NGK South Africa only list 8 for aprilia and 9 for yamaha, both for 125 bikes

A new set of regular NGKs will make your engine run better. So will a new set of Irridium/platinum NGKs.

The advantage of platinum, they will last longer without regapping every year. With the regular $1 plugs you need to check and regap them yearly.

Also they can withstand longer the ultra-high spark voltages of the new ignition systems. Won't make a differnce on the Datsun electronic ignitions, which are only medium high voltages.

I am using 8's in my car, told to use 7's on a cold day.

5's for moving the car around but once warmed slightly I'll chuck the 8's in, have to be careful changing plugs on a hot motor the thread can gall in the head.

Hey sean, what damage/problem dose running 8's caus in colder conditions or on a mild motor?

They'll foul badly, thats if it will start at all, then you might not get a complete enough burn. My road car with the dellortos had 7's but was a real pig to start, if I didn't get it right and fire up the first go I would wait a couple mins and try again, had that issue at 1am leaving work, was a little stressful.

YAMAHA YZ 125 2006 + BR9EIX
APRILIA MX 125 2004 + BR8EIX
ARE THESE PLATINUM PLUGS? THIS IS HOW NGK HAS THEM LISTED

No they are iridium. I would steer clear of a 8,9 plug unless you are spending all your time on the track.

What about a 5, 6 or 7?
not sure if I can get them though

Well what are you intending to do with the car?

Grandma putt to the shop, daily spirited street driving, occasional track use, race/circuit?

What are the specs of your motor, compression etc. webers, turbs?

spirited street driving, occasional track use

my current specs are a stock a14
I'm in the process of planning my build, have a GC head in the pipe lines
looking for su carbs
recut cam
lighten flywheel
extractors and freeflow
and 15 elec dizzy
that would probably the first stages
porting an machining will follow