Well I got told this by a good 4x4'n friend with 2 patrols so I gave it a crack.

I got an extra 20ltrs but it's up to you on how well you do the job And tank size..

What I'm about to explain is very simple. All your doing is removing the air gap in your tank. Manufacturers usually give 30% expansion gap.

So the steps..

Step one, locate some way to get into your tank... In my 91 LWB GQ it's a simple matter of lifting the floor matting, remove the cover plate and carefully take out the sender unit.. Others may have to remove your tank..

This took a bit because my plug was a bit seized and they ain't very forgiving with length... But all the hoses and bolts came out very easy..

Next to find the breather line.. Mine was towards the rear . It should run next to the filler neck..

Now as you can't fit your head I the hole the whole lot is done by feel..

Once you find the line you can get to work.. Grab a 16mm ring spanner.. Slip the ring over the line (it's metal by the way) and straighten out the bend in the last 100mm of the pipe..

Once happy with that, feel along the pipe for a bracket.. Once found simple push or pull the pipe to flatten the bracket and push the pipe towards the roof..

I got mine within 2 mm of the roof.. The closer you can get it the more you will fit...

Clean all your seals and bolt it all back together..

Warning... I have been advised not to fill and leave the car.. As you are removing the air gap you should only fill to the top and then drive... Spillage may occur...

Happy modding..

Hmmm.....That makes me a little nervous..lol

Simple really. Took me 40 mins (20 to get the plug apart...)

Is the air gap there in a diesel?

Yep.. Mine is the diesel sorry.

Sweet. I assume that it would be the same in a GU so the next time I have my rear draws out I might do my main and have a look at my sub tank too.

Dropping the tank is easy enough I have been informed if you have access to a hoist..

I guess it is there to allow for heat expansion?

Yes, as I said don't fill and shed it.. You have to drive..

This reminds me of the time I changed my diesel fuel filter with the car in the sun.... either do it in the shade or open the fuel filler cap. Ooops.

I bent mine up a bit and left room for a little expansion. I got about 11l extra in. Thats nearly 100km.

I'm happy

Both tanks can be done BTW, you'll just have to drop the secondary as the only give a small 'wire up' hole threw the body.

But the father in-law and I will be building larger main tanks.

He's expecting 110 L for his Prado, I think I'll try and push for 130-140L as I don't run a secondary..

I will look into the sub (literally) as soon as its empty