Combustion Chambers???

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.


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10 Answers

The ledge the Spark plug bottoms on is 20.73mm.

The Center Hole is a 14mm Spark Plug.

The Groove is for an O-ring.

The Radius are reall small almost sharp edges.

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The Dome on top radius is not crutical. The Base of the Dome is 66.90mm. The Top is 25.56mm. It's just an open area for water to flow around it.

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The Top part of the Insert press's up on the Head Shell to seal the water jacket.

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I have thought to simplfy the top something like this. The Groove under neath is not really needed.

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Thanks. I got the igs file to open ok. This Insert, was for a Skidoo 670HO (78mmx 70mm) 669.2cc that used a flat top piston. On the Combustion Chamber side at the 78mm OD, it drops .136mm or 0.005354315", and the Squish Band looks to be 12mm or 0.4724395" wide. The Squish Band Angles down very slighly at the bottom of the angle it is 2.22mm or 0.08740131". So it angles from .136mm to 2.22mm. The Chamber is 54mm or 2.125978" wide. The center of the chamber at the Spark Plug Hole is 13.62mm or 0.5362189" from the bottom of the Insert.

To simplfy the OD of these, I think the groove under the top part could be eliminated, and just angle up from the outside radius on top to the bottom of the top part

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Back to my original question of how to make the different size combustion chambers for the different Bores used, 62mm, 67.5mm, 69.5mm, 72mm, 76mm, 78mm, 82mm, in different CC? I will probably only make the stock CC, and two others, for each Bore size. Like Stock, High Performace, Full Race. I would like to make 1 Generic Insert, with 2 different bolt patterns. Like these Inserts above are for a 670(78mm) give a 15.2cr. Not sure what CC they are yet.

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