CYLINDER HEAD CHANGE G12

rockinron1941 · Post by **rockinron1941** » Sun Mar 27, 2011 6:43 pm

Hi all,just reading book on post war twins by Roy Bacon ,(by, the way my bike is a 1959 G12 Standerd,) and he says that the cylinder head casting was changed in 1960 to incorparate three small diagonal fins under the exhaust ports for better air flow ,my bike has these fins do you think they have been fitted to the existing cylinders or will these have been changed as well?Thanks Ron

Merlin · Post by **Merlin** » Sun Mar 27, 2011 6:54 pm

Very often the next years model was brought out in September or so yours could be a 1960 model registered in 1959.

rockinron1941 · Post by **rockinron1941** » Sun Mar 27, 2011 7:25 pm

Hi Stuart how are you, lovely day just been over to Knott End, the bike was registered in April 1959. I was just wondering if the heads where interchangeable .Ron

Eric · Post by **Eric** » Sun Mar 27, 2011 7:25 pm

Its very doubtful the new type heads would even fit the old cylinders, the combustion chamber was a different shape and the pistons also have a different head shape.

What is your engine number to be found stamped into the top nearside of aluminium crankcase at the front just below the cylinder joint.

If it starts 60G12 then its a 1960 engine.

Eric · Post by **Eric** » Sun Mar 27, 2011 7:29 pm

Just looked at your other post with an engine number so its more than probable that if you really do have the new type heads you will have the cylinders and pistons to match.

1608 · Post by **1608** » Sun Mar 27, 2011 11:39 pm

The spiggot on the early 650 barrel is narrower by about 1/16" than the later ones because the later heads underwent a few changes as already mentioned.You probably have the correct / corresponding barrels because the gaskets would have blown by now due to the loose fitting spiggot ( flame trap ).

Don Madden · Post by **Don Madden** » Mon Mar 28, 2011 12:19 am

To add detail to John's post above, 1959: Top spigot & recess in head 3in, piston "carbuncle' top. 1960-up: Top spigot & head recess, 3-1/16in, piston pop-up flat top. These must be used as sets. Cheers, Don.

rockinron1941 · Post by **rockinron1941** » Mon Mar 28, 2011 5:38 pm

Thanks Guys ,sorry but whats a spiggot.Ron

chasdev · Post by **chasdev** » Mon Mar 28, 2011 7:58 pm

It's the area of the barrel that protrudes above the gasket sealing deck.

rockinron1941 · Post by **rockinron1941** » Mon Mar 28, 2011 9:46 pm

Thanks guys I will check the spigget,now I know what it is .Ron