HOME and how to join Forum Open Area General Scott topics squished heads Re: Re: squished heads

Roger Moss

I George and I helped each other when he started in business. My recollection is that the shaped head was suggested by George and that as our family had quite a substantial machine tool business, I agreed to have the patterns made on the basis that George would have some cast to include one for me. These had a rear profile to allow for the water feed to exit horizontally rather than vertically to conserve height. After many years, these patterns were returned to me and are in my stores. The early heads were made of something like LM3 not heat treated and I had problems as the skull sections flexed and cracked needing welding up. I decided to have a second set of patterns made and to have them produced in LM 25 TF to be more durable. You can recognize these heads by the fact that the rear area that is flat for the coolant connection of the Silk Scott, is radiused. The combustion chamber shape is very similar, but the patterns were made with perhaps more close attention to the accuracy of the profile. I have plotted the shape in X and Y axis of the two cast in profiles and they are within a few thou of being equal. I have a batch of ten heads in manufacture and great care is taken to ensure that both combustion chambers are on the same plane and that the final combustion face is at the correct height. The main idea was to have a head that allowed a more even burn and so the flame initiation is within a counterbore, rather in a similar fashion to the pre chamber of a Ricardo engine. The actual depth of the combustion chamber is about ten thou deeper than a conventional Scott head, but, of course, the cast in sections reduce the overall volume. I can only speak for the machining of my castings, so when considering a Silk Scott head, I have no idea to what horizon the fire face / combustion face was machined. I have always run on petrol in my racer apart from a brief try with dope years ago and at a very early stage discovered that it was folly to alter the transfer and exhaust ports. In fact George and his son Phil were here yesterday and we discussed this matter again. We agreed that the Day cycle deflector type engine will not rev much over 5000 rpm and produce power. If arranged properly, it will develop good torque from about 2000 to 4800 rpm and that any significant alteration to original port heights will dramatically reduce bottom end torque and whilst you may get it to rev a bit higher, then those revs will not have much torque, so you have lost the intrinsic advantage of the engine. My emphasis is to reduce any impediment to cylinder filling and leave the ports alone. You thus retain the character but enhance the torque. So if anyone has a problem with an engine with such a head, I do suggest that the height of the transfer and exhaust ports are measured first to see if they have been “modified” I have all the port heights on record if needed, just email me. I would also suggest that head volumes and compression ratios are a very deceptive preoccupation. It is the volume of gas that you can arrange to get into the head space after the exhaust has closed that is important and the Scott conventional inlet tract is unduly restricted, but then, in the 1920’s, to be fair, there was very little understanding of fluid dynamics. Kind Regards Roger