New guides come in different types.
There are the thin wall "guide liners" usually made of bronze. The guide is reamed out slightly oversize and the thin liner is installed. I am not a big fan of those. Bronze is good at resisting scuffing from lack of lubrication, but it wears out quickly and has a low melting temperature.
Then there are what I refer to as actual guides. Calling them an insert isn't really correct. They have an outer diameter of about .503". The existing guide in the head (which is just the iron of the head itself) is reamed out to .500" exactly and the guide is hammered into place then trimmed to length. The guides are already the correct inner diameter for proper clearance to the valve stem, but unfortunately hammering them into the head collapses them a little and they have to be reamed back out to proper size. These guides come in steel with a smooth inner bore or steel with a partial spiral groove at the top to let oil get into the guide except for the very tip near the valve head. I prefer the spiral groove guides. All my engines have them.
'72 Jeep Wagoneer Custom, 360 V8
I love how arguements end as soon as Ristow comments. Ristow is right...again.