I am a light guy and wanted a softer spring in my 400EX shock. So I decided to try and install the stock blaster spring in the EX shock to see if I could soften it a little. I was able to install it and it turns out the blaster spring is a little softer... to figure this out, I set the 400EX spring to its lowest ride height that still had a little compression on the spring. Then I stood on the grab bar and measured the ride height to see how much my weight compressed it. Then I did the same with the blaster spring in it. The blaster spring allowed it to settle 3/4" lower than the 400EX spring. (they both had the same ride height before my body weight was added to the grab bar).
I am pretty sure the stock blaster spring is a dual-rate spring... The reason I say this is because the section where the coils are closer together is easier to compress with my hands than the other section of the spring. It's a pretty significant difference.
As far as I can tell, the springs are almost identical in dimensions. The blaster spring might be a couple millimeters longer, but not by much (It was a little tighter fit when installing in the 400EX shock). While I had them both out, I took some photos:
Same Height:
Same diameter:
Removing the spring from the blaster shock wasn't too difficult. on the top of the shock there is a little spring clip that sits in a groove (similar to a piston ring) that holds the spring in place. All that needs to be done is to adjust the spring to it's softest setting. after this, the spring needs to be compressed - it doesn't take very much. with the spring compressed, you can remove the ring at the top of the shock. Then decompress the spring and remove the part at the bottom that holds the spring. After that, your spring just slides off.
In this picture, you can see the ring and you can see the groove that it fits in on the shock body: (pictured with 400EX spring)
Here is what the bottom of the shock looks like:
Removing the 400EX spring from it's shock was much easier. you do not need to compress the spring at all to remove it. I didn't even uninstall the shock to swap the spring. Just set the quad on something to get the rear wheels off the ground and take the lower pin out. Then adjust the spring to it's softest setting. You should be able to get it soft enough that the spring is loose on the shock. Once you're here, you can remove the two half circle supports from the bottom of the shock and then the spring will slide right off. You can change the spring on the 400EX shock in a matter of minutes.
Here is my blaster spring on the 400EX shock installed in the quad:
I am pretty sure the stock blaster spring is a dual-rate spring... The reason I say this is because the section where the coils are closer together is easier to compress with my hands than the other section of the spring. It's a pretty significant difference.
As far as I can tell, the springs are almost identical in dimensions. The blaster spring might be a couple millimeters longer, but not by much (It was a little tighter fit when installing in the 400EX shock). While I had them both out, I took some photos:
Same Height:

Same diameter:


Removing the spring from the blaster shock wasn't too difficult. on the top of the shock there is a little spring clip that sits in a groove (similar to a piston ring) that holds the spring in place. All that needs to be done is to adjust the spring to it's softest setting. after this, the spring needs to be compressed - it doesn't take very much. with the spring compressed, you can remove the ring at the top of the shock. Then decompress the spring and remove the part at the bottom that holds the spring. After that, your spring just slides off.
In this picture, you can see the ring and you can see the groove that it fits in on the shock body: (pictured with 400EX spring)

Here is what the bottom of the shock looks like:

Removing the 400EX spring from it's shock was much easier. you do not need to compress the spring at all to remove it. I didn't even uninstall the shock to swap the spring. Just set the quad on something to get the rear wheels off the ground and take the lower pin out. Then adjust the spring to it's softest setting. You should be able to get it soft enough that the spring is loose on the shock. Once you're here, you can remove the two half circle supports from the bottom of the shock and then the spring will slide right off. You can change the spring on the 400EX shock in a matter of minutes.
Here is my blaster spring on the 400EX shock installed in the quad:

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