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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old June 8th, 2023, 10:27 PM
Prowbar Prowbar is offline
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Default Magnum camshaft specifications

My 1965 manual has a nice camshaft valve timing diagram as seen in the picture.
Pete Chronis told me the Magnum engines had an improved camshaft grind.

I'm looking for the 1966 and up or Magnum camshaft diagrams, to compare the two.
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File Type: jpg Camshaft timing 401-478 60-65.jpg (3.40 MB, Multiple views, 11 clicks)
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Old June 9th, 2023, 01:48 AM
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Default Re: Magnum camshaft specifications

I have this from the Power by GMC Manual

*Manual is dated 9/64 so not sure if it helps your cause
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File Type: jpeg IMG_9845.jpeg (2.69 MB, Multiple views, 4 clicks)
File Type: jpeg IMG_9844.jpeg (3.15 MB, Multiple views, 13 clicks)
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Last edited by Funky61; June 9th, 2023 at 01:56 AM. Reason: added date of the manual
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Old June 9th, 2023, 08:36 AM
Prowbar Prowbar is offline
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Default Re: Magnum camshaft specifications

Quote:
Originally Posted by Funky61 View Post
I have this from the Power by GMC Manual

*Manual is dated 9/64 so not sure if it helps your cause
Thanks, Funky, what is interesting is that it shows some variation on the camshaft grind. Figured all years before the Magnum were created equal.

Please post the 1962 478 camshaft specs, or the 1963, or the Magnum 478 specs if you have them.
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Old June 10th, 2023, 03:21 AM
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Default Re: Magnum camshaft specifications

These the numbers you were requesting? Its from the same manual

Found these numbers in a later manual about the HC engines
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File Type: jpeg IMG_9852.jpeg (1.60 MB, Multiple views, 7 clicks)
File Type: jpeg IMG_9853.jpeg (1.30 MB, Multiple views, 8 clicks)
File Type: jpeg IMG_9854.jpeg (2.51 MB, Multiple views, 4 clicks)
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Last edited by Funky61; June 10th, 2023 at 03:32 AM.
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Old June 10th, 2023, 09:24 PM
Prowbar Prowbar is offline
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Default Re: Magnum camshaft specifications

Thanks again Funky. Looks like the camshafts are all in the same ballpark. The 401 closes the intake a little earlier than the 478, which closes later. Probably to improve cylinder filling on the 478.

Added my own cam design in my build thread.
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Old June 10th, 2023, 11:24 PM
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Default Re: Magnum camshaft specifications

After reviewing my notes, I would add .075" valve lift. Base on stock valve lift of .425"-.430 (as measure when I torn the engine down), then the modified valve lift would be .500" lift (with room to spare). All of these measurements was performed using stock pistons and head gaskets.

Best thing to do is to measure you engine to be sure what the maximum the valve can open (with the piston at TDC) just in case there was some machining performed in the past.
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Old June 11th, 2023, 05:51 PM
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Default Re: Magnum camshaft specifications

Thanks, James. That would require some welding on the camshaft, or possibly they could reduce the base circle, but .500 - .415 = .085, which would be a lot to grind from the base circle. Will do some checks and see what can be done. I'll report back.
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Old June 11th, 2023, 06:24 PM
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Default Re: Magnum camshaft specifications

Quote:
Originally Posted by Prowbar View Post
Thanks, James. That would require some welding on the camshaft, or possibly they could reduce the base circle, but .500 - .415 = .085, which would be a lot to grind from the base circle. Will do some checks and see what can be done. I'll report back.
The welding should be only on the lobe and not on the base circle. Altering the base circle will change the valve geometry which would require custom parts (i.e. rocker arms and/or push rods) to get back into specs. If you do weld on the base circle (for a repair) it will need to be return to the original base circle radius.

I also added some info here:
https://6066gmcclub.com/showthread.php?t=51066&page=11
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