Exhaust manifold gasket

[Mike_from_NH]

I blew an exhaust manifold gasket/donut, passenger side, accelerating from 20-50 MPH. There was a muffled pop (backfire) and the gasket fell to the street. Prior to this I felt I was experiencing an exhaust vacuum leak - slightly louder and minimal power loss on acceleration.

I could not get back to pick up the pieces, traffic wouldn’t allow it.

2 questions

1. Is there something else I should be checking that would have caused the gasket to fail?

2. Is a cracked or failing gasket the likely cause of the pre-symptoms?

Thanks

[Royce]

A 351W should not have an exhaust gasket between the manifold and the head. Using gaskets there causes the manifold to expands and contracts in ways it was not designed to do and eventually causes failure. Take it apart, clean everything well, and reassemble with black or blue RTV on the surfaces so it doesn’t happen again.

[CRZYCAT]

I think he’s referring to the donut gasket between the manifold and the exhaust pipe? However, I’ve driven over 35,000 miles in my Torino with a 351W that has gaskets between the exhaust manifold and the head with no problems. I’ll admit I’m no master mechanic but I’ve never heard of anyone using silicone in place of exhaust gaskets.

[Royce]

The shop manual tells you to use lithium grease. I use silicone. No gasket is used because the gasket can and does fail.

If he’s referring to the donut it just needs to be replaced. When they leak, like any other gasket, they are done.

I think he’s referring to the donut gasket between the manifold and the exhaust pipe? However, I’ve driven over 35,000 miles in my Torino with a 351W that has gaskets between the exhaust manifold and the head with no problems. I’ll admit I’m no master mechanic but I’ve never heard of anyone using silicone in place of exhaust gaskets.

[Mike_from_NH]

I think he’s referring to the donut gasket between the manifold and the exhaust pipe? However, I’ve driven over 35,000 miles in my Torino with a 351W that has gaskets between the exhaust manifold and the head with no problems. I’ll admit I’m no master mechanic but I’ve never heard of anyone using silicone in place of exhaust gaskets.

Yep, the donut gasket. I wasn’t sure if just replacing it might cause other issues.

Thanks for the quick replies!

[Royce]

Typically you need to drop the entire H or Y pipe to replace one, so that means replacing both. They don’t fail unless someone didn’t tighten the nuts / studs enough.

I think he’s referring to the donut gasket between the manifold and the exhaust pipe? However, I’ve driven over 35,000 miles in my Torino with a 351W that has gaskets between the exhaust manifold and the head with no problems. I’ll admit I’m no master mechanic but I’ve never heard of anyone using silicone in place of exhaust gaskets.

Yep, the donut gasket. I wasn’t sure if just replacing it might cause other issues.

Thanks for the quick replies!

[CRZYCAT]

Well, I learned something new today. Thank you! Is that only the 351W or does that also apply to the 289 and 302?

[Mike_from_NH]

Typically you need to drop the entire H or Y pipe to replace one, so that means replacing both. They don’t fail unless someone didn’t tighten the nuts / studs enough.

Thanks Royce.

I did get 2. The PO had installed a custom exhaust about 12k miles ago.

Is there a specific torque for tightening?

[xr7g428]

It’s rare but I have seen these fail from a plugged exhaust. Blow some compressed air through it and see if it is obstructed.

[Royce]

I am sure the shop manual will have one. I use an electric 1/2" impact wrench on mine.

Typically you need to drop the entire H or Y pipe to replace one, so that means replacing both. They don’t fail unless someone didn’t tighten the nuts / studs enough.

Thanks Royce.

I did get 2. The PO had installed a custom exhaust about 12k miles ago.

Is there a specific torque for tightening?

[Mike_from_NH]

OK Guys… need some help.

It turns out the donut gaskets were intact. One of the bolts (passenger side lower) worked it’s way free and fell out. The top bolt was also loose- but I was lucky/cursed to recover it and the washer/nut.

I ordered 600-1155 from NAPA, next day in store, confirmed fit… 3 bolts, 3 fine nuts, 3 coarse nuts & 3 lock washers. Here is the difference in what was in the car and what I got. (New =silver bolt and below)

The new nuts are both fine and coarse threads…the coarse end of the bolt goes into the manifold, the fine matches up with the fine nut. I see no use for the coarse nut.

The black original bolt is 3” long, the silver replacement is 2.25 inches long. Summit recommends the exact same kit ( https://www.summitracing.com/parts/RNB-03099/applications/year/1969/make/mercury/model/cougar/engine-size/5-8l-351/engine-vin-code/h )

The difference in length is not letting me put the accompanying lock washer on. The flange barely lets the nut begin to thread.
I have a thought that by using the coarse nut is to be used to “lengthen” the exposed bolt, sacrificing depth into the manifold…but that seems wrong.

As always, I appreciate the help! Thanks