I was actually afraid that it was the newer design, rather than one like my 02/2014 SD4 (which is basically an SD3 in a taller case, with metal enclosure bits), before I got to see it.  That would have made things a lot harder, I think.

I think you'd just need to go the extra step of cutting the closed loop side belts and punching holes on their edges.

That is an older SD4 which used open loop belts, and did not use linear bearings on X. They used bushings and teflon blocks just like with Y. It's now abundantly clear that the newer SD4s acted as testbenches for the new workbench series, which makes a lot of sense to me. The printed carriages on that machine also look much, much better than the printed carriages mine came with.

I was actually afraid that it was the newer design, rather than one like my 02/2014 SD4 (which is basically an SD3 in a taller case, with metal enclosure bits), before I got to see it.  That would have made things a lot harder, I think.

On yeah, and I always love to see people printing my little dodads (the calibration thingy).

I completely blame you for having a much better set of printers, then.  That calibration thingie has helped me dial in no less than four printers now, and works beautifully for it!  Thank you! :-)

I thought I'd relate this little story, just in case anyone else needs to do what we did, and share some of the magic....

...On a side note, there's magic in watching someone work so hard to reassemble their printer, then level the print bed right, and seeing the look on their face as their printer prints something successfully for the first time.  It makes me feel all warm and fuzzy inside.  Completely worth the effort.

Good for you.  It's nice to see some success stories here rather than all the bashing and utter frustration some folks are displaying.

On yeah, and I always love to see people printing my little dodads (the calibration thingy).

     A close friend of mine, at my urging, bought an used SD4 off of eBay pretty cheaply.   Well, it arrived... in less than perfect condition.  The heated bed wasn't attached any longer, the left carriage had broken into pieces, the right one had split, and the X motor was flopping around loose inside the printer.  This was a disaster!

     Until we got to scheming, and thinking.  The bed proved easy to reassemble.  The electronics were good, motors motored, and switches worked fine.  It was mainly that the carriages that had blown out along the way. 

     I was getting ready to put Lawsy's MK5 and carriages into an SD3 I'd picked up on eBay, so I'd already printed the extruder and carriages, finished and test-fit them to my Misumi rods, and was just waiting to finish my SD3's acrylic enclosure before tearing it all apart again.  So we have the technology, and we can rebuild him!

     The real limiting factor was that the SD4's stock hot end was no longer a groove mount, and I didn't have an E3D yet.  So we were stuck with the stock extruder and center carriage for the time being.  It turns out that this is eminently doable, and results in a working product.     

Him and I started at 11pm.  I kept my SD4 and SD3 where he could see them, let him know how things -should- look and work, and I let him doing about 90% of the work while I lent a helping hand.  By 7am, we'd finished a test calibration print.

     The gotchas:

The stock X carriage belt seems to ride quite a bit higher than Lawsy's design.  This was fixed by flipping the X drive pulley upside down.

The SD4, as we all probably know, keeps its X limit switch on the X carriage itself.  When we mocked up mounting the switch on Lawsy's Y carriage (right), we discovered that the stock X carriage would hit the idler pulley long before it would actually touch the switch.  Ouch!

The solution there was to use a little metal mount to move the X switch from the carriage body out to contact the Y carriage in the rear.  Unfortunately, the only little metal thing I had lost us about a centimeter or so of X in the end.  It's also not held on in a manner I'm proud of.  But it works, and that's what matters for the moment (We're thinking of better ways to mount it now)

The final gotcha was that the belts on the stock SD4 carriages were too short, really, to reach Lawsy's carriages.  We used slightly longer M3 bolts to fix that problem, until such a time as he may want to upgrade to GT2, or purchase MXL in enough quantity to do it right. 

I've attached a few pictures to show what the printer looks like now.  I neglected to take photos when we started (or during the process), but did snap one of the broken carriages after we'd extracted them from the SD4.

The remains of the carriages:

New carriages installed, everything sitting pretty:

Closeup of the X limit switch "adapter":

Here's the first calibration/test print:

On a side note, there's magic in watching someone work so hard to reassemble their printer, then level the print bed right, and seeing the look on their face as their printer prints something successfully for the first time.  It makes me feel all warm and fuzzy inside.  Completely worth the effort.