nope not possible. 13b rotor housings have the same dimensions as 12a rotor housings but are wider. So you have to use 12a rotors with 12a housings or 13b rotors with 13b housings. The endplates and intermediate plates are the same.

Can I ask why you are so keen to go for a 13b?
it doesn't take much to make a 12a go just as hard, while staying nice and legal.
more capacity is not always better, (kinda the whole point of a rotary)

You can't "bore out" a 12a - it's a rotary, not a piston engine. Google it and you'll see some diagrams of how rotary motors work and you'll see exactly why it can't be done. It doesn't have holes and pistons like a normal motor.

You could easily get yourself a factory 12a turbo motor (they come fitted with a hitachi turbo, plenum, injectors etc) and run an aftermarket efi computer, intercooler and make TONS of power with it.

I've had two 12a turbo motors in previous cars, one an extend port with factory turbo, no intercooler and stock ECU and the other one with an aftermarket efi system and adjustable boost. I can tell you that in my rx3 coupe weighing 910kgs it spun the 235 back tyres (locked diff) in 1st, 2nd, 3rd, 4th and chirped into 5th. and that was non-intercooled.

You could make as much power as you want with big injectors, intercooling and a bigger turbo - way more power than ever needed to drag off virtually any car.

12a and bridge port, you'll have lots of fun....especially with the cooling.

looks like you can't fit a 12a turbo or 13b turbo legally due to the weight of the vehicle.
So you could go bridgeport or peripheral port but cooling is a tough one, noise is out of control and they are pretty grumpy motors that don't idle real well.

Go for the EJ20T out of a WRX - easy to bolt up to your gearbox, as much power as you like and legal. PLenty of em around.

from my experiance with the rotary scene 13b standard would be the best choice . or a 13b with a large extended port if you want more power. if you go a 12a i definitley would not be goin peripheral port as you will chew through more fuel then a blown v8 the car will not be at all drivable and you will only be getting perofmance when you ring its neck , i have driven a pp 13b in an rx2 berfore and it would be rediculous to be cruising around the streets in something like this ( not fun)
bridge port would be the biggest i would look at goin but even then you are goin to be loosing a lot of low down power and drivability not to mention the trouble you will have keeping it quiet enough for the road and fuel bills

i run a 13b large entended port injected rotary with upgraded series 5 injection and turbo housings ( better exhaust port) and i struggled to keep the car at a resonable noise level using 2 inline rotary specific baffles and a staliness cat. its still way over the legal limit but its the best i could do ( idle is just on legal db readings) with limitited room in the engine bay. so goin a bigger port on a non turbo engine would provide queit a few problems.

rotaries are an awsome and fun motor but i will agree that they are probly not the most practicle engine conversion for a vw hence my new baja is moving away from my daily rotary driven baja and will use a 2lt turbo instead even though i absolutley love the rotaries,

In my personal opinion rotaries belong in RX series Mazda's. Go sick

so if everyone keeps trying rotaries in VWs and doesnt like them, why the hell cant I find an adapter kit secondhand anywhere??

I missed the original post. even thopugh I probably could have answered whatever you needed to know.

they are really an awsome motor dont get me wrong didnt mean anything like that but porting wise i dont think bigger is better in a road driven car ( i still love my rotor baja , i am in love with the sound just time for something a little bit less harsh and some more performance)

lulu94 if your looking for an adapter kit try

Wilson V W Services
Paul N Wilson
40 Main St Bakers Creek (Flat) 4615
(07) 4952 1761

He had one about a month back complete with the motor.

I got a spare adapter
Rotories in vw's are awsome