When an engine gets old, what gets old?

I know this sounds dumb but Im not a mechanic. I know you can buy new internals like pistons, rods, bearings, etc. but what gets old and used up when you say "My engine is old". Are you just reffering to the pistons, rods, etc. or what exactly.

usually you are reffering to the piston rings, also the cams and injectors can start to die out. Pistons and the crankshaft are usually signs of more serious problems like blown head gaskets.

Friction exists everywhere. If it didn't then nothign would ever quit moving. The law of inertia states that an object remains in constant motion until something stops it.

What wears out is a long list. Every moving part develops larger and larger clearances. If the engine wears from only age, then typically rings wear, rod and main bearings wear, ect ect. Want proof? Look at your oil. Now I'm gonna give you a hint on this one. Oil doesn't wear out. It gets contaminated. Now assuming your PCV system is working correctly, what makes the oil darker over time? metal particles. Thats wear. Obviously some is blowby, but darker oil quicker will also mean more blowby hence wear.

Best way to determine engine condition is to do a compression test or a leak-down test.

dark oil can mean oil is getting burnt. for some weird reason my car has been gettin black oil every since the day i bought it. i would burn 1 qt of oil every 3000 miles but i still have perfection compression. i would change my oil then drive for 100 miles and the oil would turn black for some reason. its gettin burned somewhere really slowly and i don't from where, but when i switched over to syntec the dark oil would now take like 500 miles to get.

now for the things that wears out. usually the seals, the bearings, the cylinder walls, valve seats, the valve guides, and the rings are usual wear items. for the 240sx KA's we also have to worry about our timing chain stretching and the nylon valve guides breaking over time.

Tat, you mentioned you couldn't get to your PCV valve. May be time to change it. I know mine needs to be done.

i'd say bearings or rings eventually go. everything has a life span.

Soooo true mynismo...soo true, thats what a lot of people never think of, everything can only last so long until it dies down.

fluids
gaskets can be a problem
rings
bearings
valve seats
cams
blets
timing chain
the cylinder walls can get warpped
water pump
oil pump

for the most part under basic repairs and normal driving no races etc., the pistons, rods, crank shouldn't recieve to much damage

an old engine is basically cuased by friction, the motion of engine parts cna cause the points of contact to be shaved down over time cutting away at them, then the shaving move throughout the engine and can lead to damge else where, [like sand erroding rocks]

Just so happens nissan engines last forever when maintained.