I am going to pull the head on my Tecumseh HS40 Model: 55565K in the next few days.
I'll let you know what 21 oz. did to it. Should be interesting.
I figure, if they sell a dip stick for the purpose of using on a slant mount. it must mean you need less oil, since the safe zone is lower.
The oil level on a Tec HS40 is 21 ounces. Period. That is from "dry." If you have a "wet" engine, even with drained oil, you will not need 21 ounces of oil. If you added 21 OZ of oil to an engine that was not "dry," meaning, brand new, or freshly rebuilt and oil free, 21 OZ of oil will over fill it.
Trust that fancy Rupp Parts $15.00 dip stick. Or fill to where the oil level just barely reaches the bottom of threads on the fill plug.
Your oil level did not cause anything to go amiss with your head gasket. The ONLY indication of over filling on these engines is extra oil being pushed out through the breather assembly. It is not that big of a deal. As was explained earlier, or in other threads, these are oil slinger systems, where a dipper on the end of the rotating assembly throws oil all over the place, lubricating the rod and cam bushings.
Before you remove the head, check the torque on the head bolts. They should be at 200 inch pounds.