IH/Mcormick Model 70 plow- What size cylinder?????

On the bore side of this question, a lot depends on the gpm hydraulic output of your tractor. Since bore controls speed of lift, this is quite important in making nice even headlands. The speed you are travelling plus speed of plow lift will control making every furrow the same length. When I farmed I used to keep numerous implement cylinders around, 2.5", 3" and 4" bore. Depending on the job and the tractor would govern which cylinder was used. You will find this a trial and error issue. For the most part 4" will be used on big trailer implements as you don't want that big weight going up or down very fast. Haybines on the other hand you go 2.5" as you want speed. I had 3 and 4 bottom trailer plows and while anyone of the cylinders listed above would lift the plow, I also had tractors with 12 gpm and 17 gpm hydraulic systems. All three would pull the plows but each better with different bore cylinder. My recomendation if your tractor is less than 12 gpm go with 3" bore, if more than 12 gpm go with 4" bore. You don't want that plow coming out of the ground like a jack in the box.