Something I deal with quite a bit is people asking me if I have any used
engines for sale, or where they can get a used engine. I often don't have
exactly what they are looking for, but feel a little obligated to tell them how
risky buying used can be.
Buying a used engine, or a UL with a used engine can be risky business if you don't know what to look for. One of the things that makes it so risky is the fact that there is no legal requirement to maintain the engine to any standard, or to keep any kind of accurate log.
So the first thing to look for is a very detailed and up to date log book and service record on the engine. If it's not there, the engine might as well be worth $200. I see at least one engine every month that was bought used and was misrepresented as being much fresher than what it actually was. Nearly every such case, the buyer was told that the engine had just been "rebuilt" a few hours ago, or that it has only been "broken in." In actuality the piston clearances and crankshaft indicates sometimes as much as 600 hours on it since rebuild or since it was new.
"Rebuilt" is a relative term, and should not be relied on as an indication of quality unless there is a detailed service record describing exactly what was done with the engine. The service records that get sent back with one of my rebuilds is usually about 2 - 3 pages single spaced, and every little detail of the engine service is in there. Without that kind of records, "rebuilt" could mean new seals and gaskets, or it could mean new crank, pistons, bearings, seals, and gaskets. That is a big difference. The crankshaft alone is $580 on a 503 and $970 on a 582.
As long as the castings are not damaged, the engine is almost always worth at least $200, and can then be rebuilt with a new crank and pistons for under $2000. If you pay $1000 for such an engine, it's not such a good deal anymore.
Some other things to consider when buying used are:
The age in years of the engine: If the engine has been used less than 50 hours per year or has been stored for a period of time, the chances for corrosion on bearings is elevated. Just the tiniest piece of rust or pitting in the wrong spot can lead to disaster, and it is almost impossible to detect even with the engine taken apart. Any of the 2 strokes should have the seals replaced every 5 or 6 years regardless of the hours. If the engine has never been run, you might extend that a year or two, but the seals tend to dry out, crack, and leak. Even on an engine that is run, if it's not run 50 hours per year, there is too much down time for the seals to dry out. If it is run 50 hours per year, then the hours will wear out the seals.
Total hours on the engine: If the engine has 150 or more hours it needs seals and gaskets replaced. If it has more than 300 it really should have a crankshaft replaced, although 447's and 503's tend to have crankshafts that last longer as long as there is no chance of corrosion on them. I just finished a 503 that needed a crank at 163 hours though. In this case, the engine was 8 years old, and some pitting had developed on one of the rod bearings. The pitting was undetectable when visually inspected, but a radial clearance check on the rod indicated a worn out bearing. All other checks on the crank were within factory new tolerances, but that rod bearing was within just an hour or two of complete failure. One needle was already gone, and the two adjacent were about to go. On one of these cranks, you can only see a couple of the needles at a time, and the missing needle was not discovered until after the engine was completed and the crank core was being shown to the customer.
When looking over an engine, at a minimum, you should take a look inside the ports. On the 503 I just finished, the engine had obviously been seized by the previous owner because there was carbon built up on top of the seizure marks. The other piston had a hole starting through the crown, probably from the use of non-standard spark plugs. This damage was also carboned over indicating it had happened probably under the previous owner. A good inspection through the ports with a flashlight will reveals most of these types of problems. They are usually fixable, but at the cost of a piston and some labor for the cylinder repair.
If you really want to consider what an engine is worth to you, assume that a 503 will need $1600 - $1800 in service before it is zero timed, and a 582 will need $1800 - $2000. If you figure that is what you are going to need to spend on it in addition to what you pay for it, you should do ok.
Now that I've scared all of the buyers, maybe I should write one on selling a used engine. That's easy. Follow the maintenance plan to the letter, and make sure you have record of it. When the engine needs service, get it done by a reputable source who will provide a detailed account. Keep good records just like you would if you knew you were going to be forced to prove something because you are.