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Combustion Chambers???

By RICH GILLEN on 11 Apr 12:10 0 answers 484 views 0 comments

My need is, to turn some Billet 6061 Snowmobile Head, Combustion Chamber Inserts for Skidoo/Rotax heads, max 5" OD insert shell. I have a large 12" x 37" manual lathe to cut the Insert OD's. I'm looking at a small hobby CNC lathe, to cut the different contoured chambers, on the bottom face of the inserts, which the biggest chamber I would ever use is 84mm, or 3.30". The different chambers needed would probably fall between 26cc - 38cc. The Skidoo/Rotax 670HO Pistons(78mm - 82mm) use Flat Top pistons, so have a very small chamber, and all the other Skidoo/Rotax Pistons have a small radius dome.

Domed Chambers are for, 62mm, 67.5mm, 69.5mm, 72mm, 76mm, 78mm, 80mm, 82mm, 84mm, Bores, would be similar to this one below, in probably something like 26cc-28cc(Race Gas), 32cc-34cc(92+ octane), 36cc-38cc(stock 87/90 octane).

Since I'm looking to Big Bore most of these older different engines, I may be limited to, 62mm, 67.5mm, 69.5mm, all to a 72mm Big Bore, and 72mm, 76mm, to a 78mm Big Bore, and 78mm to a 82mm Big Bore. So I may need only a 72mm, 78mm, 82mm, inserts. But I will probably do the stock bore sizes down the road also.

My problem is how to draw the chambers for the different 72mm, 76mm, 78mm, 82mm, bores in the different cc neededed. They all have a squish band .040" - .050" around the outside of the chamber. The universal Insert is 5" OD, with a 6 Bolt pattern, with a 100mm Bolt Circle, for the 72mm, 76mm, 78mm, 82mm, bores, and some will have a 6 Bolt pattern, with a 94mm Bolt Circle for the smaller 67.5mm, 69.5mm, bores.

Thanks
Rich

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