350 Magnum - exhaust riser damaged brass plug

[DHutch]

Hello,
I have issue with a brass plug that was seized on one of my exhaust risers. Unfortunately broke this off when trying to remove it.
This hex head brass plug is located on the port side riser - facing inside towards cylinders 1,3,5,7 side of the block. The plug actually was interfering with the valve cover oil fill cap. I tried backing out the plug (its a 9/16" hex) and it broke inside. I want to drill out, tap new threads and replace plug. Figured I may be able to purchase plug from an Industrial Supply outlet.

From what I can find on parts lists, this is supposed to be part # 36382 - (.250-18 x .62 fitting). I do not understand these dimensions? The plug has a 9/16" hex head and whats left of it, I'm measuring the diameter of the threads slightly more than 1/2"?

I'm assuming it may be NPT, Can anyone confirm what this exact thread size is?

 

[Bondo]

Ayuh,..... Welcome Aboard,.... It's 1/4" pipe thread,.....

 

[Bt Doctur]

Agree, drain and remove the riser, drill and tap for 1/4 inch pipe using the correct drill bit.
If not needed to drain it ,replace with brass allen head plug for clearance

 

[Bt Doctur]

Usually it is a sign the valve covers are reversed, oil fill is usually port side between 2 and 4

 

[DHutch]

Usually it is a sign the valve covers are reversed, oil fill is usually port side between 2 and 4

Thanks - Interesting regarding the valve covers reversed. LOL, simple fix.
I bought the boat used last fall. The PO had the engine re-built in 2021.

Also like the idea of using new plugs with the recessed allen key.

New 1/4" NPT 18 tap on order.

Thanks for the replies

 

[DHutch]

Hello iboats members
Update on this issue with seized plug on the riser.

I drilled out the old plug and re-tapped threads with a 1/4" NPT 18 tap. Installed new recessed allen key type plug so that is flushed with riser.

Two issues
1. When using a "fake a lake" water feed and idling on the trailer, the plug is leaking a slight amount. Just a couple drops of water every minute. What do you recommend for thread sealant?

2. There's also a leak between the riser & manifold. Its leaking only in one location, at the front - opposite end to the exhaust elbows. I did reuse the existing gasket between the riser & manifold just for testing the new plug. I have new gaskets which I intend on installing. Do I need a sealant or compound on this gasket? Also what should the 4 riser bolts be torqued at?

Thanks

 

[Scott06]

Hello iboats members
Update on this issue with seized plug on the riser.

I drilled out the old plug and re-tapped threads with a 1/4" NPT 18 tap. Installed new recessed allen key type plug so that is flushed with riser.

Two issues
1. When using a "fake a lake" water feed and idling on the trailer, the plug is leaking a slight amount. Just a couple drops of water every minute. What do you recommend for thread sealant?

2. There's also a leak between the riser & manifold. Its leaking only in one location, at the front - opposite end to the exhaust elbows. I did reuse the existing gasket between the riser & manifold just for testing the new plug. I have new gaskets which I intend on installing. Do I need a sealant or compound on this gasket? Also what should the 4 riser bolts be torqued at?

Thanks

Don't need sealant on the new gaskets but you should take out your spark plugs and make sure there is no water in the cylinders. If this gasket leaks water can leak into combustion chambers and get into oil. Can hydro lock the engine and rust the valve stems

I would use perfect seal aka permatex aviation form a gasket #3 - it will seal but also prevent corrosion and make the plug easier to get out.

 

[DHutch]

Don't need sealant on the new gaskets but you should take out your spark plugs and make sure there is no water in the cylinders. If this gasket leaks water can leak into combustion chambers and get into oil. Can hydro lock the engine and rust the valve stems

I would use perfect seal aka permatex aviation form a gasket #3 - it will seal but also prevent corrosion and make the plug easier to get out.

Thanks Scott06 - after removing the spark plugs, how do I check for water? And if I do determine there's water in the combustion chamber - how do I remove it?

Just additional info- I had the engine idling for approximately ten minutes, also shut off & restarted twice. During this idling and restarting, no issues at all - ran very smooth without missing, stalling or smoke. This was my first started up after winterizing.

Thanks

 

[nola mike]

Just turn the engine over. If waters in there it will spurt out and drain the cylinders.

 

[Scott06]

Just turn the engine over. If waters in there it will spurt out and drain the cylinders.

What he said....also check the oil doesnt get milky.

Could be that there is no water in there but with reusing gaskets and a leak you might have an issue better to make sure there is no water than have an issue later

 

[DHutch]

What he said....also check the oil doesnt get milky.

Could be that there is no water in there but with reusing gaskets and a leak you might have an issue better to make sure there is no water than have an issue later

Update
Before pulling the spark plugs and checking for water in the combustion chambers, I want to get the leaking to stop.

I just sealed that new brass plug in the riser with permatex - problem solved.
But with a new riser gasket installed, I now have a leak at the back of the riser near the exhaust elbow. Not excessive, just a few drops every minute.
Question - how tight do the 4 long bolts have to be? Is there a torque setting?

Thanks

 

[Scott06]

Update
Before pulling the spark plugs and checking for water in the combustion chambers, I want to get the leaking to stop.

I just sealed that new brass plug in the riser with permatex - problem solved.
But with a new riser gasket installed, I now have a leak at the back of the riser near the exhaust elbow. Not excessive, just a few drops every minute.
Question - how tight do the 4 long bolts have to be? Is there a torque setting?

Thanks

I think its 35 ft lbs but they need to be flat if they are warped, pitted or otherwise damaged might not seal

 

[Bondo]

I think its 35 ft lbs but they need to be flat if they are warped, pitted or otherwise damaged might not seal

Ayuh,..... 'n both pieces have to be perfectly clean metal, 'n perfectly flat,....

 

[DHutch]

Ayuh,..... 'n both pieces have to be perfectly clean metal, 'n perfectly flat,....

Thanks guys

New update - I did some surfing around and found a merc manual - looked like 27lbs. Torqued them down and re-started engine with the "fake a lake". No leaks anywhere. Idled engine for about 15 mins on driveway, temp gauge climbed to 160F and stayed there entire time.

Before attempting removal of the spark plugs, I'm going to check the oil.

Thanks for reading my posts and replying.
Cheers