Lots of people including myself are running 4k springs with stock internals. The only must is you have to change the valve springs to HD 60lbs springs. Other than that the rods, bearings, etc will be fine. Remember just because you put 4k springs in does not atuomatically mean you are going to turn 4k. You still must have the proper combinations of fuel, timing, and air to achieve the 4k. Right now I can only turn about 36oo myself.
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