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I have found that my Birmingham Scott is fitted with Top Hat Stuffing Blocks secured into the crankcase with four 3/16″
allen screws. They don’t appear on any drawings that I have seen.
Please can someone tell me, is this a standard thing on Brum Scotts, or, a tuning mod?
It’s a tuning mod to increase crankcase compression, but be careful, I had an engine that had been prepared by a well known sprinter and racer which performed well until one of the Allen screws either sheared or stripped it’s thread in the crankcase. The stuffing block moved, smashing the piston, the cylinder spigot and knocked a hole through the crankcase!
Although Alfred Angas Scott himself used them in some of his TT engines, I have never quite understood how they can really be of much benefit, as although the primary ( crankcase ) compression is increased, surely it also reduces the volume of the petrol/air mixture that is being pumped through the crankcase ? And the greater the volume of mixture we can get pumped up into the cylinders the more power we will get ? Perhaps someone will now tell us that the flow through the crankcase will speed up due to the higher pressure invoved, and thereby increase the volume at the same time, but I need convincing.
I have in the past bought three Scotts with stuffing blocks fitted, was never happy with them, and removed them. At least two pairs are still sat somewhere in a box in my workshop. If anyone wants them they can have them for the price of the postage and packing….
Thanks for the replies. My question was prompted after experiencing a similar catastophy to Dave’s. A screw came out and found its way down a tranfer port into the crankcase, the conrod eye then punched it through the front of the case leaving a hole about 1″ by 1/2″ ,also, a hairline crack in the piston skirt.
I am now considering my options for repairing the damage. A hole would have to be cut in the outer crankcase to access the damaged area for welding. Or, I could fill with steel reinforced epoxy putty accessed via the flywheel well.
Any advice would be welcome.
Also, the best source for a replacement piston please.
Crankcase stuffers are a benefit. I fitted them, one per crankcase on the inner side to my engine quite a few years back. You have to be careful they don’t impede the gas flow from the inlet ports and into the transfer port. Mine are 25cc each and carefully shaped. They don’t reduce the volume of charge, they increase it. It’s like using a tyre pump (the piston and cylinder) on a bicycle tyre (small crankcase volume) and a car tyre (larger crankcase volume). One stroke of the pump has more effect on the bicycle tyre than the car tyre and then when the piston compresses the charge in the crankcase with a smaller volume it pumps the charge at a higher pressure and bigger flow into the cylinder. Fillng the outer part of the inlet annulus where there are no inlet ports also has the same effect so together with stuffer blocks makes a significant improvement.
The stuffer blocks have another advantage, they cause turbulance inside the piston helping to cool the crown and also increase little end lubrication.
I have seen too much carnage to trust stuffing blocks that are screwed in. I make a few engines to order with my own cases which have a larger inlet and these have stuffers cast in situ. As regards repairing a small hole, I certainly would not weld it and incur distortion. I just apply a few epoxy soaked layers on the back.
My racer has run for many years with a hole patched up at a race meeting with epoxy and a couple of patches cut out of a pair of jeans. Do not make trouble for yourself! Kind Regards
Thanks Roger, you have put my mind at rest.
I had decided to use epoxy but thought I would be condemned as a Bodger. Welding old irreplaceable aluminium parts always seems a huge gamble to me.