Pre war supercharged Comet..........Zoller supercharger.

greg brillus

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I've read most of the available literature about it, and it appeared to have 2 issues........like all small run hand-built machines, they simply did not have time to develop it enough........So with tuning as in carburetion and ignition adjustments, plus the fine-tuning of the oil delivery........too much would smoke and foul the plug, not enough could potentially destroy the blower........And second, more critically was the bad overheating........piston rings turned blue, so lost tension, loss of compression. Became all too hard and they removed the lot, returning the bikes back to being naturally aspirated. The HP gains were marginal....... I think a figure of about 36 BHP comes to mind.........If they had run alcohol fuels this might have helped but who knows now..........The cylinder head is not a very strong casting, I am not sure what the head material on those 2 bikes was, but the alloy ones are very weak.
 
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davidd

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The original aluminum Series A aluminum head had a cast-in web that contained the two valve seats and the spark plug. That was the weak point and upon running the web would separate from the casting and leak air out at the spark plug hole. This would have been on the outside of the spark plug, not the inside, as the compressed charge was finding its way around the web insert on the backside of the insert as the compression rose. The cure was to cast up an aluminum head.

There was a CI head and a Bronze head, but for shedding heat, the aluminum head was the best.

David
 

greg brillus

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"Skull heads" were pretty common back then.......Norton for one used them........Large bronze center with the alloy fins cast over it all.........interesting concept.......must have been tricky.
 

Martyn Goodwin

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The original aluminum Series A aluminum head had a cast-in web that contained the two valve seats and the spark plug. That was the weak point and upon running the web would separate from the casting and leak air out at the spark plug hole. This would have been on the outside of the spark plug, not the inside, as the compressed charge was finding its way around the web insert on the backside of the insert as the compression rose. The cure was to cast up an aluminum head.

There was a CI head and a Bronze head, but for shedding heat, the aluminum head was the best.

David
I seem to remember that the Velo"porcupine" heads were cast in Silver
 

oexing

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The Laverda 750 ohc twins had cast iron sculls in alu head, with valve seats machined in. So no seat could ever drop. But today not so great with unleaded fuel so in the long run you´d have to do something about seat wear.

Vic
 

davidd

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I cannot specifically remember the material. It was likely bronze when I think back on it. Carleton had it fail in practice, I believe. We did not strip it down as he knew what was happening. I tried to repair it with some Loctite, which was the only apparent thing to do. I think it worked for a lap.

David
 

Chris Launders

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Even Honda used them in their small racing bikes, I saw a bag full of new ones at a patternmakers where they were making patterns for replica heads, probably there wasn't room for 4 shrink in valve seats in a 50cc head.
 

delboy

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Even Honda used them in their small racing bikes, I saw a bag full of new ones at a patternmakers where they were making patterns for replica heads, probably there wasn't room for 4 shrink in valve seats in a 50cc head.
Hi Folks,
PEI talks about Cast Iron "Spectacles" [valve seats and plug boss] cast into Ariel "Red Hunter" alloy heads on pages 174/5 of Motorcycle Engineering.
Page 214 of "Back to A" has a pic' of an "A" alloy head with Bronze cast-in spectacles.
I recall being told at a Series "A" meet some years back, about the "A" Twin there with Alloy heads; had Bronze cast-in "Spectacles". It was thought that they had come loose in the heads and were leaking gas.
No Idea if it was ever rectified in any way? Would seem nigh impossible to fix
Regards, Delboy..
 
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