My first Stearman , which I call the Greek Stearman, always operates at about 60º C oil temperature. A bit more on the hot days.

My other Stearman, 77744, operates at about 72º C under the same conditions.

Although the operating oil temperature is not really a problem, this discrepancy has driven me to unusual lengths to try and find out what the cause could possibly be.

I did a very thorough calibration of  the oil temperature bulbs and meters on both planes only to find that the indicated temperatures were right on the money.

At Galesburg 2003, Don Sanders of  Sanders Airmotive, gave a seminar on Stearman engines. During his seminar he pointed out that in interesting  bit of trivia was that a W-670 with a small hole in the plug on the rear of the crankshaft would run 10 to 12 degrees C HOTTER, that an identical engine without the hole.  Needless to say, I about fell off my chair. 77744 has the hole and the Greek Stearman does not.

Don went on to say that he has never been able to find documentation for the hole in any of the drawings, but all of the floating weight 7th order harmonic crankshafts have the hole, and it seems also to be in the later serial numbers of the solid crankshafts.  However, they have seen enough examples of the difference in oil temperatures, that he is sure of the cause and effect. Relationship.

The hole in the plug is the size of a #60 drill. Yes, that small.

One might ask how such a small hole could possibly make a 10 to 12 degree difference in the operating oil temperature. The answer is so obvious that I won’t bother the readers and take up article space to go over it!!

If anyone  has  had any direct experience with this phenomena, or has run across a reasonable explanation, (other than the obvious one) we would certainly be glad to hear about it.

One of the people that I asked about this in years past said that it reminded him of a similar concern with oil temperature on seven Peitenpols with Model A Ford engines. Of these seven, only one had a worrisome oil temperature problem. The others had not seen any indications of this condition..

They analyzed every possible explanation for the differences, including  engine baffling, rpm in climb and cruise, type of oil, amount of oil carried, and could find nothing of substance, that would account for the difference.

The matter was pretty much put to bed by an outside observer who pointed out that the owner with the oil temperature problem was the only one that had a plane equipped with an oil temperature gauge.