I bought two new masts and an extension and they both seem to fit very loosely on the extension. When I measure inner tube of masts it's 32.6mm and extension is 31.6mm so it's about 1mm diameter different. When I put them together(without any downhaul tension) they seem to be very loose.
It might be a silly question but is this normal? I was always renting so this is my first real set I put together.

I'd add a video but as a new member I can't post links unfortunately.

Some play is normal, otherwise extension would wear on inside of mast, and with downhaul tension it is not loose and that is what matters.

Some brands are a looser fit than others and that can be down to slightly different build tolerances for the inside diameter of the mast or else the outside diameter of the extension.

A couple of brands - like Severne, for example - made a point of making their RDM extensions slightly bigger so that the fit was snugger and the extension stronger.

If the extensions are too snug a fit however, then sandy water getting in there leaves behind the sand and the extension gets stuck.

If your extension is a very wobbly fit then I'd suggest trying another, to see if the extension or the mast is the problem.
And a simple solution is to put a collar of electrical tape around the top of the extension, to increase it's tube diameter slightly. Unfortunately, you can't do that at the bottom section because of the length adjustment needed for different size sails

Select to expand quote

Sandman1221 said..
Some play is normal, otherwise extension would wear on inside of mast, and with downhaul tension it is not loose and that is what matters.

clarification: extension with sand would wear on inside of the mast if the fit was too tight, at least for me it is hard to keep sand out of the bottom of the mast when rigging on a beach. Never had an extension get stuck since it is relatively loose compared to how the two halves of the mast fit, that is why it is a lot easier for sand in the water to lock the mast together.

Definitely be easier with photos. It may be that the top part or cap of the extension is missing. Some neilpryde extensions and masts can be different usually smaller. Fit shouldn't tight but very close though.

It's really visible when I set the extension to full 30cm
youtube com/watch?v=G_a33bTc90g
youtube com/watch?v=jB6pYIlI_T8

I was using an old extension on a modern mast and it was really really loose. So I inserted some PVC reticulation pipe with a slit down the side into the mast. Yours doesn't look to be that loose but it might be worth trying something similar. As people have said, don't make it too tight because of the sand.

Select to expand quote

vanillaDonut said..
It's really visible when I set the extension to full 30cm
youtube com/watch?v=G_a33bTc90g
youtube com/watch?v=jB6pYIlI_T8

This is fine VD. The tolerance it normal. The most important thing is the bearing surface of the collar. This is what is supporting the mast. I've seen **** designs that only partially bear the load of the mast.

Select to expand quote

vanillaDonut said..
It's really visible when I set the extension to full 30cm
youtube com/watch?v=G_a33bTc90g
youtube com/watch?v=jB6pYIlI_T8

That's weird. It does have the cap on top? What brand extension and mast?

Both are from CAAS. It does have the cap on top but it's only slightly wider. This is the extension, ibb co/fYmXpdt or you can see it better on CAAS webpage.

Select to expand quote

vanillaDonut said..
Both are from CAAS. It does have the cap on top but it's only slightly wider. This is the extension, ibb co/fYmXpdt or you can see it better on CAAS webpage.

And mast? Did you contact them? Could be within tolerance. How about side to side play?

I wouldn't worry about it, as soon as you have applied a load of downhaul it won't move.

I just wrapped some pvc tape around extension to take the play out works a treat.