The 3.9" OD by 5" long Coupler between the 31" Booster Tube and the Transition/ISC, slides into the ISC transition 1.25".
I grabbed a few screenshots from my OR model to detail this part. I could not find a way to make a transition with both a conical section and a straight section that wasn't assumed to be an internal shoulder. So, as a workaround, I added a short section of body tube to complete the transition/ISC part in the OR model.
Screen shot of the Transition/ISC model per actual measurements of the part I received: See OR Model File further below.
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Screen shot of my work-in-pogress OR model with no motors. I'm up to 107 oz without electronics or motors so far.
See OR Model File further below.
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Here is the OR model for the Transition/ISC that I used:
Here is my current OR model... take a look at the dimensions ... I've gone through and measured all parts, so the model should be pretty good.