for using aluminium stem on a carbon fiber fork, should you still use the carbon assembly paste/ compound?

also, when you install the fork onto the frame, what do you guys use to hold the fork in place? so when you slip on the headset and stem, that you get a tight fit to avoid any play in the fork & frame?

With the steerer tube cut to the right length, a starnut or similar in place, and/or an appropriate amount of spacers, you then use the top cap to provide bearing preload. Then you tighten the stem and all is good.

Assuming it has a CF streerer, that's what I do.

ah.. but in this case, it's a carbon steer, so that's why i'm pondering how to get the fork nice and snug with any play of going up or down (when the frame is lifted up)

yes it is.. so i'll try the paste tonight. need to put it on my CF handlebar too..

"ah.. but in this case, it's a carbon steer, so that's why i'm pondering how to get the fork nice and snug with any play of going up or down (when the frame is lifted up)"

Unless I missunderstand your question-- there are inside the end of the carbon steerer devices (not start nuts) (plugs?) that both reinforce the steerer from compression as well as have a top cap bolt hole. (Compression plugs?) Some are internall expanding some are epoxied in place. Andy.

You need an expanding plug in your steerer to allow proper headset adjustment. These plugs are used in place of a starnut which is not to be used an any carbon steerer unless the fork maker specifically provides a suitable one. FSA and Profile designs both make suitable expander plugs.

Once the fork is installed, any spacers added and the stem slipped on loosely, the top bolt in conjunction with the expander, is used to set the headset adjustment. Then, and only then, do you tighten down the stem's clamp bolts.

BTW, I have aluminum stems on two carbon steerers and have never used or needed assembly paste. I tighten the stem's clamping bolts to the lower end of the specified torque range or even a bit lower and have never had the stem slip.

I got one of QBP's 'locking spacers' which is sort of like a miniature seat post band clamp.
It goes directly on top of the top headset race.. it just keeps the fork in the frame.
I got it because my bike had a integrated headset, and the fork fell out, and jumbled the
various headset parts, too easily ..

when time comes to actually adjust the headset, the stem and this thing are all loosened.

Apparently you didin't notice the "or similar" part of my post. There are expander wedges that does for carbon steerers what star nuts do for metal steerers. Get one of those, proceed as usual.

Or get a length of threaded rod. Make yourself two cross pieces out of an old broom handle or whatever. Put one cross piece at the end of the rod. With the wheel removed, insert rod through steerer from the bottom. Put spacers and stem in place. Put next cross piece on. tighten gently to create preload. Tighten down stem. Remove rod.
Pat yourself on the back for shaving maybe 30 grams worth of weight off the bike by not using an expander.

I have the expander plug inside. So last night I put on the carbon paste for the stem and preloaded the expander plug and tighten. Then I did the stem. That seem like it did the job now no more play. Going to ride it and see if it will move again.

Cute variation on the home made headset press but most carbon forks have a closed crown at the bottom of the steerer tube so there is no opening to let the rod enter.