Fixing Mercruiser 470-any tips welcome

[JasonJ]

Once again, my friends little Tahiti ski boat has arrived at my doorstep. Here is what happened. He was running it hard, tried to pull a big skier, and ended up with the motor shutting down, not restarting, and all the oil blown everywhere in the engine compartment, and it had definatley been blown out the dipstick tube.<br /><br /> Myself and another guy got it, put some more oil in it, got it running just fine, temp stays fine, oil pressure is great, not a drop leaking or flowing from anywhere, but a lot of blowby going into spark arrestor. did a compression test, three cylinders were in the 140-150 range, one was only 50 lbs. Put a bit of oil in cylinder, it brought pressure up to only 75 lbs. I am thinking broken/worn ring and/or an exhaust valve starting to burn, and too much blowby when running hard and hauling fat guy caused excessive crankcase pressure.<br /><br /> I have removed engine from boat, it is in my garage awaiting me to pop the head off for a better look. My friend is not made of money, he only paid $1500 for boat a month ago, so I am thinking if rings are broken or excessively worn, I would check cylinder dimensions and run a hone through them all, and replace rings in all cylinders, check valves, and call it good. Any insight into this particular motor or what I might expect to run into is helpfull. I understand the crankcase is a Merc casting and the head is a Ford product.

 

[Trent]

Re: Fixing Mercruiser 470-any tips welcome

Other issues with the 470 / 3.7 Liter Mercury Marine 4 cylinder:<br />1. Early model heat exchanger (3") was too small.<br />2. Charging system prone to failure.<br />3. Head gaskets easily damaged in severe overheats.<br />4. Motor mount failure due to engine shakeing when cold.<br /><br />(1) Early model 470's used a 3" diameter heat exchanger which proved to be inadequate.<br />If you have an early 3" style exchanger it should be replaced with the newer 4" version.<br />The 4" heat exchanger is 75959F1 for $604.10<br /><br />(2) The charging system consists of a stator and a water cooled voltage regulator/rectifier.<br />with the voltage regulator as a set.<br />The stator is part number 398-6231A10 for $196.95<br />The regulator is part number 99502A13 for $366.00<br />That totals $562.95 and you still have a system that is prone to shorts and failures<br />and is difficult to replace.<br /><br />(3) The engine has an aluminum block with a cast iron head.<br />These items have different heat coeffieciants (expansion rates) thus<br />the head gasket gets "worked" when the engine is overheated.<br />A new head gasket is part number 27-13709 for $30.35. A go-nogo gauge is required to get the proper push tube length<br />if machine work is done to the head and/or valves.<br />A special tool is used to collapse the lifter and the go-nogo gauge is inserted<br />between the valve stem and the rocker to test the clearance.<br />If the clearance is wrong you will need to buy the correct length push tube<br />to get the correct clearance.<br /><br />Always follow the service manuals specifications and procedures when<br />replacing the head gasket and reconditioning the head and valves.<br /><br />(4) The 470 can be a real "rough idler" when first started.<br />Proper choke adjustment will reduce this shaking.<br />Motors that have been used for years with this shaking condition<br />will often have motor mount failure.<br />A soft or failed mount can change the alignment of the engine<br />which, in turn, can ruin the drive coupler and splined drive yoke.<br /><br />Replace all worn mounts and check the engine alignment before installing<br />the drive unit.<br /><br />(5) If you tear it down make sure you replace the two cam seals that drive the waterpump. Very common for them to leak.<br /><br /> Service Bullitin

 

[airman]

Re: Fixing Mercruiser 470-any tips welcome

When my charging system failed I was able to build brackets to mount an automotive alternator (with spark arrestor) in the space on the front of the engine where the coolant tank sits.

 

[JasonJ]

Re: Fixing Mercruiser 470-any tips welcome

Pulled the motor and popped the head off and removed pistons. As suspected, the bad cylinder had broken piston rings. No damage to cylinder. Other pistons were fine, but might as well re-ring them anyway. Numbers 3 and 4 piston rod bearings were toast, but crank journals looked fine, no scoring or burning. I assume the bearings started to go when the oil was forced out and the knucklehead kept maxing it out instead of shutting down when he started to lose power. Crank bearings are fine, thankfully. He got off lucky as far as I am concerned. So, replace rings and rod bearings, run a hone in the cylinders (there was no ridge at the top, they looked fine), and all should be good. In my mind a proper rebuild is in order, but I do not own this rig, and the owner wants the el cheapo route, so he gets the el cheapo route.