Open ended question, how long on 450 engine?

If it starts and runs fine, I wouldn’t open it up for another 50 hours at least, even then if the crank has no up/down play I would just do a piston and timing chain. These modern KTM engines can go for a long time when cared for properly. 

Look up Monk on here or TT, he is/was a local AA off-road guy that documented the hours and maintenance on his 2016 KTM 350. I think he was around 600 hours on an unopened engine before he retired it to his wife, and she put a fair amount of time on it after that before it was ever cracked open. 

On that bike, top end and check timing and you're fine. The average rider can get 200 + hours on the bottom end. I've seen people on here say they are over 300 hours and still looking good. 

I’ve been burned by a bike ceasing in a bad spot. I was always 60 hours then drop the bike off for full rebuild. 

Yeah 50 or 60 hours for me, dont care how many stories of guys with "200 hours on a stock top end". I can see pushing it to 60 or 70 if your light on the revs. 

I value my safety much much more than trying to extend my piston 20 or 30 hours past its safe point. 

450's last forever if maintained properly. I wouldn't even think about doing a top end until 150+ hours. 

There are variables that will factor in here, such as the quality of oil used, how many hours between oil and filter changes, and if the air filter has been changed frequently and oiled correctly.   

How the bike is ridden in what kind of dirt is a considerable factor as well, bikes ridden on deep sand tracks will have significantly less engine life than a bike ridden on hard pack.    A simple compression test should determine how much life the piston, rings, and valve bushings have left.

That said, KTM 450s usually have a very long life span if they are taken care of, it's not unusual to see a motor get to 100 hours without a top end replacement.  I wouldn't let the bike go over 100 hours without top end service however...

As mentioned above- lots of factors can determine life.

I had a 2011 CFR450- Rode/raced it for 4 years and estimate at least 200 hours (never touched the engine- except of course the clutch). Sold it in 2016 to someone who flat tracked and mx raced another 6 years and never touched the engine .. They finally opened the engine for the first time in 2022 to rebuild pro-actively. Could have had 400-500 hours... 

I always ran Rotella  🤨

Last May I did a complete rebuild on my YZ450 at 190hrs. Crank was loose, did new cylinder and piston, as well. Valves were in factory spec and didn't require any adjustment so we did not replace. 5 minutes in on the first ride it dropped a valve on a jump and over the bars I went. Heavy pelvis injury and internal bleeding. Took me 8 months to recover and I'm still trying to come back. Point being, I will NEVER go that long on any component again! I don't care if they look good and measure in spec.

If it dropped a valve after 5 minutes of run time after putting it all back together I'd say that there's a high chance that there was something off upon reassembly. Just seems a little too coincidental.

250h on Husky 2023 450 FC is the most I have seen before doing top end. Untouched before that. Friend, Pro speed and can hang with top guys 2-3 laps on hard packed. But rides a 450 as it should be ridden, barely hear him coming if he is behind you. 

400h, national pro rider, on a EXC 250 Enduro bottom end (19-22 package). 

So frequent oil and air filter changes and you can go a lot longer that you think. 

100%

Depends on maintenance and the type of rider, but if ridden easy like a Vet (me) and maintained properly (oil changes and air filter cleaning every <5 hours), I'd do top ends around 150 hours.  Never had an issue - but I've always done the maintenance methodically and don't beat up my bikes.  Modern 450s

You might think, but not the case. The valves were not removed from the head as the adjustment never deviated from factory spec. In hindsight,it was a major oversight not to remove, inspect and ultimately just replace being they had 190hrs. It broke right at the fillet where it meets the head of the valve. I really paid the price on that one. 

The KTMs just seem to be more reliable in the engine department and with longevity. If you are doing moto at a normal human pace a piston at 150 hours is more than safe especially on a 450. Theres old dual sport guys with 600 hours on the stock top ends on 500s. 

2021 350XCF. Harescramble racing, Sprint enduros and little bit of moto. 200 hours on this piston and cylinder, it got a new piston, the updated metal oil pump gears, a new timing chain and new valve seals. 

Piston was probably ready it was smooth on the skirts in a couple spots.

Did you happen to measure the piston/ cylinder ?

I didn't measure the piston but measured ring gap on the new piston and all was good so cylinder was in spec. 

I bought and ran a 2010, 13, 15, 17 and 19 KTM 450 SX-F’s all well over 200 hours and never opened the engine. The 2019 is in my garage with 267 hours and never touched. They all were running strong when I sold them. My son is quick and he ran his own 2013, 2015 and 2019 for over 200 hours and only opened one engine. I can’t remember why we opened that one up but I know it wasn’t due to engine wear concerns.  Robert Reynard did the work and he said it looked factory fresh at 150 hours.  It was sort of funny because Robert is not a fan of KTM‘s so it hurt him to see what he saw.  The quality of the OEM components is astonishing. If I ever have to open one up, I’ll spend the money for the OEM parts in all of those bikes. I think we only put three sets of clutch plates over all those hours. I’m talking well over 2,000 hours of actual data across 8 bikes and I’ve not had an issue.

I need to sell my 2019 as I picked up a 2024 FC450.  

My son and I rode Honda’s 450’s before switching to KTM’s and we had to do a top end in the Honda’s at 40-45 hours religiously. 

I bought a 17.5 SXF 450 a few years back with 137 hours, it was a great price, one owner, respectably fast vet, religious on the oil and air filter changes. I did the top end immediately as a precaution, that piston looked great. Still had ribs visible everywhere, no shine. Sold it at 175 still running perfectly. I’m at 94 hours now on my 350 and am probably looking at doing a top end at 125-ish?