1977 Thompson Mercruiser 470 - identity please

Discussion in 'Gas Engines' started by nhhockeyplayer, Apr 9, 2014.

  1. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    the only part in the engine that can corrode is the exhaust elbow as it is cast iron and has sea water in it.
    If it fails it can allow seawater to drain back into the exhaust ports.
    It's a consumable, change it if you dont know the age of it.
    If you have water in the oil check the seal on the camshaft that is behind the freshwater circulation pump, even though it has a drain hole between the seals.
    Start with a cooling system pressure test as you would do on a car
  2. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    The Mercury 170/470 engines were a pretty good piece of work and a tough engine to boot; I've owned several of them over the years (still have one). As mentioned earlier (but with slight error), the engine is essentially "half of a Ford 460", using the high-performance 429/460 head casting and similar geometry rotating components in a custom aluminum block.

    They are all closed system engines with heat exchangers. Hence, none of the issues related to cooling the engine directly with raw (sea) water. Biggest problems I ever encountered were:

    1. Coolant leaks at the manifold (Merc did an extended warranty and dealer recall on that issue)

    2. Leaking coolant around camshaft front seal.

    3. Routine failure of crank-mounted alternator. Most (like me) converted the engines to an external belt-driven alternators.
  3. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    sorry yes later stuff
  4. mreoe4sure
    Joined: Jul 2009
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    mreoe4sure who me

    Check the compression again with a good tester, if it is still low that means something around that cylinder. The backfire through the carb might be related if there is low compression. I would think burnt intake valve on that cylinder. If the compression is good ,start looking at ignition wires crossed or someone moved the distributor out of time, these will cause a nice flame to come out of the carburetor. Pretty simple, large , 4 cylinder with most parts easily obtainable . Don't get to stressed about the project, BOAT stands for BRING OUT ANOTHER THOUSAND, and you are only in 1200. Years from now you will be enjoying this boat and won't hardly remember the work that you needed to get it to your level of desired finish. It will take to or three regular oil changes to get the oil cleaned up as the inside of the engine is still covered with black oil , and when the engine is started the detergent in the new oil will be cleaning the inside of the engine and keeping it in suspension till drained. Steve
  5. nhhockeyplayer
    Joined: Apr 2014
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    Location: nashua nh

    nhhockeyplayer Junior Member

    Thanks Steve, great words of promise and I think you might be on target.
    Its not in bad shape. So I will cozy up to it in the sunshine soon and start workin on it.
  6. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    The previous posts regarding the 470 are right on the money. I owned one for over 1/4 of a century and found it pound for pound to be my favorite marine engine. They were lightweight and pulled like a tractor! These engines required a closed cooling system (antifreeze in the block) to prevent galvanic corrosion between the lightweight aluminum block and the cast iron 460 Ford cylinder head. In order to keep them healthy an owner needed to be meticulous with maintenance, especially cooling system PH.

    BMcF gave you leads on 3 common issues. I can see that the original alternator has been retrofitted with a belt driven version. BMcF also mentioned the camshaft seals leaking. I remember this problem being crankshaft seals but it's been a few years since I owned a 470 and my memory might be wrong. Never the less there is a wear issue with the seals at the front of the engine that will require attention if you want to bring that engine back after all these years. The tiller post on the drive unit will also be square on that vintage as it was made prior to Mercruiser changing the design to a splined set up. I recall my steering got pretty sloppy in later years.

    The problem with old rigs like this is that if you have to pay someone else to do the work you'll go broke getting everything fixed. As is often the case with old boats, they can be brought back but the greater question is weather it is cost effective to do so. Especially if you lack the experience to DIY.

    Good luck whatever you do and welcome to the forum.


  7. boatbuilder41
    Joined: Feb 2013
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    boatbuilder41 Senior Member

    These engines are very impressive when its running properly. I would start by pressure testing the cooling system... if it was my guess... i would say a head gasket. Unlike most marine engines.... the 470 was a aluminum block with a steal head... unsimilar metals greatly enhances the corrosion problem at the head gasket. Not all 470s came with a alternator... most came with a charging coil located behind the harmonic balancer.... and it had a external regulator/rectifer... battery connections were critical to these engines... which overworked the rectifer or regulator causing them to overheat ... . When they shorted out... they shorted to ground and caused a very serious problem to the aluminum blocks... most people dont know about changing the zinc anodes in the block and heat exchanger... ... i would say head gasket first... ... if it were mine... i would pull the head ... if its not the head gasket now... it wont be long...i promise.... its time for a new one... i have worked on several... almost always a head gasket... buy new head bolts and dont over torque the head.... remember... its just aluminum threads . I cant recall the proper ft/lbs... but mercruiser took this into consideration and lowered the head bolt torque specifications... i do recall a few having the issue of thread failure in block...... if you fix it right....and service properly.... these engines are real impressive. ... i know its a old thread... but this is some good info i havent found discussed here.... just for future 470 owners .....
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