Modern Vespa

I'm after info about the maximum heat of a cylinder before damage please. I've fitted a thermometer to my Vespa p200e.

That's not an easy question to answer, but a good starting point is that if you're getting over 350F with an under-plug CHT, you probably need to take a look at your jetting and/or track down an air leak.

If it's an in-head CHT, then all bets are off.

I've known air cooled engines to survive glowing temperatures, enough to melt spark plug wires. In the upper limits of heat issues all sorts of issues arise, I guess this whole question is how you define damage. An example is WOT at all times, the head warps a bit, and compression is lost, thereby preventing catastrophic failure. This can happen so quickly a gauge won't even be recorded. Like an old wide body I found in the tropics, where someone thought leaving the tin off would keep it cooler. They drive it to its resting spot thinking it was done. I loosened the 4 nuts and shaved the head a bit, resealed it and fired it up.

I like to jet for 300f or under at WOT.

And really for an accurate picture of it all, I should be running a EGT gauge with the CHT gauge, but I haven't.

I remember reading a thread where OopsClunkThud explained why you should run both, at least while tuning, but I haven't...

Measuring exhaust gas temp tells you how hot the combustion is in a direct manner.

Measuring the temperature of the cilinder head tells you how hot the engine is becoming/running. This is way more indirect because the combustion first needs to heat up all the metal. It is a function of the combustion temp, the load and the cooling.
Furthermore it is the píston which is the Achilles heel.
Regardless you need to measure the temp as directly as possible to be able to correct load in time.

Ergo keeping an eye on the exhaust gas is way more effective in tuning the combustion temp.
The CHT tells you about the engine heat situation.

The one I have at the moment only fits on the spark plug so I'm looking for max cylinder temperature so I can set it so it will flash red on max temperature. Since fitting the tachometer speedometer I have found a SIP tachometer speedometer il will have it in two weeks I just got to get all the wires for it and fitting info but after Christmas that will be fitted so for now il use the one I have on.

Max safe cylinder head temp

CHT's are a good rule of thumb indicator for engine heat/health, but they can vary a lot between set ups. Their true value is knowing the temp range on an engine that's properly jetted and known to be running well. If those numbers suddenly change along with other signs of problems, it can save your engine. Can save you from blowing up a new kit when your jetting is not yet where it should be.

This. CHT's will tell you if something is anomalous in your motor, like developing an air leak, but you still need to have your Known Good baseline.

For an extreme example, I have an in-head CHT sensor on my lammy that's DEEP into the head. It reads faster and MUCH hotter than an under plug. I don't even begin to watch it closely until it's reading 450F, which has only ever happened under a combination of really high speeds/load/RPM's and high air temps (95F+), but it still gives me peace of mind, especially on long rides.

I also have an EGT, which is both more responsive and a lot more useful for letting me know if I need to back off the throttle for any reason.

Any old crate I build and drive has to prove its trustworthiness through prolonged stable performance and reasonable gauge readings. Until then, it's an ice cream getter.

The CHT gauge was really helpful for NOT blowing up my VMC 187 kit. CHT's that just kept climbing fast at moderate speeds. Time to stop and check. Continued high idle and that hot smell=probably air leak. Turned around and puttered home.

It wasn't just the numbers I was getting. CHT's should drop like a stone when engine loads are off. You can also damage a piston through poor lubrication and CHT's will still be low.

Also, (shameless self-promotion, but...) I did a video on this topic a little while back...

Maybe someday I'll buy an exhaust gas analyzer.
I do have a NIB hand held IR thermometer, if only there was a way to have a known temp at all altitudes, oil types, fuel types, mechanical ratios.

Ohh, this fun game... again