Anyone got an ATI Damper and caliper handy and can take a measurement for me?

Looking for the center to center distance between the 3 bolts.

 

Four ATI dampers, one MSD crank trigger wheel, and one dry sump arbor I found here all measure 2.7713" center to center.
(3x 120° on a 3.200" bolt circle is more accurate)

Miata ATI may be different???

 

The one I have is on a car and so is a little awkward to measure.

2.7 from the outside to outside of two bolts. Bolt width is .3 = 2.4 centre to centre.

 

A call to ATI should answer this quickly.

 

What PN is yours? I have a 918522.

I'm getting 2.375".
2.725" outside to outside. .345"(ish) bolt head size.

 

Thanks! This is correct for a miata ATI Damper. I got a confirmation earlier from a contact at ATI and he confirmed. Works out to be a 2.750" Bolt Circle with 5/16 bolts.

Thanks again everyone.

 

This is the part I'm about to have machined. Will drive a mechanical fuel pump.

 

Good to know, I'm glad you went to the source for accurate info.
Nice. ATI sells mandrels but it's easy to spin off your own that precisely fits your application. My $0.02:
1. Make the corner radius between the mandrel and the flange as large as you can
2. Make the flange contact the mounting face all around. Getting rid of your scallop cuts completely would be the stiffest but won't look as cool.
3. Add a boss to the back side that locates on the ID of the damper. This will be more accurate than relying on the 3 bolts to center your mandrel on the crank. It's hard to see from your screenshot but it looks like you may have this already.

You many not think you have much static tension on that belt (bending moment on your mandrel) but harmonics are a surprising.

 

Good points. I will think about this and revise the design. I didn't think about resonance but you're right. Having a changing stiffness as it rotates probably won't help that any either. Last thing I need is the belt flying off at high power and going lean.

I do have a shoulder on the backside to keep it concentric, it's just hard to see like you say.

One thing I could do to really improve the stiffness is to machine the pulley ID from 1/2" to say, 3/4" or so. Then I could do a larger diameter shaft on the mandrel which would be way stiffer. Thoughts?