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Problem with 390 engine |
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390AMX2
AMC Apprentice Joined: Jul/04/2013 Location: Fairmont, WV Status: Offline Points: 112 |
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Posted: Jul/31/2014 at 2:57pm |
I am rebuilding a 1969 390 engine which is bored 30 over. Everything went together find until I tried to adjust the rocker arms. The 1969 TSM states that the rocker arm nut needs to torqued to 20-25 ft lbs. So I already had the number 1 piston on TDC so I torqued it. I then turned the engine over so the number 8 piston was on TDC and torqued it. So far so good. I then turned the engine over so the number 4 piston was on TDC. I torqued it. When I tried to turn the over to the next piston the engine would not turn. It was as if the engine was locked up. Does anybody have an idea of what the problem might be? The cam I am using is a Comp Cams 280 with a .490 lift, Comp Cams valve lifters #822, Com Cams push rods #7812 which I think the length is 7.794 inches, Comp Cams #926 single valve springs with a damper and stock rocker arms. What do I need to change to make this work? Thanks for any help.
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waukman
AMC Apprentice Joined: Aug/03/2008 Status: Offline Points: 87 |
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my understanding is you do not need to turn the engine over at all when putting stock rocker arms back on a 390 non-adjustable drivetrain set up.per TSM all you need to do is torque all rockers down to spec regardless of where drivetrain is in relation to the firing order
did this on mine with no problems, but my rebuild was 100% stock
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greasygt III
AMC Addicted Joined: May/27/2014 Location: Florida Status: Offline Points: 676 |
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Can you rotate the engine in the other direction?
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iapexl8r
AMC Addicted Joined: Apr/13/2008 Location: north idaho Status: Offline Points: 639 |
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did you rotate the crank a full 360 deg before you adjusted rocker arms? I will assume so
if you did there could be a few things that are wrong. Are you running an adjustible rocker? if you are then back the rockers adjuster nut off and adjust to Zero lash + 1/2 turn. if you are running the non adjustable rocker set up you may have the pistons installed backwards the exhaust valve relief is hitting the intake valve or you have the wrong length push rods. pictures help alot . Chris
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SKeown
AMC Addicted Joined: Jul/30/2009 Location: Texas Status: Offline Points: 3085 |
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Make sure the valve springs aren't stacking and that the retainers aren't contacting the valve guide/seal. SKeown
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Boris Badanov
AMC Addicted Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209 |
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Back everything off, check the cam timing!
Turn it over a couple of times preferably by hand. If you were using the starter to turn it and it locked I will bet you already have a bent valve :-( If the cam timing is good I am guessing the pushrods are too long.
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Gremlin Dreams
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390AMX2
AMC Apprentice Joined: Jul/04/2013 Location: Fairmont, WV Status: Offline Points: 112 |
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I checked the engine to make sure he pistons were in the correct direction and they are. When turning the engine I always did it by hand. Removed all valves and they slide in and of valve guides very easy. Other than the push rods being too long ( They seemed to work when I turned the engine from #1 to #8 and then #4) could it be the rocker arms as they are the ones which came with this engine? I could not find a set of nos rocker arms or any body who makes good repos. The guy that I had helping me just told me that when cleaning the rocker arms that he got a few of the rocker arms and rocker arm balls mixed up. I know that these parts are supposed to be kept together and not mixed up. Could that be the problem? Thanks to all who are trying to help fix this problem.
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Boris Badanov
AMC Addicted Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209 |
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The wear difference in the balls is so small as to be insignicant in this situation.
do it again, but instead of torquing them down, at the heel/base circle of the cam, go to just contact on each cylinder and then count the turns till you hit bottom of the stud. As you tighten if the valve opens, and stays open while on the heal/base circle then you have a push rod problem. I suspect a push rod issue but all the other above suggestions are a real possibility....
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Gremlin Dreams
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Rebel Machine
AMC Addicted Joined: Jul/03/2007 Location: Western Il Status: Offline Points: 5118 |
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Do you have the original rocker studs or aftermarket? The stock studs have a shoulder the rocker ball will rest against. Aftermarket studs don't have those.
-Steve- |
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390AMX2
AMC Apprentice Joined: Jul/04/2013 Location: Fairmont, WV Status: Offline Points: 112 |
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Does anybody out there have a 1968 or 1969 390 engine that is disassembled where they could measure the push rods and let me know what the length is? The push rods that I have are Comp Cams 7812 which they say the length is 7.794 inches. I measured these and they are 7.750 inches. (7 3/4 in.) Comp Cams on line catalog says that for 290 thru 401 I should be using 7.950 inch push rod. Does any body have an old Comp Cams catalog that covers the 68 and 69 390 engine? I would like to get this engine running before winter.
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