Exhaust manifold gone...replace both?

[dkjbama]

Howdy folks. Been quite a while since I've frequented this fine forum.

I've got a question. My boat is a 92 Supra ski boat with a PCM 351 Ford. It is raw water cooled but has only seen freshwater. The boat has the original manifolds and risers on it, 22 years old. This afternoon I was getting the boat ready for the season and found the threads on the starboard manifold drain plug hole are pretty much gone and it will no longer hold the plug.

I plan to replace the starboard manifold. Incidentally. I have a pair of risers that I bought and never installed. I was replacing the exhaust hoses and the original ones looked decent so I kept them in service. That was about 6 years ago.

Should I buck up and go ahead and replace the portside manifold as preventative maintenance?

How many years can a person expect to get out of a set of manifolds and risers in freshwater only use?

Considering the potentially devastating consequences of a manifold leaking internally, would I be a fool for not spending the extra $230 to do both?

Thanks in advance,
DKJ

 

[cmariner32]

IMHO...if the one manifold is showing signs of replacement, you should replace both. Since you already have the risers, replace them as well. The cost of new hoses is not all that much, I would say just re-do the entire exhaust system and not worry about it for the next 22 yrs. Wrasslin with old exhaust hose on and off is no fun-I can attest to that-my elbows have still not recovered from last winter.

 

[LouC]

Water leaking in from defective exhaust, is what ruins a lot of marine inboards. Water leaks in around the joint between the manifold and riser, then it gets into a cylinder through an open exhaust valve. Even if its not enough water to hydrolock the engine, the valve stem rusts then you have a sticky valve, low compression and if valve sticks open the piston can hit the valve on the compression stroke. Just replace the manifolds and risers and go boating....

 

[HOGAN]

you are WAY overdue!! Replace both manifolds and risers every ten years.

 

[Billylll]

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Originally posted by HOGAN

you are WAY overdue!! Replace both manifolds and risers every ten years.

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+1 Mike,
Bill

 

[alk]

A lot of my friends from freshwater areas tell me they get 25 plus years out of a fresh water used manifold, and half that from risers. Besides the area around the plug rotting out, how do the manifolds look? If they never leaked at the gaskets, and don't look too rusty, you may want to try drilling and tapping a bigger drain plug? But like others said, best way to kill your motor is letting your exhaust go too far, so proceed with caution. Have any pictures?

 

[Billylll]

Pictures would be nice to see?
Good suggestion Andy.
Bill

 

[dkjbama]

Thank you all for your input.

Pictures as requested. Best I could do at night inside the storage unit with a flashlight in one hand and cellphone in the other.

Portside:

Starboard:

 

[dkjbama]

The plughole with the trashed threads is 3/4" NPT, I'm not sure there's room to go any bigger. I really like this motor, not sure I want to gamble for another season.

 

[dkjbama]

The above being said, I'm really impressed with these Pleasurecraft manifolds and risers. The last time I had the risers off to replace the hoses five or six years ago, the mating surfaces were very clean with no sign of leakage. There is significant metal between the water jackets and the exhaust passages where the risers meet the manifolds and the PCM gasket is a really thick piece of lead. They appear to be very well engineered pieces.

 

[GeeBee]

I have twin grand daughters. When they were in diapers, since I had all the stuff out like butt cream and butt wipes it's easier to diaper them both than to wait for the other to "fail". That way, they were always "dry".

 

[alk]

My experience seems to be opposite most. I had twins, and don't recall their grandparents ever changing a diaper.

But I concur, assuming you can still buy quality replacements for the Ford, and your first one is shot, makes no sense to only replace that one.

 

[Billylll]

How about Teflon tape around a new 3/4 plug? It might be too much of a gamble. For the age of those manifold they are reallly clean looking. The riser joint looks like it's been dry as well.
I am amazed that they are as old as you say and look pretty good. Too bad it's really just an issue with a threaded plug.
Bill

 

[dkjbama]

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Originally posted by Billylll

How about Teflon tape around a new 3/4 plug? It might be too much of a gamble.

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I tried a new plug and it would start to thread in by hand then stop. Using a wrench to try and turn it further, it would immediately pop out. The threads are just done.

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For the age of those manifold they are reallly clean looking. The riser joint looks like it's been dry as well.
I am amazed that they are as old as you say and look pretty good. Too bad it's really just an issue with a threaded plug.
Bill

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I agree, on the outside they look like they could go another ten years. I worry about the inside though.

My father bought this boat new in 1992 and it has been in the family ever since. I can affirm that the manifolds and risers are original.

Only two things have been done to this motor. The PCM ProTec proprietary coil pack ignition system failed and I replaced it with a Mallory magnetic breakerless distributor with a standard coil and ballast resistor. This is the official PCM fix for ProTec failures and it actually runs better than the ProTec ever did. Second I replaced the troublesome Holley 4010 "fishbowl" carb with a 4160. The 4010 suffered badly from heat soak and NEVER idled right, even after a complete remanufacturing by the Holley Custom Shop.

After these two fixes, the motor went from good to spectacular. As good as this motor has been, I really can't begrudge it $450 in parts and a saturday afternoon. It's gonna have to be done at some point in the next few years, might as well be now.

 

[jtybt15]

Manifolds look awfully good for their age. I'd get a 12 ga wire cleaning brush and chuck it in a drill and clean up the hole. Then I'd get JB weld and wipe a thin coat on the threads and on a brass drain valve and put it in. I bet it last another 5-10 years. In fact, if all you do is drain the system, think about filling up with anti-freeze over the winter...or if you're in warm climate, you cam add an anti-rust additive to distilled water for the winter.

If you want to check the manifold for rust, pop the exhaust hose and look at the exhaust and water passages.

 

[dkjbama]

quote:

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Originally posted by jtybt15

Manifolds look awfully good for their age. I'd get a 12 ga wire cleaning brush and chuck it in a drill and clean up the hole. Then I'd get JB weld and wipe a thin coat on the threads and on a brass drain valve and put it in. I bet it last another 5-10 years.

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Well shoot. Charlie I always respected your opinion when I was a regular here, but I also respected Bill's and Hogan's as well. I am tempted to try the 12 gauge bore brush trick. That riser's going to have to come off regardless so I might as well try before I order parts. I had planned on using brass drain valves in the new manifolds anyways. I've had them on the block and water pump elbow for years. I've come close to ordering the manifold drain valves many times in years past and I talked myself out of it figuring $45 was too much to pay to keep from having to remove two plugs. Sounds pretty cheap in hindsight.

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In fact, if all you do is drain the system, think about filling up with anti-freeze over the winter...or if you're in warm climate, you cam add an anti-rust additive to distilled water for the winter.

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I know I should probably fill it with antifreeze, I just never have and frankly end up lazy at the end of the season. Change the oil, fill it with gas and stabil, drain it, and put it up in storage.

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If you want to check the manifold for rust, pop the exhaust hose and look at the exhaust and water passages.

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Easier said than done. The hose run is very short. There's no way the get the hose off without pulling the risers. When I did have the risers off 5 or so years ago, they looked fine insidee with only light surface rust. That's why I left them in service despite already having new risers to install. I'll have to pull a riser anyway to get the hose out of the way to see if the hole can be cleaned up. I'll take a look into the water passages of the manifold and see what they look like at the same time.

These manifold and riser leaks that destroy engines: are they exclusively at the manifold riser junction or do I need to be worried about the exhaust runners themselves rusting through?

 

[jtybt15]

'exhaust runners'? Are you talking about the exhaust passages in the manifolds? Exhaust passages seem to be surprisingly immune to rust. Remember the old cast iron exhaust in cars? They'd mostly rust from the outside. The susceptible areas are where water and air can get to them...like the exhaust/riser joint if they leak.

The other way water gets into the engine is by way of 'water reversion'. Water gets sucked back into the risers and down the manifolds during low RPM/idle operation. There's supposed to be a film of it happening. Merc went so far as to make a SS manifold/riser joint gasket that extended into the exhaust and turned up to catch condensation and water sucking back.

Mostly, stay aware of steam coming out of the exhaust. Mostly rough running at start-up that clears up as it warms up.

Replacement is always the safe way to go and I'd be interested in what you find in the manifolds.

 

[dkjbama]

Yes Charlie, what I was referring to as the exhaust runners are what you call the exhaust passages.

Well so much for that. It was worth a shot though. The depressing part is the water passages of the manifolds and risers look every bit as good as they did last time I had them apart; only light surface rust. Again I apologize for the quality of the pictures.

Manifold:

Riser:

Threads after cleaning with Kano Kroil and a couple shotgun bore brushes chucked in a drill. No luck here: both the old and the new plug behave the same way. Kind of thread in until they meet resistance, then a little more and they pop out. Threads are Borked.

Unfortunately, it really doesn't look like there's room to get a tap in there far enough to even chase the threads, much less cut new ones in a bigger size. These two pics show the depth that i have to work with from the outside of the manifold. The first is for reference so you can see the length of the bore brush (or what's left of it), a little over an inch. The second illustrates how far the brush will go into the hole before hitting the exhaust passage.

If I want to be able to work on it this weekend, I need to get my parts order in tonight. I've pretty much given up on the starboard manifold, so the question is one new manifold or two. The extra cost for the second manifold will be $180 for the manifold and $13 worth of gaskets. That is unless somebody has any last second bright ideas that we haven't thought of yet.

Thanks again gentlemen for your advice and assistance. It is very much appreciated.

 

[alk]

Are the new manifolds of the same quality as what you have on there now? I know I had a Chrysler 440 a few years back, decided to replace the original manifolds, around their 24th bday. The replacements were not nearly the quality, I'm guessing they didn't make it half that long. My suggestion is that if you can't find equal replacements, leave the good one be. Pull the bad one off, epoxy ( jB weld) or weld a plug in place, and keep running. If you can find new ones you like, especially for 180 bucks a piece, order them tonight and be done with it.

 

[dkjbama]

quote:

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Originally posted by alk

Are the new manifolds of the same quality as what you have on there now? I know I had a Chrysler 440 a few years back, decided to replace the original manifolds, around their 24th bday. The replacements were not nearly the quality, I'm guessing they didn't make it half that long. My suggestion is that if you can't find equal replacements, leave the good one be. Pull the bad one off, epoxy ( jB weld) or weld a plug in place, and keep running.

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The replacements are from PCM and appear exactly the same in the photo on the website of the place I get them from. I already have a new replacement set of PCM risers that I've mentioned before in this thread and they are of equal quality to the ones on the boat now. PCM sold marinized Ford 302s and 351s for a LONG time and parts availability is excellent.