That was used as a pressure tap in some installations. The plastic nipple
needed to be drilled out with a tiny drill. It was hard to keep a pressure
line attached here, so it was used very little.
If you plan to use this engine DO NOT drill out the tap. It will provide
crankcase pressure which is way too high for any use other than certain
applications that require more power. If you need more power get a more current
designed engine like the Norvel or other similar powerplant that isn't a 40
year old engine design. They will run circles around the TD but I still like my
I just noticed you don't know the displacement If it has a small band around
the Base of the piston when you look through the exhaust port ( Piston at TDC)
it is a .051
If there isn't any grove on the piston it is a .049
This was done so that it was easy to confirm the displacement at model contests
since 1/2 A went to .5 and the A class was above. This was handy so that you
could fly the same plane in 1/2A and A free flight at a contest. I was so poor
back then I would just change piston and Cly and not do the engine swap.