Hi everyone!
Trying my hand at restoring an older bike. Under the BB is a piece of metal to hold the shift cables but should the guide should have some sort of protective sleeve so that the cables aren’t contacting bare metal BB?
Ideally you should use a cable sleeve there, however, many older bikes don’t have room in the guide for such a sleeve. Based on that image, it looks like it will be a tight fit and may not work.
If it doesn’t, it isn’t necessarily a big deal….you’ll just need to keep on eye on the cables for fraying and the BB shell for any corrosion.
Any small piece of tubing that fits, but note the tubing can collect moisture and gunk. If you go without the tubing, as bikes did for decades, and stainless cables, and a periodic small drop of lube you’ll be fine, and more period authentic. Do let us see the finished product please.
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If you were worried about the frame, try cutting a coke can and placing it between cable and frame as a shim. Colour may match as a bonus!
Thanks everyone for your help! I’ll post a finished picture here 
Don’t worry about a sleeve of any sort … metal guides like that were used for years and work just fine with a drop or two of oil now and again when doing maintenance. As others have mentioned, a sleeve provides an opportunity for rust to grow and inhibit smooth operation.
Jagwire slick-lube liner? Check the external diameter though
I used Jagwire liner on an old frame rebuilt for a customer a few weeks ago with the same metal cable guides under the bottom bracket.
I didn’t like the idea of running the cable without the liners, especially as the frame had just been resprayed. The liners were a snug fit through the metal guides, I.e. will stay in place. If the fit was loose, I’d have not fitted them.
It’s true not everyone ran liners on those in the 80s, but I’d choose to protect the frame. On my Montello, I use about 10cm of the jagwire liner noted above. It’ll fit through those eyelets with room to spare. You’ll also reduce the friction under the BB and get better shifting.
BITD I had a bike with similar guides under the BB which came with plastic liner tubing and little plugs to retain this. Shifting became near impossible on a ride where gritty slurry was encountered - the tubing filled up with crap. I removed the tubing and plugs and never had another problem in tens of thousands of km. ALAN frames had similar guides and the cables ran on the alu BB shell; no problems ever resulted from this that we saw, and we sold lots of their frames back then.
If it was me I’d not put anything in there; it should be fine.
I have some teflon thin wall tubing that will do the job, send me your address and I’ll post you some: maker@lyrebirdcycles.com.
You don’t have to worry about crud accumulation if you use something to fix the tubing to the cable. I use a piece of thin wire as a sleeve clamp, works well.
If you do this the sleeve will move with the cable, rubbing on the frame. Part of the reason teflon works is that it is mechanically very weak so some of the teflon will transfer onto the frame surface and you’ll then have teflon on teflon which is a very low friction interface.
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