Morris Minor Owners Club

Ive recently bought a new oil filler cap as I was under the impression I have the wrong type (vented vs not).

It looks as though I have the right one to begin with...
However, I thought it would be nice to use my nice shiny new one... but it won’t go in. Am I being dense? Have I bought the wrong one? Has my engine hole shrunk? Do I just need to push it harder. Maybe mechanics isn’t my calling

My filler cap is difficult to turn and lock, I presume that is your problem too. It could be because it’s new and/or made of a harder material with less give. Of course, it could well be a recent production issue.
Why not measure various points compared with the original one, I’d be interested to know if there is a difference.

Note the differences in how the cap seals are mounted.
The seal fitted to the cap with the securing ring/tie is correct. It will deform into a wedge shape when fitted. The cap will be at first tight to turn and secure into the rocker cover. Fitting will become easier with use.
The cap without the securing ring/tie is correct but with a very deformed seal.
Below is a photo of a used but serviceable vented cap with the seal correctly deformed into the wedge shape. Just use a bit more force when fitting the vented cap. The seal will deform into the correct wedge shape with use.

I’m possibly being a bit delicate with it. I’ll jam it in there tomorrow and see what happens