Thanks for the info folks. The instructions with the Wren thrust augmenting pipe give dimensions for two types of Wren exhaust cones, the original long cone with no inner cone at 10mm, and the newer short one with an inner cone at 30mm from the short outer cone. The J600R is a mix of both those types, a long outer cone and an inner cone. Thinking that it is the outer cone that matters, I have decided on a gap of 10mm. Len's experience suggests this will not cause problems. The Wren thrust augmenting pipes have a small intake and I guess a greater gap from the engine exhaust is more likely to allow hot expanding gas to spill over the outer edge of the bellmouth.

H.