So after having a rebore and new piston on my AT1e. (With top end 175cc) and just finishing the breaking in period, I felt the engine was quite hot.

I have fitted a "runleader" temperature meter to the plug and after a 40 to 50 mph ride for 30 minutes and a few gentle off road, the reading was up to 168c / 334F.

Anyone can confirm what an acceptable engine temperature is for a 175cc ?... 

I have read previous post but that relate to a 250cc... so I assume it is different for a 175cc ?....

 

Sound fine...  and no temp difference between engine sizes...

 

With a similar sensor on my '72 175cc I'll typically stop riding and let things cool down if temps hit 390f or so, gives it a nice safety margin as temps will go up a little more after stopping. In my opinion 330f is a nice place to be for clean combustion with a large safety margin for overheating.

I realise it wouldn't look good but air cooled bikes should all have cooling like a chainsaw & go hard all day & come up smiling.

I wish you'd tell my Suzuki that, the Yamaha rarely overheats but that Suzuki just loves to run around 380f and it makes me nervous.

Rotax fan cooled ultralight engines (aircraft) spec for CHT is 350F to 430F is normal, 480F max.

350F is very safe. 

I've seen just over 500F on one of my modified 175's during a several mile high speed run on a desert dry lake...  got to just over 95 mph.  No ill effects at all.  (It was carefully jetted a little on the rich side and timed properly with good fuel...  I do NOT recommend pushing the temp that high unless you know the engine well.)

CHT is nice to have but reacts too slow if engine tuning is "unknown" or suddenly goes lean or something.  For example, if a 160 main was correct and someone put in a 130 main and held it wide open on a speed run, the piston would probably hole before the CHT would show it was getting too hot... 

My CT1 runs around 315 to 325 F after several  miles at 55 to 60 mph, with outside temps around 70°. Just cruising around town at 40 to 45mph, it's below 300. I'm using the Runleader CHT gauge as well. One nice thing, I recently replaced the battery and it doesn't lose the settings. That's good, because it's sort of a PITA to setup. However, once I got a good one, it's been very reliable. With the first one, the buttons would not respond, but they refunded my money and let me keep the unit. That means I have an extra sensor. They are very good with customer service if you do have a problem.

I agree with MarkT, I have that Rotax 377 on my ultralight and the red zone on the head temp gauge begins at 450degF. At full throttle takeoff, it hits about 390-395, then falls back to 330-340 at cruise.

Absolutely crazy AlNarv! You must have balls of steel to go zooming about on a motorized kite!

Mine is more like a tube and fabric airplane than a kite. The short below is of early learning how to fly it.


youtube.com/shorts/6g3-lGmrizo?si=BfQu60_Bkm3cg15w