Hey Guys,
 
Just got myself an MR2 SW20 Turbo, it came with an aftermarket MOMO shift knob (looks similar to but older and worn) but not a fan of it so got a new one but I can't seem to get it off.
 
I assume it should just be as simple as unscrewing it right? does anyone know if there is anything else that normally needs doing on an SW20 or is it just that I'm not unscrewing it hard enough?

Being aftermarket it's likely to have grub screws holding it. If so it will have a cover at the bottom that will need to be unscrewed

Just to add. If it's not a threaded on type knob it's likely it will just turn. If so and usually the grub screws will be covered as the new knob is screwed on to the inner tube. What you'll need to do is pull on it tightly upwards while turning it anti-clockwise as if your un-screwing it. It should start to grip the lever thread and either the knob will unscrew off its own thread, or you'll bite enough of a thread into the plastic inner tube that will eventually take to whole thing off in one go. You'll need to keep pulling on it in an upward direction as doing so. Hope that works for you.

Is it this one: 
 
I purchased one for my car for about $20 on eBay because my old one was stupid and there were no OEM ones available at the time. My previous owner hacksaw'd the lever so I got one of these and drilled holes into the old cut lever so the grub screws could hang on, works awesome.
 
Anyway removal: hold the silver ring firmly and rotate the knob anti-clockwise until it comes off. The silver ring will then rotate clockwise to come off. After that get a 2.5mm allen key and unscrew the 3 grub screws holding it in place. It should then come off. 

Note: noobs who install these might glue them on because without indents the grub screws won't hold so just rotate it with the power of Zeus to break the bond of glue. Heat it up if necessary.

That's a shift load of power then! :-)) ya ya yah.

Huge noob here, but i bought an aftermarket one as well from ebay. 
 

 
and it came with three black plastic adapters but the inside of the adapters dont have any threading and they simply slide on. Would anyone happen to know how to install this type of adapter? 
 
Looking around youtube most install videos involve typical aftermarket ones which use the grub screws. 
 
hope someone can help me out. Cheers.

mistadeuces
Huge noob here, but i bought an aftermarket one as well from ebay. 
 
 
 
and it came with three black plastic adapters but the inside of the adapters dont have any threading and they simply slide on. Would anyone happen to know how to install this type of adapter? 
 
Looking around youtube most install videos involve typical aftermarket ones which use the grub screws. 
 
hope someone can help me out. Cheers.

Yeah, the plastic tubes should only have a tread on the outside, allowing them to screw into the knob itself.
1st select the correct size one that fits onto the gear stick lever, should be a tight fit, you will need to align it on there straight, then proceed to screw it down onto the treaded lever, creating its own tread inside the plastic tube. 
Once its treaded down as tight as possible (use multi grip pliers - as low as possible on the plastic tube, not on outside tread), then screw the gear knob down over plastic sleeve / tube.
DONE!

The momo knob I posted has adapters but they're just rubber sleeves: 
 
The idea with these is (exactly the same as what mikey said) you find the sleeve closest your size gearstick, insert the sleeve into the gear knob then screw it down nice and tight effectively cutting a thread into the inner rubber sleeve. Then using the lug nuts to tighten it further for a snug fit then finally attach the rest of the knob. However I don't believe knobs should ever be friction fit which is why I just put the sleeves aside, made some indentations in the actual gear stick then just used the lug nuts to make a tight fit. Everyone has their own method but. :)