[Solent]

I have a 2003 Yamaha 9.9 HP 4 stroke. I am in southern Argentina where spare parts and mechanics are hard to come by. 4 weeks ago I had water in the engine oil and a local mechanic took it away and replaced the gaskets in the power head (cylinder head and upper casing) and the O ring. He stuck down the gaskets with a local product called Siloc 8600 (silicon oximica) which he says he has used in this type of application for the last 25 years (although its the first time he's worked on one of these small Yamahas). After less than 3 hours of use the problem re-occurred and I once more have oil that looks like milky coffee. He has today dismantled the power unit and the gaskets seem to be fine. He now plans to do a compression test on the cylinder head.

My query is twofold. First has anyone else had a problem similar to this and second whether the use of silicon or other sealants is a good or bad idea on the gaskets.

thanks

[JUSTINTIME]

could be the headgasket

[rodbolt]

best I remember the service manual states to use NO sealants on the head surface.
make sure they check the head for surface flatness. the factory reccomends NOT surfacing the head due to no timing belt adjustments but I have heard of .010" taken off with no issues.

[Solent]

Many thanks for the replies. We've found that particular sealant isn't appropriate for head gaskets anyway and that the Seloc manual says NEVER use sealant in capital letters, so hopefully that's the problem. Will ask them to check for surface flatness.

[sdsaw]

Originally Posted by Solent

I have a 2003 Yamaha 9.9 HP 4 stroke. I am in southern Argentina where spare parts and mechanics are hard to come by. 4 weeks ago I had water in the engine oil and a local mechanic took it away and replaced the gaskets in the power head (cylinder head and upper casing) and the O ring. He stuck down the gaskets with a local product called Siloc 8600 (silicon oximica) which he says he has used in this type of application for the last 25 years (although its the first time he's worked on one of these small Yamahas). After less than 3 hours of use the problem re-occurred and I once more have oil that looks like milky coffee. He has today dismantled the power unit and the gaskets seem to be fine. He now plans to do a compression test on the cylinder head.

My query is twofold. First has anyone else had a problem similar to this and second whether the use of silicon or other sealants is a good or bad idea on the gaskets.

thanks

Hi, new here, first post,
I had a similar issue on a older 9.8 Yamaha (1988)
What was causing water to get in my oil was that my oil tank had corroded thru in the area that the exhaust tube passes down thru the middle of the oil tank which is on the underside of the powerhead.
New oil tank, powerhead base plate, gaskets, etc. reassembled, still getting water in oil Tore it down three times checking seal surfaces, o-rings, with gasket sealer, without gasket sealer without positive results.
Finally discovered that the surface of my BRAND NEW powerhead base plate was not flat!
Resurfaced the base plate while assembled to the oil tank, reassembled motor USING GASKET SEALER (spray on Permatex) and it's been good for 5 yrs.

Hope this helps, Scott

[sdsaw]

Originally Posted by rodbolt

best I remember the service manual states to use NO sealants on the head surface.
make sure they check the head for surface flatness. the factory reccomends NOT surfacing the head due to no timing belt adjustments but I have heard of .010" taken off with no issues.

I was able to take a light cut to resurface the head on my 1988 9.8 Yamaha after welding a spot that about to corrode thru. Don't remember how much I removed but it was a minimum amount.
It didn't seem to affect the timing belt enough to be a problem.

BTW, saltwater is very tough on my Yamaha from my experience.

Scott