So I did a quick search and only found one topic from 2015 that touches on this. I want to make sure I get the right size tube for the machine and I have a few options that will ship fairly quickly (so an answer today would be phenomenal so I don’t have the tube getting delivered and sitting outside my shop waiting for me to get back in town since I’m leaving town next Thursday morning… Otherwise, I will wait until it can be delivered the week that I’ll be back). I have found a few listings on ebay and a couple of other sources. The difference is the size of the tubes… some are 700mm and some are 710mm. Are either of these options ok? The other thread I found didn’t really specify if 710mm was ok or too big. I’m not in a position to cut a hole in the size of my machine to extend the tube outside of it (otherwise, I’d go for a 50w or 60w tube and replace the power supply as well 
). I did a quick measurement, but I can’t get an accurate measurement given the location of my machine and the amount of stuff I’d have to rearrange in the immediate area to reorient it in a position (safely) to measure the tube at this time (decided it’s safer to ask vs attempt to move all this stuff, get frustrated and do something stupid trying to hurry through it… I know myself well enough to just avoid moving it until I absolutely need to). Thanks in avance for the help!
I did a quick google search on “K40 laser tube replacement” to see what size sellers are offering.
It looks like LightObject is offering a 720mm tube as a replacement specifically for the K40.
Relevant text from the above link:
This high quality CO2 laser tube is made exclusively for the small K40 laser machine. Unlike other low quality tubes you may find on sites such as eBay, this tube is made by a reputable company. The output is very stable with low beam divergence. The length of the tube is 720mm or 28.5" .
Shipped from California. All tubes are tested with power meter prior to shipping. Do not over current or the life span is shorten.
Specifications
Power: Rated 35W (38W peak)
Triggering Voltage: 20KV
Operating Voltage: 15KV
Current: 16mA, suggest current: 15mA or below
Length: 720mm (28.5") +/- 10mm
Diameter: 50mm (2")
Water Cooling required. 18~24’C(64-75F)
Warranty: 3 months
Thank you for that. So 720mm is still small enough to fit the machine. I was definitely a bit worried and didn’t wanna buy anything without being at least somewhat sure of the size situation.
While that LightObject tube may be a little bit more expensive than an ebay seller, I can personally testify that the LO tube is very high quality and is tested at time of shipment.
I’m mulling over a few things in relation to this since the price of another machine has come down to where it was when I bought this one last year. I’m considering other things as well… like what to keep the water in for cooling. Current setup is ok, but I’d like to make it a bit better to get more life out of my tube. As long as it’s only the tube that’s the problem. Given the cost of all possible parts to replace here are about the same as a replacement machine, I’m considering that as an option (especially since it will leave me with a machine I can take apart and study/play with). I have a few things to chew on between now and next week when I officially decide what I’m going to do.
Here are some close ups of the LO 40 watt tube, as you can see it is assembled well and higher quality than the stock tube. (The 3D printed mounts do not come with the tube 
)
A lot of the tubes are different sizes and you should not assume that someone elses is the same as yours, lift the lid and measure the diameter and length of your tube.
Assumption makes a ass out of you and me…