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I have upgraded my v2 with the carbon fiber arms from trick laser. (here)
How do I determine my Diagonal Rod Length? In the software manual it says:

Open the EEPROM editor and check the values shown below. If the Diagonal Rod Length parameter isn't set to 290.8, please change it to that value. The starting position for the Horizontal Radius is 144.
If you're working with an older v2 or v1 with the u-joint based arms, your Diagonal
Rod Length should be 269 and your starting Horiztonal Radius should be 130.25

How do I know if I have the u-joint based arms? What do they look like?

On the trick laser website it says:

Note: OEM arm legths; v1 & v2 with "u-joint" ends are 269mm, Ball-Cup arms are 291mm.

What do the Ball-Cup arms look like?
I'm not sure whether to set my Diagonal Rod Length to 290.8, or to 269. Any help/photos is appreciated, thanks!

If you got the trick laser arms you linked, you should set the diagonal rod length to 269MM. The U-joint arms have a pair of rectangular white plastic pieces on each end, which are linked by a steel pin and a black plastic clip (These are the arms the printer came with, not the ones you ordered). The ball joint arms have a single white plastic linkage with ball on each end, and a white plastic spring.

For posterity... (Disclaimer, these are all google image searches. If they're your pictures, thanks for taking them!)

The very first Max had aluminum u-joints that were on axles clamped between the two halves of the effector platform. The first iteration of this required sanding and manual tolerancing to fit correctly, and the second iteration came pre-machines for the right fit (which saved hours of manual labor!).
[img]http://i3dtech.weebly.com/uploads/2/4/8 ... 9_orig.jpg[/img]
[img]http://mhackney.zenfolio.com/img/s4/v11 ... 9210-3.jpg[/img]
[img]http://i.imgur.com/YyFaj5F.jpg[/img]

The next version, which came about around the same time as the V2, but was a rolling change in the V1 near the end of its life, is the clip style arms with aluminum joints.
[img]https://computersourcesc.com/wp-content ... G_2035.jpg[/img]

Next, they switched to using Nylon u-joints instead of aluminum.
[img]https://thingiverse-production-new.s3.a ... atured.jpg[/img]

The arms stayed the same, but it's worth noting that they changed the mount for them when the injection molded CheapSkates came out.
https://cdn.shopify.com/s/files/1/0276/ ... 1438697840

And then most recently, they've released the ball-joint arms.
[img]https://cdn.shopify.com/s/files/1/0276/ ... 1446822945[/img]

2nd to last image, the clip is installed incorrectly.

g.

geneb wrote:2nd to last image, the clip is installed incorrectly.

Sure is, yep. They go between the yokes on the arm.

That one actually looks printed, so I probably found somebody's picture from the original prototype.