a7 head on a10

[bonny]

despite having the bike for about 4 years now i only realised tonight that the iron cylinder head on my machine is not a10 at all but a7 , i was thinking of buying a new carb and measured the port diameter and it came to only 24mm , so its a7 as far as the technical books say.
the bike is on the bench and not being run , will this cause any damage , and how much does the compression ratio rise with the smaller head on it ?   

[muskrat]

G'day Bonny,
                     The comp ratio rises quite a bit, put your thinking cap on. If the A7 has 8 to 1 comp that means the combustion chamber is 31.25cc, 250 div by 8 = 31.25. So now the A10, 325 div by 31.25 = 10.4 to 1 comp ratio. That's what I'm running on the cafe. Ping city.
If you use a dremel to blend the outer edge of the comb chamber to match the bores the ratio would drop to around 9 to 1. With te head still on, wind it over to TDC. Lean the bike over so that the spark plug hole is vertical. Now use a syringe and measure the amount of oil it takes to come up to the bottom of the spark plug hole. This will tell you what the cc of the comb chamber is to work out your comp ratio.
You could also use the dremel to open up the manifold to match a 26/28mm carb.
Sounds like a lot of work but it's not really.
Cheers
PS to get the oil out just lean bike back over the other way and use petrol and compressed air. Don't loose the tip off the air nozzle, LOL some will know what I mean.

[bonny]

cheers again muskrat , i think i'll just buy a proper cast iron head off ebay , loads of them on there and normally go for peanuts , or i might fit the alloy head i have .

[trevinoz]

Bonny,
          Is there a casting number on the head?
  Trev.

[bonny]

i presume there is trevinoz , but i haven't taken note of it , i'll look again tomorrow and stick it up if there is.

[andy2565]

are the valves smaller on the a7 to,would you have to fit the bigger valves ? as well as porting it,

[muskrat]

I hahe the 1.4" and 1.38" valves in mine, and a little porting.
Cheers