Often you'll find off-the-shelf high-quality headers will "fit" alot tighter than custom/xforce headers as the tolerancing will be much much tighter. You often see this in the bolt holes that connect the header pipe to the block. Xforce use larger holes to compensate for a non-exact fit. Toda/Spoon/Maximworks/etc will have precisely machined holes for an exact fit.
Just to add my 2c worth, having tighter fit bolt holes allows for much better port alignment compared to a header that has alot of free play with the flange.
Originally Posted by roar
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