage a teaspoon of oil would be OK, but after every flight it's a bit much.

No, stickjammer, it is not to much unless you don't want to do an adequate job of protecting your engine. How do you get it in? You have the nipple higher than the rest of the engine and you rotate the prop so that the piston travels from full bottom to top as you inject the oil, it will be sucked right in with no problem. However, it will also be pushed right back out unless you keep that nipple up while you distrubute the oil to the two main bearings.

A teaspoon is no problem. On my 52 I use a "squirt" bottle filled with oil that has a short piece of fuel line attached to it. I connect it to the breather and slowly squeeze. The oil will flow in and the prop will turn. You need to be careful about turning the prop by hand because the oil will squirt right back out of the engine. Depending on the position of the prop, air will be pushed back out of the breather, it just flows into the upturned bottle. I use enough for the oil to begin seeping out past the front bearing. I wrap a rag around the engine because over time, more oil will leak past the bearing. Its easily a teaspoon, probably more like a tablespoon.

Thank you all for the advice and rapid responses!

Just curious, why would the crankcase and bearings of a 4-stroke need to be swimming in after-run oil and a 2-stroke only needs a couple of drops to do the job?

Two strokes- a few drops in the carb and flip the prop several times, disperses it through the engine.
Four strokes- The same application, and a few drops/small squirt to get it int the crank case. Don't over do it, you'll just be wasting oil.
Don't forget most four strokes don't get a lot of lube to the rocker boxes, and so the rocker covers should be removed periodically and a couple drops of oil added to them.

For you fellows who think a couple of drops are enough in either a two stroke or four stroke, ask yourself these questions. What does after run oil do? How thin can your spread it when you can't make direct contact? You need to completely coat the bearings (two main ones) and in the process displace any nitro residue or old fuel residue with the protective coating the after run oil provides. Yes, a couple of drops may make you feel good but will do little to increase the lifetime of your engines.

Never had a problem, and I've got a couple of BB engines that are 15 years old!