[TheWolf]

OK here's the deal. The machinist in me says yes you should machine the seats to fit. Seeing it from the perspective of the cheap nissan owner who does the least and expects the best, you might not actually need to machine to install.

Basically it's like this. 1mm is .040" wide. So a .5mm oversized valve is .020" wider than stock. Since they're circular that's only .010" per side of the circle. So putting a .5mm oversize valve in a stock seat is going to result in a valve that protrudes .010" further out of the seat and the contact line is going to be .010" higher on the 45 of the valve.

This is assuming your seats in your head are brand new in stock condition. Since they're more than likely worn to some degree this may actually not be a bad deal with the larger valve. You might luck up and the larger valve goes right in and would work fine. You would need to atleast lap them to install them but there's a good chance that the contact line would be right where you'd want it.

To do the job properly. You would definitely require machining the seats. Going with the larger valve and not machining the seat will not net you any flow gain and would actually result in a bit of a flow loss due to the large head and choked seat. I would surface the head, front cover piece, and get the seat work done before installing them so you don't have a problem with assembled height. See as the valve sticks out farther out the bottom. It sticks out less at the top. Thus the valve train is going to have to expand to take up that extra .010" or less depending on wear. If you were on a bucket and shim type valvetrain you would be required to do the machine work but with the lifters on the KA style head you should have no problems with the extra lash.