I am currently involved in setting the squish on my MC18 R6k with Wossner 2003 RM125 pistons and a friends MC18 R5k with standard Honda pistons.
With standard base gasket, head gasket, and piston the standard squish is 1.1mm
With standard base gasket, head gasket, and Wossner 2003 RM125 piston the squish is 1.7mm at the bore perimeter and 1.62 at the combustion chamber edge. The difference across the squish is because the Wossner piston dome angle (approx 10 degrees) is different from the standard Honda piston dome angle (approx 8.5 degrees).
[[[Actually, I have got something wrong here when you think about it. I'll double check and get back to this.]]]
So, it seems pretty clear to me that the deck height on the standard piston is 22.6mm and the deck height on the Wossner piston is 22.0mm.
Also, the small end bearing on the RM125 is 20mm wide and the small end bearing on the standard MC18 is 17mm wide. So I need to get 2 small end bearings for the RM125 so as to avoid excessive bearing float along the axis of the gudgeon pin and the small end of the conrod.
The squish is to be set at about 0.8mm. So need to machine off the 0.2mm at the sealing face of the head.
Then need to machine off another 0.7mm at a diameter equal to the bore diameter.
The combustion chamber volume will be the last thing to be machined. The aim is to set it at 12.8cc to the top of the spark plug hole. Measured using a pipette/burette to pour ATF into the cylinder with piston at TDC. ATF = Automatic Transmission Fluid.
This is not very easy to do as it is difficult to accurately mount the heads in a 4 jaw lathe chuck. So I am getting a special ring and spacers made to bolt the head to, so that it is easy to mount the head accurately in a lathe for machining.
Hopefully this ring and spacer will be made by tomorrow (NZ$150) and I can continue with the job of setting the squish.
With standard base gasket, head gasket, and piston the standard squish is 1.1mm
With standard base gasket, head gasket, and Wossner 2003 RM125 piston the squish is 1.7mm at the bore perimeter and 1.62 at the combustion chamber edge. The difference across the squish is because the Wossner piston dome angle (approx 10 degrees) is different from the standard Honda piston dome angle (approx 8.5 degrees).
[[[Actually, I have got something wrong here when you think about it. I'll double check and get back to this.]]]
So, it seems pretty clear to me that the deck height on the standard piston is 22.6mm and the deck height on the Wossner piston is 22.0mm.
Also, the small end bearing on the RM125 is 20mm wide and the small end bearing on the standard MC18 is 17mm wide. So I need to get 2 small end bearings for the RM125 so as to avoid excessive bearing float along the axis of the gudgeon pin and the small end of the conrod.
The squish is to be set at about 0.8mm. So need to machine off the 0.2mm at the sealing face of the head.
Then need to machine off another 0.7mm at a diameter equal to the bore diameter.
The combustion chamber volume will be the last thing to be machined. The aim is to set it at 12.8cc to the top of the spark plug hole. Measured using a pipette/burette to pour ATF into the cylinder with piston at TDC. ATF = Automatic Transmission Fluid.
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| Standard MC18 head |
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| Machined MC18 head to give 0.8mm squish with Wossner 2003 RM125 piston. |
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| Adapter ring and spacers to allow placement of head in lathe chuck |
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