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Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.
is it better to take oil temp before or after
the oil cooler? To see what is coming out the engine
or going in to the bearings
The typical location for sampling oil temp is the sump (sometimes using the feed from the oil pump) or dry sump tank, as this gives you an indication of what the bulk of the engine oil temperature is. Measuring the outlet temperature of the cooler versus the inlet temp is useful to see how effective the cooler is.
The closer you can get to the actual oil temperature from the engine the better, because this reflects what the oil and, especially, bearings are running at.
As Steven said, a common location for the oil temperature is in the sump, either a welded on boss or using one of the special senders that replaces the drain plug (also available for gearbox and differentials, fill or drain plugs), it may not be the best, but it's a good guide to the average - something like this https://dubbed.co.nz/products/vdo-oil-temperature-sender-sump-plug - but don't forget to check the thread size and sender characteristics, if you go that route! There are also adaptors, such as https://www.demon-tweeks.com/nz/lma-oil-temperature-sump-plug-adaptor-244354/
Some wet sump pump and/or filter housings have drilled and tapped points where a plug can be removed and the sender(s) fitted.
If you have a dry sump, best option may be at the scavenge pump's outlet?
If you have it after the oil cooler, that just tells you the temperature entering the engine, which doesn't really indicate how hot the oil is actually getting within the engine.
I take it from the filter housing. In my instance it is between the dry sump pump and the feed into the main gallery.
Because my oil coolers are between the scavenge outlets and the dry sump tank measuring is there would give a much lower reading. Good for getting an idea of oil system cooling efficiency.
Measuring on the exit of the dry sump scavenges would give a higher reading and indicate the maximum the oil was hitting. Good for getting an idea of potential oil degradation.
In an idea world you would measure both. In reality for most us it doesn't really matter as long as you realise what you are measuring. Again for most of us the oil pressure is probably more important...oil temperature just tells you a possible reason as to WHY the oil pressure has dropped.
I take a bit of a different stance. If its a known engine configuration I like to know whats going INTO my engine, lets me know how efficient the coolers are.
To get it to a known config leave a dedicated EGT/K type thermocouple open to task as a roaming sensor to find baseline deltas for intake, coolant, and oil to get to know the chassis.