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Old 26 September 2015, 03:50   #61
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It is 100% corrosion. It is a really bad design, the mount is encapsulated by that stainless plate and rot sets in and expands and blows the whole lot apart - I can take photos of at least five scrap engines with exactly the same problem. Even ones that we fitted new replacement legs to in the mid 90's were doing it again exactly the same. It was never as bad with the manual start version, which I always put down to lack of battery current to mess things up.

Anyway I would be looking for 120psi plus ideally.
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Old 26 September 2015, 04:11   #62
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120 on a hot engine?

I'm guessing these may have been cold?
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Old 26 September 2015, 06:21   #63
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Perhaps if as you say it's a common problem, the nut is overtightened at factory which either hair-line cracks the case, creating a CASE corrosion problem. Galvanic corrosion happens when two dissimilar metals meet with or without water, with exception of stainless steel. It's often used to connect weaker metals, especially Alloy or Aluminum specifically to avoid galvanic corrosion. Overtorque however could cause a problem especially with the addition of heat which as we know expands the joint even tighter. All kinds of stuff about why SS is used to fasten such metals sealing with anti-corrision. I've used stainless steel fasteners and screws on any metal parts, boat, car or other and never a corrosion problem.
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Old 26 September 2015, 13:05   #64
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that's strange as an ex biker we often replaced our engine casing bolts with stainless for a, bling and b, the oe crosshead screws were made of cheese and often the head chewed up . but after a while the stainless allen bolts would corrode with the alloy engine and seize the fix was to coat the stainless fasteners with coppaslip grease never had a problem undoing a coppaslip coated stainless fastener
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Old 26 September 2015, 14:00   #65
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Quote:
Originally Posted by Nightfisher View Post
Perhaps if as you say it's a common problem, the nut is overtightened at factory which either hair-line cracks the case, creating a CASE corrosion problem. Galvanic corrosion happens when two dissimilar metals meet with or without water, with exception of stainless steel. It's often used to connect weaker metals, especially Alloy or Aluminum specifically to avoid galvanic corrosion. Overtorque however could cause a problem especially with the addition of heat which as we know expands the joint even tighter. All kinds of stuff about why SS is used to fasten such metals sealing with anti-corrision. I've used stainless steel fasteners and screws on any metal parts, boat, car or other and never a corrosion problem.

Aluminium/alloy and Stainless Steel are not happy bedfellows when an electrolyte is present: Galvanic Corrosion – keep those metals apart! | Anzor Australia's blog - provides technical tips, case studies, and other useful information about stainless steel fasteners
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Old 27 September 2015, 02:02   #66
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Austentic Stainless
(Type 302/304
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Old 27 September 2015, 03:12   #67
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120 on a hot engine?

I'm guessing these may have been cold?
Yes, that was readings when cold.
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Old 27 September 2015, 03:36   #68
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Worth checking your compression tester is showing the right figures. 5% difference between cylinders is ok. 100psi really isn't good, but it could easily be down to a poorly calibrated tester.
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Old 27 September 2015, 04:19   #69
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Worth checking your compression tester is showing the right figures. 5% difference between cylinders is ok. 100psi really isn't good, but it could easily be down to a poorly calibrated tester.
But also normal to test with the engine warm if I am not mistaken?
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Old 29 September 2015, 14:40   #70
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Hi guys, update time.

Got myself a near mint mid section and because it comes with a far better condition steering arm (not too happy with one that is on it now, very corroded, a problem with these engines) I'm going to take the powerhead off and rebuild. Have talked it though with a very competent outboard fixer and he will help me if needed, but says it's mostly just bolt off then on again.



This obviously means more work than before and includes undoing the 4 bolts that go DOWN from the powerhead as well as the 8 that go UP into it. All of the 8 bolts come away freely, but 2 out of the 4 (the 2 at the back of outboard) are a pig to get to. Any tips? see pic. This one is not too bad and should come away with a bit of heat treatment, but the other one is buried, looks like I'll have to take a few bits off the powerhead to get in.



Referring back to my first picture, I'll be needing 2 new gaskets for either side of the small section that connects the powerhead to the mid section (the bit that gets covered by the wee plastic apron). I assume that a local yamaha dealer will have such gaskets even if it is a 25 year old 1990 outboard?

Thanks for all the help again
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