Oil temps above 260 really do require an oil with an HTHS of at least 3.8, and better still 4.5.
Is 260F too hot for oil?
- Thread starter oil pan 4
- Start date
240 is a fairly normal oil temp, a bit warm but it will work.
I'm thinking just cool it down.
It depends on the oil. Some are quite comfortable at that temperature. HPL's Euro oils see peak friction coefficient at 300°F.
Might be a good idea to check the shroud and cooling fins for obstructions as well as checking valve clearances.
Switch to a full synthetic 10w30.
Although the Honda 390 is probably the best OPE engine ever made, when it's run hard, and above rated RPM, that is probably not the best choice. I lost 2 new Honda powered water pumps using the recommended 5W-30 oil. Like the OP, the engines ran hard, and the oil was not able to protect the connecting rod big end. Catastrophic failure was the result.
One of my first cars was a '67 Chevrolet Corsa convertible. It had an air-cooled six cylinder 164 CID engine with four one-barrel carbs. It also had a head temperature gauge in the instrument cluster. I recall seeing 425 F when running 75 MPH on the highway for extended periods. The recommended oil was SAE 30.
I just rebuilt the engine, I put the valves at almost zero lash as I expected the new tappets to wear a bit.
I'm using synthetic 20w-50 oil.
I would use 10w-30 if I could get the oil temperature down.
20w50 is thick
Have you looked into a valve adjustment
Its a new rebuilt engine with around 10 hours on it, old cam new tappets.
Possible a mechanical situation
It runs hot because it operates at nearly 4,000rpm and constant wide open throttle for up to 90 minutes at a time.
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Zero lash is not good. Valve clearances will tend to tighten, not loosen over time. Valve seats receed faster than the cam and tappets wear.
Also tight exhaust valves spend less time fully closed and therefore have less time to transfer heat from the valve to the head. This holds more heat in the engine and also risks burning the valve.
I would urge you to set the intake valve to 6 thousands (.15mm) and the exhaust to 8 thousands (.20mm).
I set them at one peice of paper. So maybe 3 thousands..
Straight 30 was the default choice for air cooled OPE engines for decades. Due to better results than the 5W or 10W-30's. The minimum HTHS for a straight 30 is (by memory) 2.9, but most offerings were about 3.8. Also by memory.
