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reaming valve guides

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i am in the UK and currently rebuilding my Chrysler Wimbedon (Plymouth P4) engine and will be changing the valve guides. The factory specification gives reaming dimensions in decimal points of an inch , 0.342-0.343 for inlet and 0.344 - 0.345 for exhaust. Converting these to imperial fractions they both end up as 11/32".

Can anyone tell me if this is corect?

All help gratefully received!

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I can't comment on the dimension as my math sucks.  Just a note that if you look carefully you will notice the same guide is used for intake and exhaust, but they are installed differently.  One end is square shouldered,  the other rounded.  I don't remember which is which but they go one way for exhaust and opposite for intake.  My manual uses the term counterboreld, and indicates the counter bored end goes down on intake, up for exhaust.


This book says intake clearance. 001 to .003 and exhaust .003 to.005.  It says to micrometer the stem and ream as needed to achieve specified tolerence.

Don't know how critical the countebored thing is but something to be of concern for dealing with heat, so for peak longevity of your rebuild its a concern. It mentions chilling the guides to make installation easier. But making sure they are at shop temp before reaming. 

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Thanks for info. i have not removed existing guides yet but will note which way up they are!

The sizes I have are taken from the Pymouth service manual 1936 - 1942 (one with red cover)


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What I have found with modern replacements is this, the external dimensions are the same, intake and exhaust.  But, they have different part numbers as they are internally finished to the correct size.  And, being thick walled cast iron, and don't distort when installed,  no reaming is needed,  That last may change if you're using new valves that may or may not be exactly sized.   In my case the valves were used, just reground and were a perfect fit in new guides.

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