valve cover grommet for twist-on cap

The grommet that you linked to on Summit would work. BUT, I see they are extremely helpful in providing ID and OD measurements.

You might be able to find what you need at O'Riellys, I just took one of my M/T covers right into the store last week and fitted what I needed.

I have had one heck of a time getting the correct sizes, because a lot of places don't provide that info, just like the one in the Summit link.

The only way I felt comfortable in getting the correct ones was to do what I explained above.