Will'smotobikes19
Well-Known Member
Do you think I could make a flat plate of aluminum into the shape of a gasket and put gaskets above and below the plate? Is that what decking is?
Can I use a big bore piston/cylinder on a stock crank?
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gary55
Well-Known Member
Yes I do think you could do that. I also think it would not adversely effect the port timing as the piston is rising so high. Hopefully others brighter than I will chime in on this as it is something I have only thought about and not tried myself.
Decking is the removal of material from the top of the cylinder to adjust squish clearance and increase compression.
Another thing you are going to need to take into account is depending on how much larger the big bore is as this will cause the head to encroach farther into the cylinder before it begins to rise. Even the after market heads only go out to 47mm on the OD of the squish band. The stock ones are closer to 44.5 and would hang inside the 50mm bore over 2.5mm so you can see how this will cause a prob. with the dome of the piston. For me a stock head would be out of the question.
You should use a few different thicknesses to check clearances before actually cutting the plate. Another factor is now with trying to pack more volume into the same sized head chamber compression is going to rise a lot. This is all doable, but it's going to take some thought as to how all the parts relate to one another. Patience is going to be the key on this little gem.
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Will'smotobikes19
Well-Known Member
Ya the piston is 1mm larger in diameter so head gasket will have to be modified I'm thinking of getting a thick copper one. I'll have to source some aluminum plate. I might be able to make it using a hacksaw, drill and file. I don't have any machines. I really don't want to take the cylinder off because it's hard getting the rings in. I'm going to get a VM18 carb to go with the reed valve.
gary55
Well-Known Member
Head gasket ID is 49mm. No need to alter for a 48mm piston. . Still would go with after market head and spacer at base. Short rod bottom end probably won't work with your piston. I should have known it was th 48mm piston from your 72cc calc.
Will'smotobikes19
Well-Known Member
Mock up of the engine. I realize with all those gaskets and the spacer as well as the washers for the head I will need longer studs.
gary55
Well-Known Member
I didn't even think about that.
Will'smotobikes19
Well-Known Member
I think I'll need atleast 1/4 inch stock for the spacer if you look at picture # 6 and if not I could sand it down. I still need to get some grease for the clutch gear since there's none on it and loctite. The SBP studs I got are 112mm so I think they will work.
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Will'smotobikes19
Well-Known Member
Update: I need to get different studs because stock ones are too short and the SBP ones I got is the wrong thread pitch for the new block. UGH. I seem to need longer studs with coarser thread. I think the ones I have are 1 mm pitch while the new block is tapped for 1.25 or something like that.
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gary55
Well-Known Member
I'm sure their are other suppliers but here's a thought. First take the studs that match the threads in your block to verify the pitch, then look in this direction for graded allthread with the proper pitch. Google is your friend.Class 8.8 Steel Fully Threaded Rod, M8-1.25 Thread Size, 1 m Length, Right Hand Threads: Fully Threaded Rods And Studs: Amazon.com: Industrial & Scientific
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