What does a rebuild actually cover?
A rebuild is not one job. It is a list, and the list changes with how the engine was used. On a four stroke motocross single, a full rebuild normally covers the piston, ring, wrist pin and circlips, the cam chain and tensioner, the valve seals, a valve lap or a full valve job with new guides if the stems are worn, the main bearings, the big end and small end bearings, all gaskets and seals, and a cylinder hone or replate. On a two stroke the same logic runs shorter: piston and ring, small end bearing, crank seals, main bearings, and a bore or replate.
The parts that get inspected and only replaced if they fail the measurement are the crank itself, the rod, the transmission gears and shift forks, the clutch basket and hub, and the head casting. A shop that quotes a rebuild without measuring those parts is quoting a top end job with a rebuild label on it.
If you want to understand how a professional rebuild is structured, from teardown and measurement to the break in period and the mileage you can expect afterward, the same process is documented for car engines at engine rebuild and exchange, and the sequence is nearly identical: clean, measure, machine, assemble, torque, run in.
What signs decide it?
Compression
Check it cold, throttle wide open, kick until the gauge stops climbing. Write the number down. Then add a teaspoon of oil to the cylinder and check again. If the second number jumps well above the first, the rings are the problem and the bottom end may still be fine. If both numbers are low and close together, the leak is at the valves or the head gasket.
Oil consumption
Fill to the mark, ride a full practice day, top up to the mark again and measure what it took. A healthy four stroke single on a track day uses very little. If you are adding oil every moto and there is no puddle under the bike, it is going out the exhaust or past the valve guides.
Metal in the oil and filter
Cut the filter open. Fine grey paste is normal clutch material. Bright flakes, magnetic fuzz that stands up on the drain plug, or anything that looks like bearing material means the bottom end is coming apart. That is the sign that turns a top end job into a full rebuild.
Noise and smoke
A rattle that follows rpm and does not change when you pull the clutch points at the crank or the rod. Blue smoke on deceleration points at valve guides. White smoke that smells sweet points at coolant, which means the head gasket or a cracked head.
Hours
Track bikes live on hour meters, not odometers. A race four stroke is usually due for a piston around 20 to 30 hours and a full rebuild somewhere between 60 and 100 hours, depending on how hard it is ridden and how clean the air filter stays. If you are past those numbers and the bike still runs, you are riding on borrowed time, not on a healthy engine.
Rebuild or replace: which one is cheaper?
Price both paths before you commit. A rebuild costs parts plus machine work plus labor. A replacement costs the price of a used or new engine plus the labor to swap it, and then whatever the replacement needs once it is on the bench.
A used engine from a wrecked bike is a gamble. You cannot hear it run, you cannot check the oil, and you do not know its hours. If the seller will not let you pull the valve cover and check the cam lobes, walk away. A new crate engine is the known quantity, but it is usually the most expensive option and it may not be legal for your class if the rules require the original cases.
Rebuild wins when the cases, the crank and the head are reusable. Replace wins when the cases are cracked or the cylinder has already been bored to its last oversize, or when the bike is your only race bike and you need it running for a date you cannot move.
What does a warranty cover, and what does it not?
A rebuild warranty covers the work and the parts the shop supplied, for a stated period, and almost always only if the failure traces back to their assembly or their parts. It does not cover wear items, and on a race engine that list is long: piston, rings, valve seals, clutch plates, cam chain, bearings and gaskets are usually excluded after the first few hours.
It does not cover damage from a lean condition, from dirt through a poorly sealed air filter, from overheating, from running the wrong fuel or oil, or from missing the break in procedure. Most shops will ask to see the hour meter reading and the oil you used before they honor anything.
It does not cover consequential damage. If the engine lets go and takes the exhaust, the radiator or the gearbox with it, the warranty pays for the engine, not the rest of the bike.
And it does not cover labor to remove and refit the engine unless the shop wrote that in. Ask for the warranty terms in writing before you drop the bike off, and ask specifically how many hours or how many months, and what voids it.
How do you break in a rebuilt engine?
Heat cycles first. Start the engine, let it come to temperature, shut it down, let it cool completely. Do that two or three times before you ride it. Then ride it at moderate load, varying the rpm, without holding it at one throttle position for long. Do not lug it and do not bounce it off the limiter.
Change the oil and filter after the first hour. Cut the filter open and look. A little fine fuzz is normal. Flakes are not. Change the oil again at the interval the shop gives you, and keep the receipts, because that is your proof if a warranty claim ever comes up.
What to write down before you decide
Before you spend anything, put these on paper: cold compression, hot compression, oil added per practice day, hours on the meter, what you found in the filter, and the quote for both paths. That page is what turns a guess into a decision, and it is also what a shop needs to give you an honest number instead of a range.
If the numbers say rebuild, book the machine work early, because cylinder plating and head surfacing queues are the long pole. If the numbers say replace, buy from someone who will let you inspect, and budget for the seals, gaskets and bearings the replacement will need anyway. Either way, the engine you end up with is only as good as the measurements you took before you started.