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The starboard motor on my '69 FBC w/1978 454s has an exhaust leak where the outside exhaust manifold bolts onto the head. Since it has to come off, and they're old, I'm going to replace the manifolds/risers on both sides with new.

She has the log-style manifolds now. Any advantage/disadvantage to going with the newer center riser style? Will they bolt right up? Will they fit in the box OK? Any particular brand better than another?

I'm asking because the center-style are easier to find and look like they'll be cheaper.

Thanks-

John F.

Hey John
Attached here are pics of my 454's in a '66 Express. The engine boxes closed but as I remember it was close.

I don't know much about differences in the engine boxes of earlier or later model 31.....but they worked good for me. By the way....I did not install these manifolds....they were on the boat when I purchased her.





I have always had success with OSCO products.

john,
Pay real close attention to the exhaust port on the head when you remove the manifold and gasket. It should be flat and smooth.

I haven't seen a problem with center risers over aft in box fit. There is usually lots of room above the carb.

They will fit but the exhaust hoses will come up short and need replacing.

Bruce/Harry-

Thanks for the replies. I'll take a look at the gasket surface. I thought someone here had commented that the center-risers were too wide for the box. The log-styles go right up to the inside of the box--outboard I have plenty of room.

John

Be sure to use Mercruiser manifolds that don't contain any metal or they will rust out and fill the engine with raw water. I don't like the center riser style manifold as they seem to be the ones that you hear all of the horror stories about. i have log style on my chryslers with the exhaust exiting the back of the manifold at a down angle. Less chance of a leaking manifold filling a cylinder with water.

John,
She may run a little better. The center riser design gives both front and back cylinders equal distance to exit. The logs tend to load up the forward exhaust ports, and dump the aft ports...this creates uneven exhaust port pressure. Typically the center riser design is "more-better" for performance.

Although your engines are 30+ years old!!! so how much better could they be!!

Rawleigh...Correct! if the riser is not cared for, or replaced at regular 5-7 years intervals, the riser WILL leak into the exhaust ports. If they are fresh water flushed after each run they may live a little longer. Althought I had "heard" that most of the corrosion occurrs during engine operation. So I suppose it is a years + hours event. This does not apply to the manifold itself...they can live a long time, it's the riser that needs replacement.

I see you point on the design. Some of the Mercuiser elbows have a SS plate between the riser and the manifold. The plate (gasket) extends slightly into the flow path. The intent of the plate is to get hot, and "steam off" any water running down the flow path. Many of the Mercruiser Risers are SS, vs grey cast iron (what 95% of all exhaust manifolds and risers area made from)

The Mercrusier center risers will not tolerate as much cam overlap as some others. The longer PCM style risers inject the water farther down stream and can handle more cam=more power...if an engine is modified.

John wrote:I thought someone here had commented that the center-risers were too wide for the box. The log-styles go right up to the inside of the box--outboard I have plenty of room.

John, I remember the post that you are talking about....the boat that the post refered to is in California..I think.. and the problem with the manifolds interfering with the engine box was due to broken mount or bad stringer or something like that....the engine was shifted to the inboard....I may look for the post tonight....

As far as fit I think we all agree that you will be Okay.

Harry

Thanks for the advice. I've got it all apart except for the manifold. I'm just going to go back to logs--they'll bolt right up.

John, that would be mine.Who ever installed the starboard engine somehow "turned" it inboard. I just straightened it and then aligned the shaft. I did also machine the risers at an outboard angle from the machine shop and now they clear fine. You should not have a problem on yours though.
Rocky

I stayed with the logs. What about the one inch spacer between the logs and the riser? John, do you have the spacer? I was told you can't get them anymore and some body on this board said you don't need them.

I intend to replace the risers this winter and am asking around about those spacers. One machine shop said he could make them but I didn't get a price.

Rocky-

Thanks.

Dick-

I'm about to go out and do battle with the remaining manifold bolts. My B31 doesn't have the manifold to riser spacers.

John

Manifold off. One of the best things about B31s--access.

John,

Looks like the hard one is done, good luck on the other side.

BTW, don't forget to check out those elbows while you have it apart.

br,
JohnD

I had one hell of a time getting the outboard manifold off on the stbd engine when I replaced them four yrs ago. As the heads of the bolts were worn away and coundn't get a wrench on and couldn't get at it to grind the head off, I took a sawsall and cut the manifold lenght wise. The manifold took the saw blade well as there are many water ports. Then I ground off the head of those two bolts.

Now there is a trick to putting the gasket on when matching the new manifld to the block. And guess what? I can't remember. I think it's about cutting away a piece of the gasket and then dropping the gasket down between the blck and the manifold oon another block stud. Maybe some body will know.

I'll get the whole manifold/riser kit, so everything will be new. The bottom 4 bolts (1 per cylinder) came out easy with a 6-point socket. Two of the top 4 came out by using a breaker bar and these things I bought at HD that look like easy-outs for sockets. For the last 2 I had to drillthe heads off. The top bolts all have a portion of their thread exposed, and I'm guessing that's why they locked up.

To install the new manifold, I'll match the gasket with the exhaust ports, use some sort of gasket sealer (permatex 2?), and get a couple of studs to guide help guide it on. I'll also use all SS bolts. Sound OK?

Going to clean up all the surfaces today and check for the gasket surface for straightness per Bruce's instructions.

John
You have a clean setup....looks like it would be a pleasure to work on this puppy.

Is that a Carter AFB carburetor you have there.....I don't think I have ever seen the Carter Carb's on GM marine engines....I'll bet she screams......I am familiar with the Carters on the Chrysler's....a sweet setup.

On your manifold bolts.....be sure to apply liberal amounts of Never-Seze on the threads as well as the bolt shanks. A lot of times the bolt body will rust up in the thru hole of the manifold and not so much on the threads....Never-Seze will help prevent that from happening......I always use the Silver kind of Never-Seze.

Getter going.....

Harry

Harry-

Thanks for the reminder on the bolts. The carbs are 600cfm Edelbrock marines. They're a copy of the Carters from what I understand. When I got my B31, she had leaky Holley automotive carbs--they had to go. the Edelbrocks have been great. She doesn't scream anymore--kinda' loafs along at 2800/20 knts., or on one motor at 2 or 3 knts for slow-trolling.

John

i was told not to use never seze in salt water environment, supposedly it hardens, not sure what to replace it with.

It's never done that for me.