A zero deck is when the pistons are level with the top of the block at TDC. A negative deck CLEARANCE is when the pistons stick out of the block. A positive CLEARANCE is when you have MORE clearance, such as when the pistons are down in the hole at TDC. The key word here is CLEARANCE. If you are still confused, ask yourself "what do the pistons have to clear at top dead center"? The answer is THE HEAD. I'm pretty sure that Doug Roe had it right. He designed the aftermarket head for the Vega 4 cylinder. I bought one and sent the block out to be machined for a negative deck clearance (.020 stick out at TDC) and with the engine together I actually measured the CLEARANCE with a feeler gauge through the spark plug holes. That was the only change that I made to that engine and the results were: 55 horsepower on a chassis dyno and the mileage went from 21.4 mpg stock to 43 mpg with the Doug Roe head (all freeway). I drove the car for 94,000 miles before I put a V-8 in it. I have over 50 years of experience with builds like this (the current 350), so please don't tell me that positive piston stick out @ TDC is positive CLEARANCE, because if you build one like that you are going to hear some noise when the pistons hit the head.