This is a brand new steel flywheel that fits 364 / 401 / 425 Buick nailhead engines. They are drilled to accept either a 10" 1/2 inch or 11" inch GM clutch assembly. They are balanced for which ever one of these engines you tell us you have.
The 401 and 425 have the same balance. The 364 is 1/2 the weight at the same radius. These flywheels are machined in a way that you do not have to do any grinding on the crankshaft for them to work on all nailhead engines from 1957 to 1966.
If you are buying a flywheel for a 1957 thru 1963 engine, you will have to use a starter from a 1964 to 1966 Buick nailhead.
They will NOT fit the 264 or 322 engines made from 1953 to 1956 in the cars. The 322 was also used in GMC trucks til 1959. Check to make sure you ID which engine you have before ordering. The flywheels all have new ring-gears on them.
Remember that since the Buick nailhead engines are externally balanced, you will have to take care to correctly orient the flywheel to the crankshaft. Failure to do this will result in a severe vibration.
There is a sight hole machined in the flywheel so you can see the alignment hole in the crankshaft while bolting the flywheel on.