There are also 'X'-Rings that seal very well. I don't have the size at hand but can get it and return here.
Otherwise, you can likely measure diameter and ask the folks at Oringsandmore.com (804.741.1521) and get one that fits the opening/threaded area.
mike
PS, I looked up the 'X'-Ring size and its: 3.63 x 2.62 x NBR-70. At least for an FX Impact.