On my Nishiki Tri-A with the original quill stem and handlebar (I believe), the handlebar will turn forward down after riding on the hoods - specially when I stand up for climbing or give a break to my butt.
I can re-adjust it, but wondering if there is something that can be done to make it to not turn forward. Both stem and bar are aluminum.

Tighten the handlebar clamping bolt in the stem. If you've already done that, as tight as possible, I'd wonder whether you've got a mismatch of clamp diameter and bar diameter. I suppose you could try carbon assembly paste, but if the clamp is too big for the bar, that won't fix it.

I would loosen the stem clamp bolt and check for wear to the handlebars first. If all is good, then tighten the clamp bolt.

You can always try the Robert M. Pirsig trick of making a shim out of an aluminum beer can.
"Zen and the Art of Motorcycle Maintenance"

If your handlebar is slipping then the problem is mechanical. A stem clamp, working properly, does not have to be super tight. Have a look at the bar for wear in the clamp area, as suggested already. Look at the gap where the stem clamp closes. If it has bottomed out, then your handlebar clamp diameter is too small. Also, make sure that the stem inner bore and handlebar clamp area are really clean and free from burrs or scratches (either will interfere with the clamping action to be optimal. In the picture below, you can see the stem gap is still well open. On some stems, the bolt will thread directly into the stem. I such a case, make sure that the bolt is not bottoming out.


I have run up against this, now and again and, dealing with any or all of those possible situations usually does the trick. A shim, as suggested, might do the trick but I, personally, try to avoid make-shift repairs. But that is just me.

Good advice above. Try loosening the bolt completely. Is there play now or is the bar still in with a nice snug fit? If there is play, there's a good chance that a 25.4 bar snuck into a 26.0 stem. Beer can shims work wonderfully to solve that. (Most of the time. All three, bars stems and beer cans are made to tolerances and vary,)

I've used those shims for some runs of years. I cut rectangles exactly the width of the stem with good scissors. Very hard to see.

I read somewhere, a long time ago, that that's the one connection point on a bicycle that doesn't get grease.

I've never greased the bars or stem there. I do grease the stem bolt threads well. Sometimes I have bars that creak. Enough and I pull the bar partially out of the stem and look hard for cracks. Repeat other side. If no cracks or gouges, I try to tune out the creak,

Thank you for the advices. I think I see a good gap there and I have tried to tighten well the clamp. Will get some pics.

I had cases where the clamp bolt is a hair too long and bottoms out. I’ve added a thin washer to the bolt to solve the issue.

I've removed the bolt and is do see some room. Measuring the handlebar it was 25.4, so I added the beer can shim and lets see how it goes.

Ditto, the aluminum can shim (also via the book John E mentioned, which I read as a teenager). I used that on a Salsa two-bolt stem I used to replace the original stem on my Ironman. It was slipping a bit on my first ride. About 10 miles from home, I scrounged a roadside beer can, cut a shim with my pocket knife, fixed it on the spot and it's held just fine since.

One thing you might want to check is when you use a soda/beer can for a shim, it is best if you can get the plastic liner off the can interior.

Also grease the bolt for the clamp. They can bind up where it feels like you tightened the clamp all the way, but it's just seized up a bit.

Looks to me you have plenty of space to not need a shim--just good tightening on a greased bolt.

In the last pic, the shim red parts on both sides of the stem can be seen. It was very tight and I used a screwdriver to wide the clamp a bit and it got in easily.
Next 20 mi ride will tell me if it is fixed or not

Once the bar has slipped any knurling on the bar or stem is suspect.
I would unbolt and slide over to check conditions.
renewing the striations is possible.
to really accomplish that you need to be ready to take the stem off.
at that point I would consider a new stem and or bar.

modern bike shops have friction paste, for old guys, valve grinding compound.

personally I do not like stems of this design, Nitto, Cinelli, 3ttt, Modolo all are guilty.