I found a smokin' deal on a set of forged 3V pistons. Will they work in a 4V motor? Specs are the same except for the top. I thought I could just notch them for valve clearance?
No. 3V are notched for 3 valves, if you notch them further (if they survive) you will alter the compression. But the correct pistons, don't be penny wise and dollar foolish
You shouldn't have to notch 4V pistons unless they are flat top or dome. The valve geometry is such that the 4v's don't really need it. The notches in the 3V pistons are for the exhaust, and they typically do one notch on the top and one on the bottom so the piston can be used in either bank. I think that some 3V pistons don't have notches as well. I can't really see the valve notches impacting the flow in a 4v setup. If they don't have any ... all the better.
The one thing you want to be carefull of is to make sure the deck height is correct for what ever kind of 4v you are putting it in. (low compression, high compression, 4.6/5.4, etc)
Any 4V guys out there let me know if I'm wrong on this, but I think he's o.k.