boring gx390

No. A 420 wont allow it reliably either without putting in a larger sleeve. If you want 92mm, the Vegas Carts and/or Parson 10 side bolt block is your cheapest bet.
 

Armilite

New member
Can you bore gx390 to 92mm without resleeving it?
On my Skidoo/Rotax 2 Strokes I was told by LA Sleeve you need a Minimum 2mm / 0.07873992" Cylinder Wall. Find a Site that makes Sleeves, they give the ID/OD of the Sleeve. Most Sled Cylinders can be Over Bored 2mm+. Like my Rotax 277 (72mm) Bore, Sleeve ID is 72mm/2.835" and Sleeve OD 3.77". So 3.770 - 2.835 = 0.935"/2 = 0.4675" Wall = 11.8745mm. For Max Bore if Sleeve OD 3.77" / 95.758" - 4mm = 91.758mm if your Head/Cylinder Bolt Patern will allow it. With the 277's (4) Bolt 100mm Bolt Pattern 82mm is the largest Bore you can use. I have seen a 670 Big Bore 82.5mm which use's a (6) Bolt 100mm Bolt Pattern.

Power Bore Cylinder Sleeves
http://powerbore.com/sleeves/

What is the GX390 Head/Cylinder (4) Bolt Pattern based off in mm/in?
 

Armilite

New member
Honda offers 3 oversize pistons .25mm .5mm .75mm they are typical rebuild parts.
Do you know what the Max these GX390's can be Big Bored too? 96mm is the largest I have seen.

Does anyone know what the Head Bolt Circle is? The easiest way to figure Bolt Pattern is to Measure (1) Head Bolt Hole, then Measure diagonally across Hole Edge to Hole Edge, then add the (1) Head Bolt Hole.

Just like the PTO Case has (2) different (4) Bolt Patterns Based Off a 5.0" Circle and a 6.5" Circle for attaching different things.

Bolt Pattern Calc.
http://www.doov.com/cgi-bin/bolthole.cgi?d=76&num=16&xoffset=0&yoffset=0&angleoffset=0&bolthole=Calculate

Bolt Pattern Calc.
http://www.doov.com/cgi-bin/bolthole.cgi?d=76&num=16&xoffset=0&yoffset=0&angleoffset=0&bolthole=Calculate

You can use the Free, Simple, 2D CAD from www.emachineshop.com to Draw up Parts. It also lets you view them in 3D. Can Open and Export, iges, step, dxf, CAD files. Most CNC(Water Jets, Plasma, Laser) Machines use dxf files for cutting things you Design. Here is a Torque Plate for a Skidoo Twin and a Adapter Plate, 1st Concept for adding CNC to my Manual 12" x 37" lathe using different Size Stepper Motors.
 

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