My bikeīs stem and handlebar have non-matching diameters. The handlebar (a Nitto B135) is 25,4, the stem (Shimano Ultegra) is 26mm. This all happened because of an ebay seller failing to properly list the stemīs specs. But I want to keep it, for one it is beautyful, and more importantly it is short (80mm, not easy to find plus it would probably cost me more than the 40€ I gave for the Shimano), something I need as the bike itself is a bit long for me.

First I tried with a beer can, then I bought a VO handlebar shim and tightened the bolt as hard as I could but given enought time the whole system will still loosen.

I was wondering if some assembly paste would help me keeping everything in place or is my only option getting a 26mm handlebar?

No. If a shim hasn't worked, assembly paste is unlikely to work either. You need a matching bar and stem. I'm a big fan of the B135 bar, so I'd replace the stem with one in the proper size.

You anticipate that the beer-can fix will eventually fail, but it didn't on my bike. The advantage of beer cans (besides their contents) is that they are thin, and you can adjust the number of layers to your specific need. Also, you can cut them with ordinary scissors.

Another option is theslightly thicker aluminum "flashing" material used by roofers. (Gibt es kein Home Depot oder Lowes Geschaeft in Berlin?)

Nitto actually makes a shim for that specific case. Being that they also probably made that stem, your chances of success as likely to be higher.

The Nitto stainless shim works great and it can be trimmed with an abrasive wheel so it doesn't show.

26-25.4=0.6, so use 0.3mm shim stock, which is what Nitto's pieces offer.

Use your shim but add a few layers of aluminum foil. I had the same problem with aero bars on a mountain bike handlebar. It was a minor problem that I ignored for the last 10 years, but I finally took a few minutes last week and used the aluminum foil trick. Rock solid now.

I fail to understand how my use of the past tense induced you in believing that I yet have to try either the beer-can or the shim solution. Anyway it's not a big deal and me being the foreigner, maybe I just have to review my grammar.

I also doubt that there can be much of a difference between the Nitto and the VO shim so I will refrain from spending 15-20€ in the former (yes, that's the price I would pay to get it, skyhigh)

The alu foil wrap could be a good idea, I'll give it a try.

Your grammar is just fine. (It's better than some of the native English speakers here, in fact!)

A shim of the proper thickness should do the trick. If you've got a properly-sized shim and parts are still slipping, using something like carbon assembly paste to add friction isn't unreasonable. I'd be inclined to measure parts and inspect closely as well, since properly sized parts (including properly fitting shims) shouldn't be prone to slippage when the clamp is tight.

as long as you add that .6mm of diameter and the bar center comes out to Measure 26.0mm over all .

I have used The red Loc Tite the cartridge bearing retaining compound, to stop the slipping of those 2 surfaces,

Stem to shim, & shim to Bar.

With red Loctite it will never slip, even when you want it to, without the application of a MAPP gas torch.

nah it came loose with the bolt loosening . I used some red on a smaller difference
to use a Cinelli 1R stem (26.4) & a 26.0mm Nitto (MB1 take-off) dirt drop stem

On my AlAn CX Super bike, in the 80's..

laying a soldering Iron on aluminum heat transfers nicely, maybe save the torch for the crank/camshaft bearings in a cast Iron engine block.

Try 2 beer can shims. Are you prying the stem apart at the clamp with a large screwdriver (put masking tape over it first to protect the stem)?

Use enough shims that you have to force the stem apart to get them in. The advice about the Nitto shim for the job is worth noting, but so is the fact that bars and stems each have enough variance in tolerances that success is not guaranteed. (I suspect the Nitto products will be pretty close, but other manufacturers not so much. My framebuilder won't make me a shim until he has actually measured both parts.)

Ben

Even a hair dryer will generate enough heat to free up the red stuff...

Good call, BTW...

OP states he tried the Velo-Orange shims, which are essentially the same as Nitto's:

VO Stainless Steel Shim 25.4-26.0mm, 35mm or 45mm width

Not sure why he still has problems, but the proper size stem should be a solution.

I think the confusion is that you said that "the whole system will still loosen", which implies that the loosening event is off in the future and you are just anticipating it. Had you said that it eventually did loosen, that would imply that you tried it and it failed. The fact that it loosens eventually, but not right away makes me think it's a retention of the bolt issue rather than a shim issue.