The 848 head was cast with a smaller combustion chamber to increase the compression ratio. The earlier head can be milled .090" to get the same compression as the 848, but a few other modifications have to be made along the way. The intake valves need to be recessed into the head the same distance as the milling job to prevent piston/valve interference, and also to protect the edges of the valves from the flame front across the surface of the head. Then the rocker arm stands have to be shimmed up the same distance to keep the pushrod length and geometry correct, and a lash cap needs to be added on top of the exhaust valves to keep that geometry right. It's not a terribly difficult project, but attention to detail is important. As the 848's get harder to find, I believe the old school head milling procedure will get popular again.
Edit: It would also be possible to just shorten that tall intake valve stem the same amount the valve is recessed, but if you choose to do that, run a lash cap to avoid the possibility of getting into a "soft" area of the stem and promoting rapid wear. Be sure to account for the thickness of the lash cap when you shorten the valve.
Jerry
