1977 70 HP 70773S Rod Bearing Rod Bearing Failure

[Trackbolt]

Earlier this season my 1977 70 hp died slowly at 3/4 throttle and near locked up. I just finished disassembly and I found what I had suspected that the rod bearing on #2 (center) had dissintegrated. I could not tell needle from cage. The crank is scored at that journal to boot. #1 and #3 are fine. I am happy that the rod cap held and did not let the rod damage the block. I am not sure why this happened but I suspect a lean mixture or no fuel from #2 carb was the problem. I would like to rebuild the engine. I probably will need a new crank, The block has no deep scores and looks like reboring will clean things up. My machinist will let me know. There are a couple of other issues that concern me It looks like a piece of spark plug electorde got to bouncing between the top of the #2 piston and the head pitting the head. I can clean this up but can I re use the head or do I need to try to find a good used head? What are a good used head and crank going for these days? I am trying to put together a budget for the rebuild. I have a well equipped shop and enjoy the rebuilding process as much as I enjoy being out on the water. At leas when I'm done I know what I have.

Thanks

Tony

 

[Trackbolt]

Re: 1977 70 HP 70773S Rod Bearing Rod Bearing Failure

Here are a couple of pictures

 

[boobie]

Re: 1977 70 HP 70773S Rod Bearing Rod Bearing Failure

Getting a rebuilt power head may be cheaper. Check around.

 

[F_R]

Re: 1977 70 HP 70773S Rod Bearing Rod Bearing Failure

The fuel will certainly be blamed, but in reality that probably was not the real cause. Lack of lubrication will destroy the piston long before the bearings. Much more likely the bearing cage broke for no identifiable reason. OR, and this is a likely suspect, water has been being ingested through the exhaust ports, and found it's way to the crankcase, rusting/pitting the bearing surfaces, and the rest is history. That is an all too-common problem with the 3 cylinder loopers. You MUST inspect the exhaust cover inner baffle plate for warpage in the exhaust port areas. The warpage allows the gasket to leak, squirting water into the cylinder through the ports. Also, closely inspect all the other bearings for any sign of pitting. If not perfect, replace them.

You can clean up any sharp edges in the head and re-use it. I've done it many times.

 

[bonz_d]

Re: 1977 70 HP 70773S Rod Bearing Rod Bearing Failure

As you probably know already these cranks are only available used. I have been watching ebay myself for one to fit a 1988 70hp and there hasn't been one yet that I would concider. I believe your engine used the same crank up to 1984 then there was a change in the splines for the driveshaft.

Might be cheaper to look for an engine with a blown cylinder, which is what I'm also doing.

 

[Trackbolt]

Re: 1977 70 HP 70773S Rod Bearing Rod Bearing Failure

F.R.
Thanks. I always look forward for your advice. I did a little checking and you are right on the water intrusion. Not the first time this motor had issues with the exhaust cover. I bought the boat and motor about five years ago and the PO said he had replaced those gaskets by a marine shop. Two seasons ago they were leaking in the #2 area. I replaced the gaskets then myself. Either the damage was done then or it continued to leak after I replaced the gaskets. Anyway the damage is done. For what I paid for the boat motor and trailer i cannot complain as I got a lot of use from it before it failed. Deep pockets I don't have so I will have to weigh rebuilding this one or finding a good used motor.

Tony

 

[F_R]

Re: 1977 70 HP 70773S Rod Bearing Rod Bearing Failure

Yeah, replacing the gaskets without dealing with the warped plate doesn't do any good.