I’ve just bought a new set of Z rods for my Makerbot to improve the quality of my prints. If you look closely at prints from my Makerbot (http://www.flickr.com/photos/49325300@N07/4778528559/) you can see that there are lines about 1mm tall that go in and out of the object. This is called Z wobble, waviness, etc and its from the Z rods on Makerbots not being perfectly straight. This is mostly a design flaw that could have been avoided, but it also could have been avoided by using high quality ACME rods instead of plain old threaded rods. The threaded rods it uses are not meant to move things accurately. They are meant to hold nuts that hold stuff. Therefore the threads don’t have to be very accurate or very straight. The Z axis on a Makerbot is directly driven from the threaded rods which means that any imperfections in the rod will directly affect the print.
When you’re someone like me who is obsessed with getting $15,000 dollar quality on my Makerbot, this Z wobble is very annoying. So I went for it and bought some replacement ACME rods off of ebay. For anyone else who is interested, here’s the link: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330462274481 You might have to search for “makerbot upgrade” on ebay because this ebay auction is going to expire soon. I made an offer of $60 bucks and they took the offer so I ended up getting it for <$70 bucks, with shipping. I see this as a pretty good deal.
I haven’t received my new rods yet (I just ordered them a few days ago), but I will post when I do.
Makerbot #448 
Any way to get a McMaster-Carr link to the rods?
They aren’t from McMaster and I don’t think you can get ones like this from McMaster. They have the ends turned down to 8mm to fit the bearings. I wouldn’t recommend buying the ACME Z rods. Instead I would suggest trying out this instead: http://www.thingiverse.com/thing:4740 It yields better precision and it is much cheaper.