Well I have been playing around with preload springs & spacers (to learn more and because I can't leave it alone) I found a few times that my preload was not there (sticking) so when checking the fit of the spacer and spring in the tunnel I found the spacer to be tighter than I would like.
Having spoken to Smithy who said it should really fall out under it's own weight, I decided to put the spacer in the lathe, turn it down and polish up the finish. It now runs must more freely in the shaft tunnel.

Having got a new tunnel in the post recently from wren for my Wren54 I noted no rear O ring groove.
Talking recently to a friend who has built a KJ66 plus reading a few things on the GTBA forum I thought about removing my rear O ring as every time I remove the preload spacer I find bits of O ring on the side of it. I dont think the groove in the Jetjoe tunnel is deep enough for the O ring as it normally needs to be shaved off a bit when new.

I took a fit on Saturday night before I went to bed to take it out (knowing I was going flying next day)
I have one of the mick reeves quick release turbine clamps on my Boomerang so taking the turbine out is easy, I just then take of the front starter cover (leave it connected in the plane) and unplug my rpm wire and festos.

With a permanent marker, I mark one of the turbine blades to NGV, and one blade of the compressor to intake, then I mark the spinner nut to the compressor, I remove the spinner nut (left hand thread), and then mark the shaft position to the compressor, tap the shaft out through the compressor withdraw the turbine wheel and shaft.

I then pulled out the O ring and cleaned it all up and re assembled by fitting the compressor onto the shaft and lining up the marks. I have found you need to offset the compressor on the shaft about 3mm as the tightening of the spinner nut makes it turn slightly on the shaft. Check the turbine blade to ngv marks and the compressor to intake marks all line up and you are good to go, if not loosen it and do it again.

Had three flights yesterday and noticed a big drop in EGT's about 60deg, with no drop in thrust, so I am assuming because my preload is actually now working much better. So for now like the idea of no rear O ring.