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Is it normal for my gear case to be still fairly hot to touch an hour after I finished riding? Is there a point while riding that I should stop and let the thing cool? It doesn't ping or anything. Just curious and a bit concerned.

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No harm, just do regular oil changes.

XRsOnly has a thermometer dipstick that will tell you the oil temp, I've been pleasantry surprised at the low oil temps on my XR218. 

I also have a Vapor with the temp sensor under the spark plug and I've observed some scary high temps, sometimes going above 400F but mostly in the high 200s and low 300s.

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It will get hot enough to give your hand a nasty burn. Temps can easily be over 240 degrees.

 

Absolutely.  Oil temperature will always be fairly high.  Even in a liquid-cooled engine oil will stay 10 to 15 degrees hotter then the coolant under "normal" conditions.  Under extreme conditions, like racing, the oil temperature will get much higher.  You always want your oil to get above 212 degrees to eliminate water from combustion and condensate.  This is one of the many reasons starting an engine and then shutting it off is not a good thing to do.

 

This is in the context of a car engine but still has relevance:

"For a dual-purpose car, engine oil needs to be at least 220 degrees F to burn off all the deposits and accumulated water vapor. For every pound of fuel burned in an engine, the combustion process also generates a pound of water! If engine sump temperatures rarely exceed 212 degrees (water’s boiling point), the water will mix with sulfur (another combustion by-product) and create acids that can eventually damage bearings.

As for ultimate power potential, the general consensus among most racers is that hot oil and cool water make more power in most engines. Cold engine oil causes excessive frictional drag on the bearings and cylinder walls. A quality conventional motor oil will tolerate oil sump temperatures of up to 250 degrees, but starts breaking down over 275 degrees. The traditional approach is to try to hold oil temperatures between 230 and 260 degrees. Even on a short-duration, drag-only combo where oil is frequently changed, I would not want to routinely see under-200-degree oil temps.

A full-synthetic oil will withstand sump temperatures in excess of 300 degrees, and for hard-core professional racing, some oval-track race teams are experimenting with ultra-thin, specially formulated, race-only synthetics operating at 350 degrees or even higher.

Also remember that a high-end engine is built as a total combination. Piston-to-wall clearances, piston ring end-gaps, and bearing clearances are specifically tailored to match the engine oil’s characteristics and intended operating temperature."

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