I have a very old pair of 440 Source chrome steel valve covers. There is a 1 3/8 hole that I want to use for a oil filler cap. What was the stock size hole for a oil filler cap grommet?

None of the stock Mopar steel valve covers used push in breathers through rubber grommets before 1970
1970 used a rubber grommet on the PCV and push in breather caps, not sure of that size, sorry Those motors had oil filler caps that had to be twisted into the valve cover to seat and stay on, no grommets either on those

Well, I have a 1 3/8 hole that I can't make smaller but could make bigger for a grommet that would fit a oil filler cap. I don't care about year, just want a grommet and cap combination.The grommet I have found from McMaster Carr is 1 inch ID of which I have also not found any caps that size.

Most of the after market push in breather caps are 1.0 inch, O.D. with a little larger retainer groove on the bottoms, look at Jegs and Summit to see if you can find some you like

I have one of those caps. I can file off the lip at the bottom so it is 1 inch all the way. If that doesn't push in nicely, I will take a steel cap, drill out the rivet and screw a piece of wood dowel to it. If it is a little undersize, add some shrink sleeving. Thanks.

Check for a grommet kit that has both the 1.0 inch I.D. and the adapter grommets from 1 3/8 down to 1.0 inch