Dave Ett wrote:Depends. Water temp is more accurate since water reacts to temp changes much quicker. You're right in that the oil will take longer to reach 'normal', but it will also take longer to reflect and change up or down.

Oil is also designed to work effectively over a large range these days, so even though it's not fully warm, it's still protecting.

Oil is also not pumped round the head to cool it, whereas water is. The water should be hotter as that's what cools the combustion chamber.

Basically, water temp is the crucial one!

Quick science lesson - Actually Dave it's the opposite - I think you'll find that Oil will warm up ("react to temp changes" as you say) much faster than water as the Specific Heat Capacity of Oil is about half that of liquid water ie. It takes twice as much energy (Joules) to heat water by the same amount as oil.

The water probably does heat up quicker in the case of a l/c 2-stroke though as it is being pumped around the heat source (cylinders) while the gearbox oil is warmed more slowly by conduction through the engine.

I think you're right re. water temp being more important though - this will tell you if the piston / cylinder area is warmed up sufficiently - afterall this is where the vast majority of 2 stroke failures occur.