My original head was broken so I bought a replacement head (part no. 67-381) which did not have the bottom valve sping cups fitted (part no 67-0033). Having procured a new set I find that they will not fit over the valve guides Do they have to be heated prior to fitting. Measuring, to the best of my abilities, the guides have an exterior diameter of 15.69 mm and the spring cups have an interior diameter of 15.47 mm a difference of about 0.22 mm. Any advice would be gratefully received. Thank you