Originally Posted by ezveedub

The K engines have two relief ports under the exhaust port. The head button is also changed. It runs the same balanced crank though.

Originally Posted by houston

the vertical transfer ports have been switched to the other side of the horizontal transfers , on p5, p5x, p5xs, plus 21-5 the verticals are closest to the exhaust , on 21-5k they are closest to rear main intake with diverters in the block to match . new style thick top ring sleeves to hold pinch at peak performance pinch longer is also another difference .
plus 21-5 is a great engine and if you wanna buy a 7 port piston/sleeve or 421b (821b) p/s set and make it faster you can .
hope this helps ,
monty