507F90C Force Outboard - Blown Head Gasket

[Bob La Londe]

I recently blew a head gasket on my 1990 50 HP 2 cylinder Force outboard. (507F90C w/ D lower unit).

No big deal. These old Forces were known for blowing gaskets. I've had it happen once before, and the previous owner had it happen once to him. I just bought a new gasket and re-installed it the first time for several years of problem free operation. Its really a great little motor.

I figured just buy another gasket, replace, and good to go. I did. Put the new gasket in, torqued with my inch pound torque wrench per factory specs, and fired it up. It ran great and the cylinders don't leak, but the head gasket oozes water out into the cowling all the way around. Thinking I had the torque wrong I backed off the bolts and retorqued. Same thing. Water oozes out in the cowling. Dang it.

Not knowing if somehow I screwed something up I pulled off the head and took a look. I think I found the problem, but I don't know what to do about it. The metal ring that goes around the top of the cylinders is much thicker than the fiber gasket material that seals the water jacket. This is not the case with the old head gasket. I compared them, and the old one was definitely manufactured to be closer to the same thickness for both metal and fiber.

I'm not sure what to do. I ordered another head gasket from another source and its the same way. Much thicker metal ring than fiber gasket. I'm considering trying a high temp silicon gasket material on all the surfaces except the edge of the cylinder and cylinder head where the metal cylinder ring meets the edges of the cylinder and head hemisphere.

Bob La Londe
Director - Yuma Pro Am

 

[Frank Acampora]

Re: 507F90C Force Outboard - Blown Head Gasket

In a case like that, where the fibre (fancy English spelling) portion of the gasket just won't seal, I prefer to use non-hardening (red) Permatex. It just seems to seal better than silicone. But, did you clean both sealing surfaces? Sometimes a thin layer of gasket material sticks to them and can cause leaks.

And, to help prevent blown head gaskets in the future, tighten to factory specs, run engine for about an hour, loosen head bolts, and retighten to factory specs. (Do not tighten dry--torque to specs with anti-seize.) Since you have already inadvertently done this on the existing gasket, reuse the one you have already installed. It may very well seal the water passages better the second time around.

Yeah! Yeah, I know!--Don't re-use an old head gasket Bada-be, Bada-ba. I reuse them all the time on my own personal engines without a problem. BUT: When I rebuild for someone else I always give them a new one. If it blows or leaks, it wasn't because of shoddy workmanship.

 

[Bob La Londe]

Re: 507F90C Force Outboard - Blown Head Gasket

Sounds like we both have about the same logic in trying to fix this issue. I have high temp red Permatex, but I don't understand the non-hardening part. It forms into a rubbery material when set. I also have aviation sealer (the paint on Permatex), but I can't see that filling the miniscule voids well enough if torquing the head down (twice now) does not seal it up. I like the paint on sealer for difficult to mate surfaces like valve covers on a car where its almost impossible to get the metal cover flattened back out perfectly and tightening the bolts further just distorts the metal.

I also wonder about the difference in the gaskets. Did maybe later model yers chamfer or groove the edges of the cylinder and cylinder head to get away from the problem with blown head gaskets? Hence the thicker metal ring?

I too have re-used gaskets on my own stuff in the past when I needed to get it in service NOW, but since I already received a new gasket I figured I would go ahead and use it.

Your comments on cleaning are certainly apt. I too have had problems with that in the past when getting in a hurry and not thoroughly cleaning gasket surfaces.

Bob La Londe

 

[Bob La Londe]

Re: 507F90C Force Outboard - Blown Head Gasket

Looks like I found the problem. The replacement gasket I had used was a Sierra 1884. I went to a marine shop, and asked them for a gasket for my motor, and that is what they gave me. The one I had ordered as a second replacement I looked up on-line and ordered myself. It is a Sierra 1883. The 1884 says Force 50 on the package, and the 1883 says 45 & 50 Force.

Now, what does all this mean? Up until 1989 the torque specs for the 50 HP are 270 inch pounds. For 1990 it says 225 inch pounds. The 1883 gasket has a rubber coating, and the metal ring seems to sink into the rubber more easily than the metal ring int he 1884 sinks into the fiber gasket material.

In other words. The marine shop just saw "Force 50" on the package and gave me the wrong gasket.

I cleaned up the head and the block and installed the 1883 gasket today. It sealed up perfectly. The we took the boat out and ran it on the river for a while, and it ran great. Force 50 on a 16' flat bottom with two large bubbas and we consistantly ran 30+ on the GPS.

Frank, you suggested retorquing it after running it. Do you break it loose and retorque, or just put a torque wrench on it and retorque as is. (Obviously after it cools down of course.)

Bob La Londe