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For most RC nitro engines (aside from some lower RPM engines for air use), the seal of the combustion chamber is done with fitment of the piston inside the sleeve. From the bottom of the sleeve to the top, the sleeve tapers. If you had the piston/sleeve in your hand, you could push the piston only about 3/4 the way towards to the top of the sleeve when cold, without a lot of force. When the engine warms up, the sleeve expands ever so slightly that the piston is allowed to move all the way to the top of the sleeve more easily, but still with a tight enough tolerance that it's sealed and making compression.
The taper of the sleeve is called "the pinch", since your piston is being squeezed as it is forced up the cylinder.
Over time, the piston wears down (sleeve probably does a bit too, but more than likely less since it's coated with a chrome lining) and when warm, the sleeve expands and the fitment/seal is loose enough that the combustion of the fuel/air mix goes around the piston vs pushing the piston down. This causes flame outs, bad tuning, blown plugs... This would be an engine that is worn out.
This is also partly why many people don't let engines sit and idle during break-in anymore, now many do what is known as the "heat cycle" break-in. With the overly rich setting of the carb combined with low RPM's of idling, your not generating much heat, so the sleeve doesn't expand and you cause premature wear of the piston as it slams into the pinch of the sleeve.
So, your main goal for the first few tanks is to get the RPM's up enough to move the vehicle and generate heat, but rich enough to ensure the engine internals have plenty of lube to take away the filings as the piston and sleeve wear into each other creating your good seal. Usually the heat doesn't get up to 200F until tank 2 or 3. While doing the heat/cool cycles, your also tempering the metal of the engine by getting the temps up. As the piston/sleeve wear into each other, friction lessens and the engine is fighting less and less with itself to stay running. After 8+ tanks or so, the wearing of the parts is close to the point you want them, so you can start tuning for better overall performance without worrying about damage. By the time you put an entire gallon through an engine, it should be fully worn in and ready to be beaten and ran all day without issue.
There's a few schools of thought on the heat cycle doing anything with tempering, since we aren't getting temps high enough in theory to affect the temper of anything. Well, the measurable head temps anyway. As far as the temp of the fire inside the engine, there is likely to be some tempering being done.
After so many gallons are run through the piston eventually wears down regardless what you do. If the engine is still solid (bearings are still good, carb isn't binding on you, con-rod doesn't have much slop in it), you can send it to a few places to get the sleeve squeezed in a die to get some pinch back in the engine. This will usually let you get a few more gallons out of it after a quick couple of tanks of break-in again.
Here's a thread I posted a long time ago on our sister forum with before/after photo's of different piston/sleeves:
https://www.rctalk.com/forum/threads/had-a-few-sleeves-serviced-by-rayaracing-photo-intensive.56420/
Probably all way more info than you were looking for. Just a lot of stuff from what I've read over the past 20 years and what I've experienced personally with the 20+ engines I've broken in.
The taper of the sleeve is called "the pinch", since your piston is being squeezed as it is forced up the cylinder.
Over time, the piston wears down (sleeve probably does a bit too, but more than likely less since it's coated with a chrome lining) and when warm, the sleeve expands and the fitment/seal is loose enough that the combustion of the fuel/air mix goes around the piston vs pushing the piston down. This causes flame outs, bad tuning, blown plugs... This would be an engine that is worn out.
This is also partly why many people don't let engines sit and idle during break-in anymore, now many do what is known as the "heat cycle" break-in. With the overly rich setting of the carb combined with low RPM's of idling, your not generating much heat, so the sleeve doesn't expand and you cause premature wear of the piston as it slams into the pinch of the sleeve.
So, your main goal for the first few tanks is to get the RPM's up enough to move the vehicle and generate heat, but rich enough to ensure the engine internals have plenty of lube to take away the filings as the piston and sleeve wear into each other creating your good seal. Usually the heat doesn't get up to 200F until tank 2 or 3. While doing the heat/cool cycles, your also tempering the metal of the engine by getting the temps up. As the piston/sleeve wear into each other, friction lessens and the engine is fighting less and less with itself to stay running. After 8+ tanks or so, the wearing of the parts is close to the point you want them, so you can start tuning for better overall performance without worrying about damage. By the time you put an entire gallon through an engine, it should be fully worn in and ready to be beaten and ran all day without issue.
There's a few schools of thought on the heat cycle doing anything with tempering, since we aren't getting temps high enough in theory to affect the temper of anything. Well, the measurable head temps anyway. As far as the temp of the fire inside the engine, there is likely to be some tempering being done.
After so many gallons are run through the piston eventually wears down regardless what you do. If the engine is still solid (bearings are still good, carb isn't binding on you, con-rod doesn't have much slop in it), you can send it to a few places to get the sleeve squeezed in a die to get some pinch back in the engine. This will usually let you get a few more gallons out of it after a quick couple of tanks of break-in again.
Here's a thread I posted a long time ago on our sister forum with before/after photo's of different piston/sleeves:
https://www.rctalk.com/forum/threads/had-a-few-sleeves-serviced-by-rayaracing-photo-intensive.56420/
Probably all way more info than you were looking for. Just a lot of stuff from what I've read over the past 20 years and what I've experienced personally with the 20+ engines I've broken in.
