Big Bore


Why do flathead motors have such small bores? Why is everything you read state that the bigger you bore a flathead the hotter they run? This is just not true. Remember, with a small block Chevy or Ford normally you can only bore them once and that is because they cast them with thin walls. A thinner wall will transfer heat to the coolant quicker than a thick wall. In fact a thick wall is like a heat sink that retains heat.


Why did Henry cast the flathead with such thick walls? Most overhead valve motors only have ten head bolts per side. That minimized the amount of bolts right next to the bore. Bolts directly next to the bore can cause upper bore distortions which results in the rings not sealing properly, excessive crank case pressure and loss of horse power.


In a race motor they use torque plates to hone the block. Torque plates simulate the head being bolted on allowing you to torque the bolts, distort the bore, then hone it round.


With a flathead, because of its valve being in the block (not in the head) we have twenty four head bolts on each side. That means twelve bolts on one side are directly against the bores. Henry was smart enough to know that he could not use torque plates in production so the next best thing is to make the bores thick so the bores are not so easily distorted which is what he did. Also, there was a real packing problem because the exhaust ports ran through the block (around the cylinders) which doesn’t leave much room.


That leads to our new design. First we cast the block without cylinders. That allows us to make sure we have all the core sand out. Then we machine and install special sleeves. These sleeves are centrifugally spun during production which means a very dense material, no porosity like a cast one might have, very uniform thickness all the way around. This also means very good material compatibility for ring sealing.


Best of all, we cast in provisions at the bottom of the block and around the bore so our head stud goes through the deck to the bottom and screws in so when we torque our head nut it pulls from the bottom totally eliminating upper bore distortion. Now we have a bore stronger, made with better material and no upper bore distortion with optimum ring sealing.

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