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502 Mag Refresh

Old 03-07-2020, 07:27 AM
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Default 502 Mag Refresh

Looking for input from others who have been down this road. I have a pair of MK IV 502 Mags carb engines that are getting close to needing refresh. Want to maybe boost them up a little to say around 450 HP. They were originally 385's. Do not want to get too crazy with wild builds or anything like that. I can stay with the stock Mercruiser Silent choice or install a some Gil/Merc's that I bought several years ago from a pair of new engine take offs. My first thoughts are to have the heads reworked and add some roller tip rockers, fresh valve job, general clean up. Have the blocks cleaned up, new bearing, rings etc, and a newer design cam. The old Quad Jets are still working pretty good, but most likely need rebuilt.
If you have been down this road, would like to hear good and bad. Thanks in advance.

Last edited by Helmwurst; 03-07-2020 at 07:30 AM. Reason: Forgot to add last comment
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Old 03-07-2020, 12:53 PM
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Your on he right track but do a search for 502 builds and you'll find a ton of info. JMO ditch the QJ for tunability.
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Old 03-09-2020, 06:49 AM
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are you going to rebuild motor, new pistons, rings etc, or just do top end???
i would go complete roller valve train,
DUMP the QJ...
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Old 03-09-2020, 07:10 AM
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Originally Posted by flashgordon View Post
are you going to rebuild motor, new pistons, rings etc, or just do top end???
i would go complete roller valve train,
DUMP the QJ...
Any issues discovered during initial testing, will be addressed. If there is a need for complete rebuild it will be done. Carbs will most likely be changed to a more tunable option.
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Old 03-09-2020, 08:59 AM
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how many hours on motors??
doing a leakdown??
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Old 03-09-2020, 09:04 AM
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Originally Posted by flashgordon View Post
how many hours on motors??
doing a leakdown??
From his original post
"My first thoughts are to have the heads reworked and add some roller tip rockers, fresh valve job, general clean up. Have the blocks cleaned up, new bearing, rings etc, and a newer design cam"

Why a leak down?
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Old 03-09-2020, 06:48 PM
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I've seen estimates for rebuilding a 502 here ranging from a few thousand dollars all the way up to a ridiculous 20k. Do as much of the work yourself that you are able to do.
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Old 03-11-2020, 07:24 AM
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Originally Posted by thirdchildhood View Post
I've seen estimates for rebuilding a 502 here ranging from a few thousand dollars all the way up to a ridiculous 20k. Do as much of the work yourself that you are able to do.
I can do it all, except pulling the motors and the machine work. Going to do a compression and leak down as soon as weather permits.
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Old 03-11-2020, 11:13 AM
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a leak down is just another tool to show you if you have one bad cylinder, etcc
so if you are to replace pistons, would give you option for bigger cam,,and go fuller roller valve train..
just my 2 cents

and 20 k for one motor rebuild is crazy, built both my 547 for 16-17,new crank , rods , pistons, complete valve train,,timing chain oil pump,,,
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Last edited by flashgordon; 03-11-2020 at 11:15 AM. Reason: speeling
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Old 03-25-2020, 09:21 AM
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Probably getting to the compression/leak down test in a week or 2. Talked with guy who I trust to do a complete engine diagnostics and recommendations. Now curios of what others have used for QJ carb replacement that will work on the stock GM dual bolt pattern intake. Will likely have to use and adapter with a square bore carb. I was looking at Holley, Edlebrock and Quick Fuel. Ideas matter, but real field experiences would be appreciated.
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