EGT

what is a safe EGT figure not to exceed?

what is a safe EGT limit?

This is dependent on the components of the engine. For gasoline it might be 750 - 820 degC. Diesels try to stay below 675.

What engine are you running and what EGTs are you currently measuring?

As David said, there are a LOT of factors to take into account, diesels are running much higher than that, though. Possibly the most important ones being exhaust valve material and, assuming it's turbocharged, the turbine material. For the latter you can find the recommendations on the manufacturers web site - some of the modern exotic materials will be perfectly happy at temperatures that would have instantly melted some older alloys, but $$$s.