J4 rims fit perfectly to the Series MM axle. 4.5 inch for the J4 and, I think, 4 inch for the Austin Cambridge wheels with the exact same centres but thinner rims. Later Morris 1000 hubcaps fit perfectly, too. Currently has a 185x14 Commercial tyre on it, hence has a half inch increase in rolling radius but I'll get the radius back to normal with a tyre change.