What’s the best way to determine/measure a bolt circle?

[Janderso]

I need to finish this project.
A customer asked me to machine this flywheel front and back to fit for the engine conversion while expanding the pilot hole as well.

The lathe work is done, now I need to drill for this bolt circle.

So, once I determine the bolt circle I can move to my mill, center it on 1x2x3 blocks, clamp and set up the DRO for the coordinates.

Thank you for your help.

 

[WobblyHand]

I need to finish this project.
A customer asked me to machine this flywheel front and back to fit for the engine conversion while expanding the pilot hole as well.

The lathe work is done, now I need to drill for this bolt circle.

So, once I determine the bolt circle I can move to my mill, center it on 1x2x3 blocks, clamp and set up the DRO for the coordinates.

Thank you for your help.

Get a pair of pins that fit the bolt holes tightly, measure the OD pin to pin, then subtract out the diameter of the pin. Do it three more times and take the average. Ought to get you close. How close do you have to be?

 

[Janderso]

Like this?
I measure 2.692” from pin to pin. The pins are .420”.
So 2.692 plus .420 = 3.112” or 79.04 millimeters.
How close? As close as I can get.
Usually bolt circles are a pretty round number. In my experience anyway.
It’s probably a metric size.
79 millimeters is 3.110. Pretty darn close.
I bet that’s it??. = 79 millimeters

 

[mmcmdl]

Another way with pins ..............use 2 pins in opposing holes and clamp in a trammed vise . Tram the 2 pins for an accurate measurement . Without pins ........................measure the outside of 2 holes . Measure the inside of 2 holes . Split the difference .

 

[Janderso]

Another way with pins ..............use 2 pins in opposing holes and clamp in a trammed vise . Tram the 2 pins for an accurate measurement . Without pins ........................measure the outside of 2 holes . Measure the inside of 2 holes . Split the difference .

Hey, idea number two is easy!

 

[mmcmdl]

Hey, idea number two is easy!

Of course it is . I'm lazy !

 

[Janderso]

Another way with pins ..............use 2 pins in opposing holes and clamp in a trammed vise . Tram the 2 pins for an accurate measurement . Without pins ........................measure the outside of 2 holes . Measure the inside of 2 holes . Split the difference .

The range is within .0008”
I’m going with 79 millimeters or .3110.
Thanks man.

 

[Janderso]

Get a pair of pins that fit the bolt holes tightly, measure the OD pin to pin, then subtract out the diameter of the pin. Do it three more times and take the average. Ought to get you close. How close do you have to be?

Thanks man!!

 

[WobblyHand]

Now just to be a contrarian, I mentioned doing this with the other hole pairs. We are assuming this is a perfect bolt circle and the bolts are perfectly spaced - usually they are. But they may not be, or the part was made wrong. Been burned by assuming things, so I always check for things like this.

 

[mmcmdl]

Good luck and have fun Jeff ! Get yourself a Hartford Super Spacer if you ever see one around . Makes quick work of those jobs .