#### OVERDRIVE

##### Well-known member

- Joined
- Jul 31, 2017

- Location
- Brooklyn, Neu York

I whipped up this low-res diagram. The cam gear has 38 teeth and a new Gates belt measures 14.25" across the circumference of the gear. This is not a tensioned measurement, the cam gear circumference when using 4.5" inches as the diameter is 14.137" so I'll just call it 14.2". 14.2" / 720 (remember we're calculating degrees at the crank) is .0197" for every degree.

The equation to get crankshaft degrees retarded at the camshaft where D is the distance removed from the deck surface in inches is

(12.25 - (√((11.863-D)² + 9.339))) / .0197

This is just a simple Pythagorean theorem equation to find the change in the distance from the auxiliary shaft to the camshaft, then that distance is divided by our constant, how long is the distance on the belt for every degree.

Mathematically this isn't a linear equation but when calculating for thousandths of inches removed on such a large triangle it's moot.

.040" removed calculates to 1.98°. Remember that this is crank degrees, so the camshaft centerline will need to rotate clockwise only 1° to correct this. This is pretty close to every .020" being 1° which is half as much as I've heard mentioned many times over.

Infrequently asked questions:

Q: What about a Pinto belt?

A: If you need to run a shorter belt, that slack will be removed from the idler/tensioner side, this does not impact the distance from the crank to cam gear on the auxiliary shaft side of the belt so it doesn't impact the camshaft angle.

Q: I have an STS gear and shaved my head .020", so I advance it one tick mark for 1 degree?

A: No. Each tick mark on an STS gear is 2 crank degrees.

Q: But I degreed a cam when installed and got a different result.

A: These are quite small measurements and there's too many variables for valve events like valve lash, how worn/stretched the belt is, if the roll pin in the cam and gear are a little sloppy, if the roll pin in the cam isn't 100% at the centerline that you see it advertised at etc.