OzVMX Forum
Marque Remarks => Spaniards (Bultaco, Ossa, Montesa etc) => Topic started by: firko on August 14, 2013, 01:44:40 pm
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Do all Bultaco Astros have this style combustion chamber? Are any Pursang models similar? I have an aftermarket Jones DT1 head with a similar design..........just curious :)
(http://i1112.photobucket.com/albums/k495/firko2/Astrohead_zpse513966f.jpg) (http://s1112.photobucket.com/user/firko2/media/Astrohead_zpse513966f.jpg.html)
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My #106 350cc pursang-astro is the same. Two spark plugs, this one has the decomp tapped between them. I read once that a hot up mod on the road racer models was to fill in that bit.
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Mark,
All the Pursangs have this Bathtub style combustion chamber.The compression ratio on these heads was altered at the factory simply by changing the size of the tub.( the comp ratio is usually stamped under the head at the front)The Frontera's used the same theory but with a different shaped tub,presumably for more torque.The Astro and Pursang motors are just about identical.
On your three hole head,because of their close proximity and different expansion rates of the plugs/compresion release they usually cracked towards the centre hole.The twin plug heads were ok.
I think your Jones head was probably just a copy of the latest Bultaco 60's technology :-\
The modern single plug spherical head design mod as you have had done in the US is the way to go.
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The modern single plug spherical head design mod as you have had done in the US is the way to go.
Absolutely, I've had one done for my Maico and I'm thinking of doing a couple for my 360 Yamahas....
I was just curious as I'd never noticed a Bultaco combustion chamber until I saw the pictured head for sale on vft.org today. I'd say you're correct that Don Jones would have adapted the Bultaco design to suit the Yamaha. When I eventually get my 250 Hindall going I plan to try the five different aftermarket heads I've got to see which one came the closest to perfect back in the early seventies.