XS Breather Valve mod

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Oct 1, 2007
Hi everyone.

Can someone please assist me. I have been looking for the post on which the topic is about installing a XS breather valve to a Commando, the particular post which has the link for the part actual number.

Thanks guys. I have looked but cant seem to find the post.

Google Mikes XS and look on that site I think it was under fuel and carbs? Give that a try, Chuck. :)
Thanks Chuck. I have found the website!http://www.mikesxs.net/mikesxs-fuel_system.php?category_id=4.7 . they call the thing a reed valve. It is in the Carb section.....
However, I would still like to find the post that was somewhere here..... on this forum.. It had info of its installation on a Norton and a pic of it fitted!

Thanks mate!


T'was me with the XS breather mod. It works great. After all, it was designed for a 650cc four stroke twin motorcycle. I mounted it on the backside of the air cleaner. I shoved the battery to the aft most position and mounted a small angle iron on the bottom of the battery box for a stopper. This is to prevent the battery from sliding forward and making contact with the breather.

If need be I can email you some pics.
Thank guys, you have been very helpful!

Jim C, If you could send me some pics that would be great... also could you please explain where the hoses are connected to and from the XS Breather?
I assume that one hose connects from engine breather tube situated on the right hand crankcase to "XS breather unit", I am not shure about the destination of the hose from Xs breather though?


The 72 Commando was vented to the oil tank. In other words, you put the XS breather inline with your current breather hose. The large spigot to the engine, the small spigot to the oil tank. In my 72 there was a reducer in the breather hose. I think it is 1/2" to 3/8". This is the same as the spigots on the XS breather. You may have to lengthen and/or shorten the hoses.

Brewer,PM your email address for pics.
No Worries Jim, I sent you a email with my email details.


Just realized I was somewhat misleading about the output side of the XS valve. It has a flange instead of a spigot. I tapped threads and fitted a brass barb connector on the output side. IIRC, the threads are 1/4" NPT with the barb at 3/8". Sorry about the misinfo. Sixty-four year old minds do that, sometimes.
I'm missing something with this. You're putting a breather on your breather? I thought the breather hose went from the timing side crankcase up to the oil tank. And the oil tank has a breather hose, too, so it should be open to the atmosphere. So why doesn't that work to keep pressure from building up in the crankcase? Sorry if it's obvious and I'm totally missing it...

this valve sounds like just the thing im looking for ive been toying with the idea of a reed valve breather for quite a while, not so much as to prevent oil leaks but to try and gain a few extra horses (i have an atlas vintage racer) because what the standard breather does is just blow and suck air and to my mind this takes power to do, any thoughts on this lads and does anyone know where i can get one in the uk
This breather mod is a one way valve, the air can only go out so after the evacuation caused by the pistons going down the air does not return as the pistons raise up, this leaves an engine without excessive ring blowby with a slight negative pressure in the crankcase which kinda sucks the oil back in instead of forcing it out of the joints.
I've made and used a quite a few over the years, I just liked the look of the XS valve and so far it works as good as it looks. Not that you can see it cos it's stuck up near the oil tank. You will need to make a tail/barb for it though.
Hi Cash,

How is your breather plumbed, where does the hose outlet of the breather go to? does it connect back to the oil tank ?

Hi Brewer,
It's fitted in the pipe that goes from the timing case to the tank. I have fitted them in the tank to atmosphere pipe and they worked just the same. I prefer the former though, for support and oil tank pressure remains ambiant.

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