I have never worked on 915 heads before. I am putting 2.14-1.81 valves in and I would like to cut inside the throat area to take full advantage of them. how much can I cut [mainly] the area between the intake and exhaust valves ? just worried about getting thin in the water passage area. any ideas?

Your best friend will b an ultrasonic thickness gage.
I plot mine out then put them on my Bridgeport n rough them out n finish blend everything by hand. I have a German boring head n put a nice shallow taper big at the top n smaller at the valve guide. I have found water a time or 2 before getting the gage!!!

what size is the bit you used ? [just trying to get a general idea on thickness ]. thanks for the help... advice taken ....

The boring head is adjustable so no size. I'd take a cut then go around with the gage n find my thin spot n work to that. I like to keep .100 wall thickness at the thin spot. You have to remember on the other side is a 40 yr old cast wall that has seen water n will vary from casting differences n rust from the water,also beware of casting shift.

I use a cutter called a bowl hog, it is set at 70 degrees, I purchased mine from Goodson, but they stopped selling them. http://www.silver-seal.com/category/shop...ting.bowl_hogs/ still has them. The heads are pretty thick in the seat area, but get thin quick as you move down the throat. I have cut a bunch with the bowl hog, but if the head has core shift, it may still go to water. Only had one do it, and when I cut the head apart to find out what the deal was, it was core shift.