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Accucraft GS 4 Chimney&Petticoat

2207 Views 8 Replies 4 Participants Last post by  Charles

After carrying out some of the modifications recommended for the alcohol-fired version of Accucraft's GS4 (incl. Gordon Watson's combination levers and nozzles and new wick material) I am still not REALLY happy. The performance is quite OK but could be better.
So I referred to some textbooks and articles on the subject, e.g. Martins Evans' books and J.v. Riemsdijk's articles in the G1MRA Newsletter.
As regards the front end, i.e. arrangement of nozzle, petticoat (chimney extension) and chimney, both authors recommend the same geometrical dimensions. I built two templates to check these dimensions on my Accucraft GS4, see first picture

The template tapered 1:6 governs the height of the chimney top above the nozzle, which appears to be OK, see second picture

However, after applying the second template tapered 1:3 which determines the extension of the petticoat, it appears that the petticoat is at least 1/4" too long!

How's this? This template has 8 stripes green/yellow each 7/64" wide. The template shall sit in the blast nozzle with the tapered part still inside the chimney extension or petticoat by about 3/16". So at least six of the eight stripes should show underneath the petticoat, however, on my GS 4 only three and a half are not covered by the petticoat. That means it is too long and most likely impedes draft!
Has anyone else observed this?
Has anyone unscrewed the petticoat? How is it fixed? I would like to shorten it to make another test.

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               You dont state the reason for your search for an improvemnet in the Gs4 running since you are working in the draft area I assume its poor steaming?   I can see from the photos that you do not have a reducing bush in the central flue, without this bush the gas flow is unbalanced across the flue bank , effective heating area is smaller and steaming not optimum,, turn up and install the reducer bush , first and measure the change.

The petticoat unscrews from the stack body and can be shortened in the lathe .but there is no real need, if all else is correct.

RE: Accucraft GS 4 Chimney&Petticoat

The reducing bush should have the same diameter as the 2 outer flues:ie : 3 holes all the same area...have you got a fire arch in the firebox?

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