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Different front and rear head intake port dimensions..

charles d cannon jr

Active Forum Website User
VOC Member
Still working on the Shadow basket case and just noticed that the rear head intake port opening is about 31.5mm and the front head is 29mm (standard). The rear head looks very neat inside the intake port and whoever ported it obviously knew what he was doing. However, can I expect troubles from the different opening dimensions and should the front be opened out to the same size as the rear? Any thoughts or comments would be appreciated greatly. Since I don't port and fixing thiswill requires shipping it to others, I'd rather leave it as is if it is not that important. Thanks, Charles Cannon.

greg brillus

Well Known and Active Forum Website User
VOC Member
Hi there Charles, I had the same issue with a Shadow about two years ago where one head had an oversize port and the other standard at around the usual 1 1/8 inches in diammeter using original carb's and manifolds. It seemed to run fine at that, i can only guess that if you had the throttle open almost fully, then maybe the cylinder with the larger port may have more output and effectively drag the other cylinder along as best it can.....truth be known, how often does one have a twin on full throttle for any great period of time....I'd just put it together as is making sure that the head with the large port has a manifold to head gasket that covers/ seals both surfaces and you should be fine....cheers for now....Greg.


Well Known and Active Forum Website User
VOC Member
The internal diameter of the carburettor will limit the flow so although the gas flow might not be as beautiful as one might wish it will cause no trouble at all. Just make sure to use the same type and size of carb front and rear.


Well Known and Active Forum Website User
VOC Member
Having one Rapide with Standard ports and Shadow sized carbs and another with two front heads both ported to 34mm, big carbs, high compression and Lightning cams, I would say that the pot with the Standard port will spend most of its life dragging the ported out cylinder around.

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