Thanks EC. What model 750 are you referring to?
I thought I had the original genesis air filter, so I could try the lid for size, but it is not even a Yamaha air box. The K&N has a height of 105mm and the one I have is 90mm.

YZF750 I think, but not sure.

The 1WG airbox is the same as the FZR600 and there's a K+N for it. The 3TJ is lower so there's no K+N for it

as far as i understand it, the standard air filter on the tj is high flow anyway as the tj is designed for racing, the only advantage with a K&N is you can clean it, but at about 5 times the cost is it worth it ??

either of these should do the trick

http://www.wemoto.com/bikes/yamaha/fzr_ ... ter_hiflo/

http://motorcycleproducts.co.uk/catalog ... 61518.html

the first one being perforated steel is a bugger to squash down to fit, (i know thats what i bought )
the second one can be easily squashed down as its got mesh side's,
off the top of my head their about 10mm to tall, however the top and bottom diameters are the same, i'd check before you order to make sure,
whatever you do don't use the screws in the lid of the airbox to squash it down you'll split the box, squash a bit at a time until correct then fit the lid

Well thanks for the info guys. I was about to order that filter, the first one from Wemoto, since they say it is for the 3TJ1, but that must be wrong if you have to squash it. What a pain.

phone em and ask em to measure the height, don't just take their word for it that it's right
they may just have the right one, but i very much doubt it

Ok so we have established that the 3TJ1 filter is about 10 mm shorter and the HiFlo is also a 1WG, despite what Wemoto say on their website. I am thinking that the really simple way to solve this is to glue a plastic ring onto the sealing ring that protrudes from the top of the airbox. Just extend what is there. Then use longer screws with some spacers when fitting the lid and the problem might be solved. Assuming the tank does not hit the taller lid. Does anybody have both a 1WG and a 3TJ air filter element so that we can get an exact difference?
I don't want to use the squash element method.

you should be able to get an original from fowlers for alot of £££
or make something up for the lid, sounds like a plan,
don't forget the pics
either that, or get the mesh one and squash it down

The plan is to wait for the reply from HiFlo about making 3TJ filters. If that fails I will use an extension piece that will take the same large O-Ring on the airbox and the 1WG filter. If I go that way pics will be provided.