Moroso made the coolant restrictor washers also. I do not leave out a thermostat or restrictor washer. Here's why... as your rpm increases, so does the flow of the coolant. Now, once the rpm gets up there, as it passes thru the radiator, it does not have sufficient time in the radiator to exchange the heat, therefore in some cases, during a long cruising night, temperature will go higher than it might outta be. The thermostat, or restrictor washer slows that flow down to give ample time for the coolant to get rid of the heat in the radiator. The restrictor washers come in various hole sizes, this takes some expermentation to figure the exact one that does your ride the best. Also, dont forget, just cause someone can get an engine to stay at 115 degrees, doesn't mean it's all good, as the air/ fuel mixture is affected by the temp of the intake ( on carb rides ). Low temps can lead to sag, stumble, hesitation, hard starts, and rich fuel mixtures.