MGN12H Mini Linear Rail, 250/300/ 400/500/ 550mm Linear Sliding Gide with MGN12H Carriage Block for DIY 3 Dimensional Printer and CNC Machine (300mm)

goods were as described and arrived in a good time frame

I bought 8 of these. It turns out everybody's reviews are accurate. There's some that are terrible and some that are great. It's a gamble. 2 of them were amazing fits. 1 was a decent fit, 2 were kind of tight, and 3 were too tight. I thought maybe it was preservative gunk, but even after cleaning nothing changed. I could swap and block to the tight ones and they'd always run tight or any block on the good fitting rails and they always fit well. So it's the rails, not the blocks.I bought these because they were offered on a super deal at $11 and I needed more blocks for rails that I already had so figured I'd buy the blocks for $11 and get a free rail. Word of warning, either way, these rails and blocks are out of spec for use with other manufacturers rails or blacks labeled MGN12H. Another manufacturer's depth of cut into the slide was 9.30mm and these ones were about 8.90mm.Would give two stars, but I got them for such a cheap deal that 3 stars is warranted for the value because I'm not sending these back. I'm going to keep them. It ended up being an okay deal for me. The super tight rails I can use on lead screw driven axis and the other 5 are great or decent for belt driven ones. If you're just buying one or two rails it's not worth it because you might end up with all crappy ones and you can buy some other cheap ones from another manufacturer that might cost double but they're still cheap and you end up with what you wanted.

I've ordered 4 of these and received 4 rails made by 4 completely different machines/people/companies.. 1 of them is great, high quality, clean bearings, properly lubricated, and well more than I expected for the price. 2 of them are ok, with some roughness. Both of these needed to be cleaned and lubed properly before use, but are working well and still a good value. 1 of them is hot garbage with mismatched pot metal balls, poorly machined rail, bad wipers, no lube port etc and is now in the trash.I'd return it but I don't know which was which and don't want to penalize the 3 folks that sent me acceptable products..It's really hard to write a consistent review on what are basically 4 different products.

A quick inspection showed the rails had several bearings missing right out of the package and it doesn't even come with spare bearings. An easily fixable issue but still annoying for a brand new part. Did an initial alcohol wipe down & re-lubrication, added the missing bearings but it was still binding. So I checked the carriages and both metal clips securing the bearings were loose due to shoddily designed carriage parts. No wonder the bearings kept popping out. At this point this has already taken an hour of my time, not worth investing any more, its going back. All of my cheapo aliexpress rails are better than this garbage.

I didn't like the fact that I sow so many loose ball bearings in the blue plastic package. The carriage was not connected to the rail and there were some holes in the package caused by the sharp edges of the 400 mm rail.I seemed that one side of the carriage was fully populated. It was easy to insert the balls into carriage with a magnetized small screw driver. The end result was that there were not enough balls to make smooth movements. I will purchase more MGN12 carriages (or ball bearings) to make this to work.In past I have been using MGN25 rails and the success ratio has been higher.

This is the worst quality MGN Linear Rail and block that I have ever seen. The block will not even slide on the rail without forcing it. Absolute Junk! Sending back pronto. Stay away.