140 hp Exhaust / Intake Manifold Gasket Failure

[The Manatee]

1974 Merc. 140hp : Approx. 12 months ago I replaced the gasket between the Head and Combo Intake/Exhaust Manifold. The middle exhaust ports had burned through the old gasket. I thought I did everything correctly: bought a Merc. Factory Gasket, cleaned the surfaces, applied a 1mm thin layer of gasket adhesive, torqued the bolts in sequence and to the correct torque values. It ran great for about 15 hours...now the same middle exhaust ports have burned through the gasket again. The gasket was charred and brittle. What am I doing wrong ?

 

[Bt Doctur]

If you mean the exaust gasket that joins the manifold to the head you have either a warped exaust manifold or a warped head or a very corroded exaust post.Exaust gaskets are normally installed dry.

 

[The Manatee]

Thanks for your prompt reply! The head was professionally installed 5 years ago so I assume the head is true and the surface is flat.. Perhaps the manifold is warped, I will take it to a machine shop for inspection. I will install the new gasket without adhesive.

 

[Bt Doctur]

remove the manifold and have it checked for flatness. post a clear pic of the head especially the ports causing the problem

 

[Bt Doctur]

may even be using the wrong gasket too
http://www.boatfix.com/merc/Bullet/98/98_14.pdf

 

[The Manatee]

Thanks for your prompt reply! The head was professionally installed 5 years ago so I assume the head is true and the surface is flat.. Perhaps the manifold is warped, I will take it to a machine shop for inspection. I will install the new gasket without adhesive.

 

[The Manatee]

Opps! I reposted my previous reply. I will report back after the machine shop inspects the manifold.

 

[Scott Danforth]

put a straight edge on both the head and the manifold. never assume its flat. always verify

 

[The Manatee]

Here is an update : I took the manifold to a machine shop and in one minute of thorough inspection he found a 2" long hairline crack inside the middle exhaust ports. He said that sometimes residual moisture "hides" the cracks and it takes a day or two to completely dry out and see cracks. I will replace the manifold. Thanks for everyone's insights and comments!

 

[The Manatee]

Here are 2 photos of the blown / burned gasket.

 

[fishrdan]

Looks like the gasket is paper (green color in picture). Use the OEM grey metalized gasket when installing the new manifold, they are MUCH better than the compressed paper gaskets.

 

[The Manatee]

Thanks fishrdan, you are correct! I just got the new gasket and it is a shiny grey metallic finish on both sides! Here is another question: should I use split washers in addition to flat washers for the manifold to head bolts? The previous owner had split washers AND flat washers but it appears the split washer's "gap" expands as I torque the bolt to 23 ft/lbs (as per tech manual) ....split washers, yes or no?

 

[The Manatee]

Update : I hope others can learn from this! As I was reattaching the manifold to the head I discovered one bolt was slightly too long and did not seat correctly. In my defense when I installed the too long bolt last year I inserted it in the hole that is hidden from view...I hand threaded it into place, torqued it and never visually confirmed the bolt head was not even touched the manifold. I will buy the correct length bolt...and visually confirm EVERY bolt is seated properly after final torque! See photo.

 

[fishrdan]

If they were spreading out (growing) when torqued, they weren't the right type. I'd use grade 8 lock washers. I don't remember if I used them or not. When I replaced my manifold (Barr) it came with all the necessary hardware: bolts, washers gaskets...

Re-torque the manifold after the first heat/cool cycle.

 

[The Manatee]

Thanks again fishrdan! I will buy the correct Grade 8 bolts, flat washers and split washers. I will torque to 23 ft/lbs; run a full heat / cool cycle then retorque.

 

[Scott Danforth]

Thanks fishrdan, you are correct! I just got the new gasket and it is a shiny grey metallic finish on both sides! Here is another question: should I use split washers in addition to flat washers for the manifold to head bolts? The previous owner had split washers AND flat washers but it appears the split washer's "gap" expands as I torque the bolt to 23 ft/lbs (as per tech manual) ....split washers, yes or no?

NO.... Never use any type of split washer under anything except bolting sheet metal to sheet metal. any mechanical bolted joint is a proper hardened washer, and proper torque.