Reply To: Shaking at High RPM Running 1955 Evinrude Fleetwin Aquasonic 7.5HP

[Phil Budne]

Don wrote:

One more thought….Did you use the correct base gasket when you replaced the powerhead? There are a couple of different base gaskets used depending on the year, the wrong gasket could create a leak or restrict water flow…
Sure hope I am wrong about this thought….What year powerhead did you use? Just like the gaskets, the powerhead castings are different between the 55 and earlier vs 56 and later models also…Unfortunately, the hole patterns are the same, so the wrong gaskets/powerheads will bolt is place just fine….

Good thought!! I’d heard about the possible gasket mismatch in a MaxW article, and asked about it (on the blue board before the blueboard before this one):

http://www.omc-boats.org/phil/5.5+7.5-gaskets/

Here is the notes file from that folder:

http://www.aomci.org/cgi-bin/yabb2/YaBB.pl?num=1367261609

Me:
I’ve heard the OMC 5.5 hp and 7.5 hp exhaust gaskets can be mistakenly used (with bad or no cooling resulting). Can anyone post pictures of both and point out the difference?

FR:
There are three kinds, The main determining factor is the motor’s mounting systems. Spring mounted motors use one kind and rubber mounted use another. You can tell the difference by seeing if they fit all the passages.

303467 fits 5.5 & 7.5 with spring mounts
304314 fits 5.5 & 7.5 with rubber mounts
313065 fits 5.5 & 6 with rubber mounts and thermostat.

You can use a 313065 for a 304314 by enlarging the water inlet hole. 303467 is too much different to substitute.

FR:

Sierra still makes both 304314 and 313065. As you can see, the difference is the hole (arrow). I agree with BRP if they don’t use the 1/4″ hole in the 313065 any more. I never could see any sense to it.

Mumbles:

Here’s something funny. The last time I picked up the 304314 and 313065 gaskets at the dealer, they were priced slightly different even though they are identical.
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http://www.aomci.org/cgi-bin/yabb2/YaBB.pl?num=1385399477/1#1

Don:
OK, so the gasket that is completely cut out if for the 56-58 models. The gasket with the little hole is for the 54-55 spring mount models. The little extra flap on the gasket with the little hole isn’t really used…I guess this way they can simplify the manufacture of the two style gaskets.
OK, this was very helpful, thanks!!

Don:
The gasket with the restrictor hole is for the thermostat models, the gasket that is cut out is for the non thermostat rubber mount models…
I would think that the non thermostat gasket would work fine on the thermostat models, because the thermostat is the “restrictor”…
BUT, the best way to figure it out is to pull a thermostat powerhead off, which I will be doing soon…
I’m thinking that Bill used the thermostat gasket, the one with the restrictor hole (313065), and just cut the tab off…Of course, the little hole in the gasket must line up with the hole in the exhaust hsg/powerhead. I guess the engine would have overheated instantly if the holes didn’t line up..To me, that would work fine on the 54-55 spring mount models….

But, like Mumbles says, if you used the non thermostatl rubber mount gasket (with the cut out) on the spring mount engines, there would be a leak…OK, I’m even confusing myself now….
Thanks for the help, alot learned….

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