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600HRS on 365 Mags...Rebuild heads, leave them alone?

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Old 12-01-2010, 10:30 AM
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Default 600HRS on 365 Mags...Rebuild heads, leave them alone?

I have a pair of 365HP mags with just over 600 hours on them, they start up, run, and idle perfect. They barley use any oil, and seem to be very strong. I have spoke with some people who say 600 hours is time to rebuild the heads, other say rebuilding the heads will blow out my rings from sealing the top end up too tight. What is everyones thoughts on this? Also some other people have said to just replace the valve springs and be done with it. I have always lived by the theory if it isnt broke do not fix it! What to do...? The engine have not been ran hard I usually cruise around at 3200RPMs and change the oil twice a season.
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Old 12-01-2010, 10:54 AM
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If you keep an eye on oil consumption, and the compressions / leakdowns are good, I would leave them alone. If you keep the revs down, and take care of them, they will last a long long time.
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Old 12-01-2010, 11:36 AM
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I'm up against the same thing with my 365 Mags. They are in fantastic shape internally and run great but starting to get up in hours. My thoughts are if your gonna go through the effort of the upper end, freshening up the lower end isnt that much of a stretch on an engine thats being kept basically stock and is in good shape.
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Old 12-01-2010, 11:44 AM
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The lower hp big block Chevys are capable of rediculous hours as long as they are maintained properly. My old Baja 280 has well over 1000 hrs on the orig 330's.

Last week while working the Pacific Marine Expo (commercial fishing) in Seattle, I met a guy who is still running a pair of 1987 Merc 330's (FWC) in a crab boat. They presently have 6000 hrs on them, never even had the vavle covers off and still run perfect.

I say if they are running fine, leave it alone and change your oil every 20 hrs. You'll be surprised how long they will last.

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Old 12-01-2010, 11:53 AM
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I would atleast do the valve springs on it, it will be alot cheaper than when it drops a valve at 3500rpm.

But most will tell you at 250 hours on that eng. do the valvetrain then at 500 do the whole eng. rebuild.

Now everybody has to adjust to the way you use your boat, most people with 1075 Mercs. dont need to rebuild at 100 hours but if you race or only do poker runs and run up front you need to rebuild. Engines dont know hours they know RPM so if you are 6000rpm most of the time the engine wont last as many hours as at 3000 rpm.

My buddy had the same motors in his boat never ran hard took care of the boat with service stuff like oil and filters spark plugs but never even took the valve covers off. At almost 1000 hours he broke a valve spring took out the whole engine this was years ago but I think he spent like 6500 for a new long block and then 4000 to rebuild the other plus all the little thing he spent over 14k and missed a season of boating. Then sold the boat for 28k a year latter.
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Old 12-01-2010, 11:57 AM
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Quote:
Originally Posted by LapseofReason View Post
I would atleast do the valve springs on it, it will be alot cheaper than when it drops a valve at 3500rpm.

But most will tell you at 250 hours on that eng. do the valvetrain then at 500 do the whole eng. rebuild.

Now everybody has to adjust to the way you use your boat, most people with 1075 Mercs. dont need to rebuild at 100 hours but if you race or only do poker runs and run up front you need to rebuild. Engines dont know hours they know RPM so if you are 6000rpm most of the time the engine wont last as many hours as at 3000 rpm.

My buddy had the same motors in his boat never ran hard took care of the boat with service stuff like oil and filters spark plugs but never even took the valve covers off. At almost 1000 hours he broke a valve spring took out the whole engine this was years ago but I think he spent like 6500 for a new long block and then 4000 to rebuild the other plus all the little thing he spent over 14k and missed a season of boating. Then sold the boat for 28k a year latter.
would atleast do the valve springs on it, it will be alot cheaper than when it drops a valve at 3500rpm.

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Old 12-01-2010, 12:06 PM
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So just change the springs in the boat? Many have said if something was going to break it would be a valve spring.
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Old 12-01-2010, 12:14 PM
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If you do nothing else, change the manifold/riser gaskets if this has never been done or it has been several years since they were done. If you plan to keep the boat awhile, I would consider pulling the heads and going through them. At that point you have to consider maybe upgrading the camshafts since those old flat tappet cams all seem to eventaually lose a lobe with today's oils that contain lower levels of ZDDP. And if you go that far why not do the bearings and rings while you have the motors pulled. Then you start to mull stroking them to 496's...
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Old 12-01-2010, 12:24 PM
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Quote:
Originally Posted by onestrghtridaa View Post
So just change the springs in the boat? Many have said if something was going to break it would be a valve spring.
Kinda depends on how easy the valve covers come off, some times the exh. is in the way and has to come off. On my Nortech I had to pull the valve covers on one of the back engines so to not have to remove the exh. manifold I took the front and rear bolts out and put some 6-8 inch long ones in so when I took the rest of the bolts out it just slid down the bolts out of my way. I would have to see your boat to see the room you have it might be faster to pull the motors and do them but I dont know in your case.
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Old 12-01-2010, 12:45 PM
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I put 800 hrs. on a set of 420's. They were ready to be rebuilt at that point. These engines with low lift hyd. cams rarely break valvesprings, If it's running good and has good compression and leakdown, leave it alone.
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