Tons of variables........
But lets say on properly (or reasonably closely jetted bikes)
~Bikes that have a bunch of carbon build up (carbon acts as an insulator but also adds a lot of weight), the first thing to happen is burn up that carbon build up, or if it is really excessive on both piston and valves/head... then it'll overheat the cylinders and usually warp them, could be a couple thou could be several thou.......... But it could also burn out the seating surface of the valve as the carbon is getting "burnt off" and as the valve face/seat is exposed to clean great heat transfer area in one spot but carbon insulating the valve from transferring the heat out of the valve and into the head in the bulk of it- it can then burn right through that spot and the seat.... You usually also change the ring seal of that top ring as it becomes "soft"
you can get carbon to "flake" and get piece(s) stuck between valve face and valve seat...... ruins compression and seal and gets burning of combustion going into the port too.they still run but with less power until and unless that piece ever departs, which sometimes they do not
Bikes without that build up... will nearly always overheat the piston and exhaust valves first- warped piston and soft valve/seat are the first to occur typically..... but you also can burn through or around the valve seat and/or through the edge of the valve before doing anyhting else
Kind of the list of bads;
overheat the crown of the piston and score the cylinder wall
blow out a head gasket
warp the cylinder head
collapse exhaust valve guides
burn up the wrist pin
burn the valves and or seats
etc.......
On a bike that is running too hot because of a lean situation...
Most of the time you have exhaust valve face or exhaust valve seat problems or score the cylinder due to too much thermal expansion at the top of the piston
but you can have the rod top/wrist pin/piston all get so hot to score that wrist pin, blue or blacken the rod top and then later down the road have those fail
you can get the warped head
you can get the head gasket issues
you can have the heat sit in the head too much so that you have guide issues or even guide seal issues
pretty much all the same issues in both scenarios- but the carbon bikes slow some of the issue orders but create other ones to happen sooner
I have seen a few of the racebikes over the years come in after the dumbfucks crashed and lost a bunch of water or overheated them and pushed a bunch of water and then continued to race the weekend with a 1/2 full or 2/3 full radiator and they bring them back and everything from the base gasket up is scorched, and I have even seen the rods discolored down to the main bearing
I have had a few street bike guys bring me bikes in with water levels so low the sensor would not even pick up the water temp, rather it was just giving a bare aluminum cavity temp (far lower on the gauge), and some of those you could smell the burnt oil immediately, some not so much but many of those when adjusting the valves were found to be close to zero clearance or below zero clearance- a few had to simply get new used engines and upon disassembly rod tops were black, wrist pins scored, cylinder walls out of round .005" or more, pistons (while typically out of round due to thermal expansion properties of these partial skirt pistons used in most every sportbike engine) measured even more out of round, but once I had one actually measure damn near perfectly round when it should not have been)
One of the major factors I think is relavent in those severely damaged is the water level........... if you push some water during the time it is far too hot but as it cools it draws it back in and you keep repeating that cycle for brief periods of time there is less damage than those bikes that push so much water they cannot draw enough back in as it cools to completely fill the radiator again and then you start with an ineffective cooling system to start
way back in the day.......... The 85/86 Ninja had about half the bikes on the grid that would push out pretty much all the water in every 10 lap sprint race, it was more an issue of taking a bike in production form with just adequate cooling and then building it into a race bike and adding about 10-14hp (which was rather easy to accomplish in the day -nearly 20% more power) anyways, those would warp heads, blow head gaskets and burn up valves routinely...... I saw guys go through an engine or two each and every weekend (that was really when you could easily run 500 race miles in a weekend on just sprint weekends or even 1000 miles on endurace weekends............ between entering that one bike in about 10 ~ 10 lap races and then either a 100 mile race, a 3 hour race (averaged about 275 miles) or a 5 hour race (averaged about 450 miles) (obviously bikes are built better than they were in the mid 80's with many having much better valves and much better tolerance controls, but they are now pushing about double the HP through the same displacement...... hp equals heat, and radiators need lots of airflow to remove that heat plus have no air in the system.............. Many of those Ninja 600 guys would shut off the bike as soon as the crossed the finish line and coast back to pin in (which pit exit was used at the time) and coast as far back to pre grid as they could then fill radiator immediately and go right back out and race again the very next race or maybe have a one race reprieve between their races........ I routinely did 18-20 races a weekend back then on just the 600, but could get in 28 races if I entered every race I was eligible for on just that one bike........ 28 races in 2 days was really rushing through both days and you had to have zero issues and atleast me........ I was completely wiped out and worthless for about 2 days after that, it defintely tested your mental and physical fitness....
bike did just fine. It did get toasty sitting on the bike. Bike needs 30 to 35 MPH to cool down. My BMW F800GT had a much “Better” cooling system and would keep the engine cool even if significantly low on coolant. Thanks to “excellent “ servicing by the dealer I discovered that it could be so low on coolant that there was not enough airflow from the fan to keep it cool but 20 mph would cool it down in 95 F ambient! Guess it was a generously sized radiator.
