|Posted on Wednesday, June 15, 2016 - 12:20 pm: ||
I bought an 1190 bore kit and know that there is a difference in the cylinder base gasket thickness to raise compression. Does anyone know the gasket part number for the thinner gasket? Or the thickness of it uncompressed?
|Posted on Friday, June 17, 2016 - 02:18 am: ||
The answer you seek is not so simple. The standard compression ratio of the 1125 is 12.4 to 1, but because of the cam timing the effective compression ration is about 8.5 to 1. This is due to the very late closing of the inlet valve. (60 degrees ABDC. This is why you can run the lowest octane fuel available without problems.
I could easily calculate the new CR is if I knew the piston volume of the new pistons, but I don't. For your info, the cylinder head volume is slightly more than 25cc and the standard 1125 piston volume is slightly more than 15cc. I normally use 8cc to allocate volume for the base gasket, head gasket and deck height. (distance between piston top and cylinder deck at TDC). If you wanted to maintain the same CR as factory, the piston volume would have to increase to about 22cc.
I would guess that if the piston head volume of your new 106mm piston is the same as the 103mm piston, the CR would increase about one full point assuming the same gaskets were being used. Since you are not using different cams, your effective CR would not exceed 10 to 1. This could be an issue of fuel choice on an air cooled motor, but I think you will be safe using the standard gaskets, as long as you don't allow your engine to get too hot.
|Posted on Friday, June 17, 2016 - 08:54 am: ||
The standard base gasket for the 1190 big bore kit for 1125 is exactly the same as an 1125 base gasket, both in thickness and size.
It's the green paper material, and it compresses to about .017".
If you ran a thinner base gasket, it was a metal gasket and was .010", but this required modification of the dowel holes in the cylinders to allow the cylinder base to seal.
|Posted on Monday, November 16, 2020 - 11:19 pm: ||
Could you explain more on this modification of the dowel holes??
I'm considering doing this on a stock bore 1125