12g940 head on this head i hear there are the nornal valve size and a bigger valve size one is this correct also what if so what are the inlet and exurst size on each head
12g940 Head
Started by
miniclubmanman
, Jul 03 2009 05:58 PM
1 reply to this topic
#2
Cooperman
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Uncle Cooperman, Voted Mr TMF 2011
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Posted 03 July 2009 - 07:12 PM
There are various valve sizes which are standard for the 12G940 on different.
For example, the Mini with the 12G940 normally has 33.5 mm inlet valves and 29.5 mm exhausts. However, the MG Metro with the 12G940 has 35.6 mm inlets.
Interestingly, the Haynes manual gives the Mini as having the 35.6 mm, which is incorrect, even on a 1990 onwards Cooper, SPI and MPI.
The 35.6 mm is an ideal size to use on a gas-flowed 12G940 and you can fit a 30.5 mm exhaust. At those sizes it is still possible to fit unleaded seats.
If the combustion chambers are re-shaped to de-mask the valves and the throats are gas-flowed, you can end up with a very nice gain in power so long as you skim the head to set the correct compression ratio, ideally about 10.2:1 or slightly higher.
For example, the Mini with the 12G940 normally has 33.5 mm inlet valves and 29.5 mm exhausts. However, the MG Metro with the 12G940 has 35.6 mm inlets.
Interestingly, the Haynes manual gives the Mini as having the 35.6 mm, which is incorrect, even on a 1990 onwards Cooper, SPI and MPI.
The 35.6 mm is an ideal size to use on a gas-flowed 12G940 and you can fit a 30.5 mm exhaust. At those sizes it is still possible to fit unleaded seats.
If the combustion chambers are re-shaped to de-mask the valves and the throats are gas-flowed, you can end up with a very nice gain in power so long as you skim the head to set the correct compression ratio, ideally about 10.2:1 or slightly higher.
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