I'd have loved to make this a video guide. Sadly my Graphics card isn't good enough to do that, so pictures will have to do.

I've kept the build simple so it's easy to work. But believe me, this is far from the limit of what you can do.

The first thng we're going to need is a frame and a couple of pistons. That's pretty easy.


Then start building the blast door onto the pistons. (leave a gap, you'll need somewhere to put the merge blocks)


Now, you'll have to extend one of the pistons before you build in the merge blocks. (The collision detection won't allow you to place them next to each other whilst they're on seperate pistons.

It's very important that the merge blocks are sideways to the pistons. If you place the merge blocks facing the same direction as the pistons then the merging force will tear one or both piston heads off.

Now you can extend th other piston and bring the merge blocks into contact.
The merge blocks may not lock straight away. You may have to move them over each other a few times before they decide to merge. Just keep at it, and they will merge sooner or later.


Now they've merged this is the last time you'll ever want to operate either piston individually. So go ahead and make a control group for the pistons.


Now that is done all you have to do is finish the blast door.

You can take the merge blocks out as soon as you've built around them. Once you have other blocks connecting the two halves of the door the merge blocks are no longer needed.

And then you end up with a blast door looking a little like this.



Here's the workshop link to the door that was created during the writing of this guide
http://steamcommunity.com/sharedfiles/filedetails/?id=298115617