Re: force 150 1993 piston replacement
Welcome to the forum.
First, the top piston is #1.
Second, buy a manual--Clymers or Factory. Either one will be invaluable.
Melted pistons on outboard engines are common. They are usually due to a carburetor running lean for whatever reason. So: no matter what you do to the engine, be certain to check that the carb is not clogged and the low speed needle is not set too lean before running it again.
Also, some things are normal maintenance and you should assume that they are bad. So: replace the water pump impeller, the fuel pump diaphragm, and the lower unit oil as a matter of course before you run the engine. Since you do not know the condition of the carbs, clean all three.
In your case, if the piston is melted and no damage has been done to the cylinder liner it is easy to replace just the one piston. Once the head has been removed, if you unbolt the rod big end, the piston can be pushed out. This job requires no special tools except a torque wrench. If you are careful, VERY light scoring can be honed with the engine assembled and a standard piston installed BUT-be aware that the smallest overbore is .010 which is 5 thousands on each side. That means a score of only 5 thousands depth will require boring. That's not much and honing to that size would be 1. uneven 2. out of factory specs. So you can see that it does not take much scoring to require a boring and an oversized piston.
However, if the cylinder liner has been scored, then a complete disassembly of the engine and an overbore job on the affected cylinder is necessary with an oversized piston installed. And, yes--if only one piston is affected, only that one cylinder need be done. If, for example, three were scored, then I would suggest that it makes sense to do all five. These engines are so small, that they don't even notice the extra displacement of an overbore on one or two cylinders and the pistons are all balanced to standard factory weight.
If you have access to an internal micrometer, it makes sense to "mike" the bore and see exactly where you are. If you drive the engine to your local machine shop with the head off, you may be able to persuade them to mike it for you. Standard bore is 3.375 with a tolerance of .002 out of round, barrel shaped, or tapered. The shop man will know how to measure it.
Once you determine the amount of damage and what you want to do, come back and we will give more detailed instructions. However, instructions on the forum are by necessity brief and assume a rudimentary knowledge of the engine itself, so again, a manual is needed.
