[ENG] - Diameter Alternative Air Filter

Hello everyone,
first of all I realize this is a German forum, altough at the moment my German skills of writing aren't good enough to write what I'd like to ask, so sorry about that.
In case someone would be so kind to answer me, thank you a lot.

I recently purchased a Zephyr 550 '91. The bike hasn't moved for 3 years, has 34k km and it's in an overall good shape. I'm taking it this saturday to service for the general things you do after a bike hasn't run in a long time, filters, oil, chain, carburator cleaning and a general check up.

I have two specific questions (I tried to check some open threads but was difficult to get what I wanted to know):
1) Can I apply (before getting the TUV done) external air filters instead of the boxed one that has at the moment (like the one in this thread 301 Moved Permanently)? If so, does it need an ABE? If so, any brand that in Germany has it?
2) Anyone may know the exact diameter of this filters to apply perfectly on the carburator? I found different durchmesser available, 48, 52, 54.

Thanks a lot to whoever may help me, and sorry again to have written in English.

Tony

Hi Spartan,
thanks a lot for the answer.
These days I've been calling (with the help of a friend) Dekra and asking to different sources (including K&N customer service). It all takes down to the idea that installing an open Air Filter is illegal and there is no way to make it legal.

Dekra said that they don't have a way to measure the noise and even if I pay (a lot) to make it check, probably I won't pass.

Do you know anything about that?

Following the answer of K&N:
Hello, thank you for your email.

Unfortunately there is no TUV approval on any of these universal clamp on air filters. Reason why there is no approval is that the TUV regulations are too strict regarding sound change. you are only allowed to raise the sound level with 0.5dB and these universal air filters are much louder. Also these tests need to be performed on each specific vehicle and since these filters are universal these is no option for us as a manufacturer to get an approval.

With kindest regards,