The Puzzle of the Perfect Fit

If you were building a door for a house, you would want the edge of the door to be perfectly flat, and the door frame to be perfectly flat. If they match exactly, they close tight.

In engine repair, you might think the same rule applies. If the valve seat is cut at a 45-degree angle, you should grind the valve face to exactly 45 degrees, right?

Actually, no.

Expert engine builders often use a trick called the Interference Angle. They grind the two parts so they don't match perfectly—usually by just 1 degree. This tiny difference is the secret to a seal that lasts for years.

What is an Interference Angle?

It is simple math.

• The Valve Seat (in the engine head) might be cut at 46 degrees.
• The Valve Face (the moving part) is ground at 45 degrees.

Because the angles are different, they don't touch across the whole surface. Instead, they only touch at one very specific, sharp line.

The "Sharp Heel" Analogy

Imagine walking in muddy grass.

• If you wear flat sneakers, you might slide around because your weight is spread out.
• If you wear soccer cleats (shoes with spikes), all your weight is focused on those tiny points. You dig in and get a grip.

The interference angle works like the soccer cleats. Because the valve and the seat only touch on a thin line, the pressure on that line is huge. The valve spring pushes down, and that "sharp" contact point digs in. This creates a high-pressure seal that traps the air and fuel inside the cylinder much better than two flat surfaces could.

Why Do We Do It? (The Benefits)

There are three big reasons to use that 1-degree difference:

1. Instant Sealing: A perfectly matched "flat-on-flat" seal is hard to get right. If there is even a speck of dust, it leaks. The interference angle creates a seal immediately, even on the very first start-up.
2. Crushing Carbon: Engines get dirty. Carbon (burnt fuel) builds up on the seats. A flat seal might get held open by a piece of carbon. A sharp interference angle acts like a knife—it cuts right through the carbon and closes tight.
3. Faster Break-in: As the engine runs, that sharp line will eventually wear down and become a wider band. By starting with a sharp line, the valve "seats" itself perfectly into the shape of the head.

When Should You NOT Use It?

There is one trade-off. Engine valves need to get rid of heat. They do this by touching the seat and transferring the heat into the engine head/coolant.

If the contact line is too thin, the valve can’t get rid of heat fast enough. For standard passenger cars and trucks, the interference angle is great. However, for extreme racing engines or heavy-duty engines that run incredibly hot, builders might skip the interference angle to get maximum cooling.

Setting Your Machine

This is where having a quality Valve Refacer matters. To get this right, you need to be able to set your machine to exactly 45 degrees (or whatever your target is) without guessing.

Machines like the SVSII Deluxe are designed for this. They allow you to dial in specific angles easily. If you are trying to grind an interference angle on an old, wobbly machine, you might end up with 2 degrees or 3 degrees of difference, which could ruin the valve. Precision is key!