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12G940 Inlet Valve Size Increase


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#1 super6al

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Posted 18 January 2021 - 07:01 PM

Hello All

1275 A+ mg metro unleaded head from a 1990 carb cooper. Valve seats are only installed on the exhausts. 

1) It has the 33.5mm inlet valves. Is it a straight forward machine job to fit 35.6mm valves?

2) Valve guides - the original guides fitted look to be some kind of bronze material - will the cast iron bullet type fit?

Thanks

Alan 



#2 Turbo Phil

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Posted 18 January 2021 - 08:16 PM

You can cut the head to fit the bigger valve, but don’t just cut a wider 45 degree seat, this will kill airflow. A multi angle seat needs to be cut, with a nice bottom angle blended into the throat for best effect.
If you’re using the standard road spec valves, adding a back cut gives a substantial gain for the small amount of work required.
No heads were fitted with bronze guides from the factory, so unless they’ve been changed at some point they’ll be iron. These are fine for most road applications, but can’t be used with the stainless valves if you were thinking of using these for any reason.

Phil.

Edited by Turbo Phil, 18 January 2021 - 08:17 PM.


#3 super6al

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Posted 18 January 2021 - 08:53 PM

Thanks Phil,it will be standard road valves- by cut back I assume you mean on the valve itself?
the engines ever been to bits so the guides are whatever the factory fitted. I could be wrong but they seem to have a yellowish colour to them.
Alan

#4 Turbo Phil

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Posted 18 January 2021 - 09:32 PM

Yes, back cut on the valve as shown here:

 

Phil. 


Edited by Turbo Phil, 18 January 2021 - 09:34 PM.


#5 Turbo Phil

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Posted 18 January 2021 - 09:33 PM

Seat mods as shown here:

 

Phil. 


Edited by Turbo Phil, 18 January 2021 - 09:34 PM.


#6 super6al

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Posted 18 January 2021 - 09:43 PM

Cheers




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