If you have not had both the 3100 and the 3500 gasket side by side, you might want to see the differences. I am guessing that coolant ports were made smaller on the 3500 so that a second sealing ridge could be added to the gasket to keep coolant out of the engine. You can see the smaller coolant port and the double ridge on the 3500 gasket (the one with the black sealing ridges) as compared to the single reddish ridge on the 3100 gasket.
Here's a comparison of the intake ports, both the shape and the position is changed quite a bit. The 3500 gasket (with the green sealing ridges) has the port reshaped and raised about 2/4 inch above the location of the 3100 port.
Here's a comparison of the intake ports, both the shape and the position is changed quite a bit. The 3500 gasket (with the green sealing ridges) has the port reshaped and raised about 2/4 inch above the location of the 3100 port.
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