Reader’s question: While I had the 350 out of my ’69 Corvette to detail both the engine compartment and the engine, I replaced the dented-up oil pan with a new GM P/N 360866 Corvette service pan I found on eBay. I already had a ’69 gasket set, and installed all four pieces very carefully, with particular attention to the corners. Shortly after starting it, I had a major leak between the timing cover and the pan at the front. A friend suggested I try one of the new Fel-Pro one-piece molded gaskets instead, and it didn’t help – it leaked in the same place. The parts store verified that I had the correct Fel-Pro gasket for a ’69. Why would I have the same leak with both correct gasket sets?
Response from John Hinckley (Corvette Enthusiast Magazine)
The leak you have is the result of a change in the oil pan front-seal radius Chevrolet made in 1975. The production oil pans were changed in 1975, enlarging the front seal radius, which required a thicker front seal (0.41”) in place of the previous thinner (0.22”) front seal.
This front seal radius change was also made in all Service replacement oil pans manufactured after 1975, regardless of their pre-1975 intended applications; GM sealed-box Service oil pans included a gasket set with the new “thick” front seal those parts required. Your 360866 oil pan needs a ’75-up gasket set, or the Fel-Pro #OS-34510-T one-piece gasket for 1975-up applications molded with the “thick” front seal; pre-1975 oil pans use the Fel-Pro #OS-34509-T one-piece gasket molded with the “thin” front seal.
Unfortunately, there is no part number identification on GM production or Service oil pans. To determine which gasket set you need for an unknown-vintage oil pan, flip the pan upside-down on a flat surface (with no gasket) and measure from the flat surface up to the 12 o’clock position on the flat portion of the front seal surface. If it measures 2-1/4”, it takes the “thin” pre ’75 front seal, if it measures 2-3/8”, it takes the “thick” ’75-up front seal. The one-piece molded gaskets do a great sealing job, but make sure you get the right one to match the vintage of your oil pan.
Your 360866 oil pan will be leak-free with a ’75-up gasket set incorporating the correct “thick” front seal it was designed for.