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Cr Call Help!


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#1 super6al

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Posted 06 August 2018 - 06:20 PM

I'm a newby here so apologies if what I'm asking has been covered before. I have searched but cant seem to find a reliable method for what I need to do.
I'm rebuilding my rsp cooper engine. It's a 1275, 12A2AF53 type with a 12G940 mg unleaded head (ie not the large valve) & 21963 std Pistons. I've reworked & cleaned up the valve seat area in the head as per Vizards book. Nothing too drastic just the basic bumps & seat machining cuts. I've calculated the CR using method detailed in this forum & it comes out at 9.5:1. According to Haynes my engine should have a CR of 10.1:1. I've measured the chamber volume at 23.5cc & using the forum calc I need to reduce the volume to 21cc to get back to 10.1:1 Converting the 2.5cc difference in to how much I need to shave off the head is where I'm coming unstuck. I've put 21cc of paraffin into the chamber but no matter how I try to measure the gap from the liquid to the head surface it 'grabs' the vernier or the steel rule. I've resorted to working out the chamber area in square cm & dividing the 2.5cc by this value. It comes out at 30 thou required to skim. Can anyone confirm if this sounds ok or is there a better method to measure. One thing concerns me is 2.5cc seems a lot of metal removed for the work I've done. In addition the head has already been skimmed after work to repair the brass insert between 2 & 3 which had sunk into the head. So this should have offset some of the increase in chamber volume.
Thanks Alan

#2 ACDodd

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Posted 06 August 2018 - 07:48 PM

Skim it 0.010" only. The engine will run better on modern fuel at 9.75:1 CR.

Ac

#3 super6al

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Posted 07 August 2018 - 12:12 PM

Thanks for the prompt reply AC




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