If i remember correctly i used a no3 and a no6 internal bush, I cut them both i half and superglued them together. Worked ok for me.

no need to use 2 bushes and mess about with glue! just pick the size bush you require and put it into the power kit from the inside so that the bush part is on the inside, its very easy to do! then trim the flat part of the bush that is sticking out of the kit to about 3-4mm and simply flang it over with a hot screw driver or similar, to keep the bush in place while you do this just shove something up the end like an old section that will wedge the bush. to get the bush in get an old piece of hollow elastic ( about 2ft) pass it through the drilled hole from the outside then thread the bush on and tie a big knot behind it roughley in the middle of the elastic. pull the elastic back through the hole the knot will help pull the bush through, hold the sticking out bit of bush and remove the elastic from the base of the kit. the whole process can be done in under 5 mins