Aggh 6" system inconsistent with existing chimney

Hi All,
This is my first post, so go easy on me.
I am putting the pieces together to begin a RMH build to replace an existing woodburning stove and have come a cropper after finding that all the 6" ducting I have ordered is not the exact same dimensions of the existing chimney.    The collar on the existing woodstove measures 140mm (5.5") and the flue terminus sized to fit that.  The ducting I have for the RMH is 150mm, will this constriction in flue diameter for the final 10ft of the exhaust run (it will be a relatively short duct length through thermal mass of 16ft) create issues for correct drafting of the system?
My thanks in advance for any assistance that can be provided.

Welcome Philip to the rocket mass heater forum!
Thank you for making the decision and doing your part in helping to keep the air a bit cleaner in your neck of the woods.
My first main question is why you don’t just remove that existing collar/reducer so that your pipes stay at 6”?

Tell us a little bit more about what you have in mind as a whole: J tube or batch box, size of house, what your building the heater with? Etc...
Pictures and drawings are helpful too.

The stack going through the roof on mine is a 5.5" natural gas exhaust that was already in place. I have a 6" single wall duct going from the mass to just below the ceiling where it joins the 5.5" double wall one. I've never had any issues. The gasses should shrink as they shed heat, and that can make up some difference in the final exhaust diameter.

Gerry Parent wrote:Welcome Philip to the rocket mass heater forum!
Thank you for making the decision and doing your part in helping to keep the air a bit cleaner in your neck of the woods.
My first main question is why you don’t just remove that existing collar/reducer so that your pipes stay at 6”?

Going from a visual inspection, it appeared to be that the flue was of the same (5.5") diameter but now that you mention it, the photo does make it seem as though it is a reducer.

Gerry Parent wrote:Tell us a little bit more about what you have in mind as a whole: J tube or batch box, size of house, what your building the heater with? Etc...
Pictures and drawings are helpful too.

It's going to be a J tube, very much according to the templates given in Ernie & Erica's book.  
The living space it will heat is 19ft x 19ft, very well insulated & winters are very mild in my neck of the woods.
A big asset in my mind will be the reduction in smoke which using the ordinary woodstove drifts into the neighbour's yard too much for my comfort.

I will try and get around to doing a scale(ish) drawing of the planned layout in coming days.

Thanks Gerry for your input and Jordan for reassurance.

I have an 8" J-tube RMH with bell, and a temporary 6" stovepipe chimney. It has excellent draft and once I was able to get the top of the chimney 2' above the roof essentially never smokes back.

Echoing Jordan's point here. It's usually fine to gradually and modestly reduce the flue diameter toward the top to maintain stack velocity. Going from 6" in the guts of the system to 5.5" near the exit is unlikely to cause problems, and more likely to help keep things flowing.