1985 OMC 3.8 v6 exhaust manifold questions

[Friendliest_catch]

Good afternoon !

Have a 1985 Marlin with an OMC chevy 3.8 v6 with a cracked starboard manifold. Only one crack and only on that side.

I have found plenty of replacement new manifolds for the 4.3 v6 but I can't find any in stock for the 3.8 . I understand that I may have to do a mid riser conversion but also have had little luck putting a kit together that has all the parts.

Any manufacturers make 4.3 manifolds confirmed to fit 3.8? Will the Sierra manifolds for the 4.3 fit no problem?
This is one example I have seen on ebay but I'd need the riser and associated plumbing to go with it.

MerCruiser Marine Exhaust Manifold 4.3L 262 V6 99746 1985-2002 Sierra 18-1952-1 | eBay

Includes new bolts, plugs, block to manifold gasket and manifold to riser gasket. Sierra Marine 18-1952-1. Barr Marine MC-1-99746. Note 2002 was a split year and may have used the newer dry-joint style manifolds or these manifolds.

www.ebay.com

Am I safe to pack the current crack with high temp rtv after terminating crack with a small drill bit while I wait for the new parts to come in ?

First I/O motor boat I have owned and I'm learning about some of the difficulty in finding old boat parts.

The engine runs great and has good compression between 133-140 on all cylinders. 5 inch long hairline crack from fore to aft on the bottom of the manifold . Some rust streaking . No visible leak when the boat is running from what I've seen so far.

After looking around the forum and online for a few hours and not finding an updated answer to my questions I wanted to post here. Really hope someone can point me In the right direction !

Thanks for the help

 

[hugh g]

RTV won't work as a temp fix especially on a 5 inch crack, imho.

 

[matt167]

The problem is a double whammy. You have an OMC stringer drive, built in the last year they were manufactured, AND you have a 3.8L which was used pretty much only by OMC, and also 1985 would have been the last year for it.

The 3.8L and 4.3L are very different. The 3.8L is a Buick engine, where as the 4.3L is a Chevy engine. You will spend more than the boat is worth trying to find new manifolds and risers to work with it.

Drill the crack, smear in some JB weld. paint it black, sell it with a disclaimer and buy a new boat

 

[Lou C]

1985 OMC Stern Drive 3.8 [384STMRCOM] - Parts Lookup - Crowley Marine
I believe these 3.8s were actually Chevrolets, based on the distributor being in the rear vs the 3.8 Buick which OMC also used (but a good bit earlier I think) which had the distributor in the front.
To replace those exhaust manifolds, you may have to seach on ebay, or other sites that sell obsolete parts. Judging by the spacing of the exhaust ports, it looks just like the 4.3 V6 and if that's the case, it may be able to be converted to a center riser style with the right combinatin of adapters and hoses, but it's going to be a custom job so be prepared to do a lot of research and searching. I converted my 4.3 V6 from the one piece manifolds (used right after these log units) to the two piece later style but I had to buy some Volvo Penta parts to make it work. This was a straight replacement, that I knew would work. For the 3.8 with the log style manifolds, I have not seen those V6 log style manifolds made in the aftermarket, so your best bet is to start with a set of OMC/Volvo 4.3 V6 manifolds and then figure out how to match them up to the stringer Y pipe. GLM (quality is spotty though) made a conversion kit for the Cobra one piece manifolds that just might work on this engine, I chose to use Barr marine aftermarket (better quality) and Volvo Penta OE 90* exhaust pipes and hoses (expensive but fit perfect). It won't be a cheap job either way. Make sure the boat is worth it before investing the cash.

Edit: I have seen 1985 3.8s as listed with the log manifolds, or the one piece units I had. If you have the one piece units, then the way I did it should also work for you.

 

[matt167]

1985 OMC Stern Drive 3.8 [384STMRCOM] - Parts Lookup - Crowley Marine
I believe these 3.8s were actually Chevrolets, based on the distributor being in the rear vs the 3.8 Buick which OMC also used (but a good bit earlier I think) which had the distributor in the front.
To replace those exhaust manifolds, you may have to seach on ebay, or other sites that sell obsolete parts. Judging by the spacing of the exhaust ports, it looks just like the 4.3 V6 and if that's the case, it may be able to be converted to a center riser style with the right combinatin of adapters and hoses, but it's going to be a custom job so be prepared to do a lot of research and searching. I converted my 4.3 V6 from the one piece manifolds (used right after these log units) to the two piece later style but I had to buy some Volvo Penta parts to make it work. This was a straight replacement, that I knew would work. For the 3.8 with the log style manifolds, I have not seen those V6 log style manifolds made in the aftermarket, so your best bet is to start with a set of OMC/Volvo 4.3 V6 manifolds and then figure out how to match them up to the stringer Y pipe. GLM (quality is spotty though) made a conversion kit for the Cobra one piece manifolds that just might work on this engine, I chose to use Barr marine aftermarket (better quality) and Volvo Penta OE 90* exhaust pipes and hoses (expensive but fit perfect). It won't be a cheap job either way. Make sure the boat is worth it before investing the cash.

I think your right.. I had forgot about the Chevy Odd fire V6's. Still based off the SBC but very different in some ways to the 4.3L

 

[Lou C]

The odd thing is that some catalogs show the log style for the 1985, some show the one piece "batwing" style for the 1985. If that 3.8 did come with the batwings, then any manifold for the 4.3 should fit, because the batwings were used from 1985/6-->1990 I think, then they changed to a 2 piece manifold/elbow and that was used by both OMC and Volvo, all the way till Volvo changed to cat converter exhaust.

BRP -- Parts Catalogs
if you look it up as a 1985 in the BRP catalog, which should be the most accuate, it shows the batwings. However, a 1985 boat could have a 1984 sterndrive set up in it, and that might be the log style. If you look it up as a 1984 it has the log manifolds.

Only way to know is for the OP to post up pix.

 

[Friendliest_catch]

Wow! Didn't expect so much help so quick!! Yes it is the chevy style not the Buick. Rear distributor (aft end of boat). I found that some of the rubber couplings we leaking when it was running and causing some water in the bilge...tightened em up and fixed it, but they were sort of dry and beed replacing. As far as the manifold it is two pieces. I was planning on just swapping the manifolds out for the 4.3 center riser style and buying elbows and hose runs to diy a custom fit. Looks like there are a lot of those pieces available.

Here are some photos of what she looks like and the crack in question.

 

[Friendliest_catch]

Also found that the engine hour meter is at 165. The bottom end (possibly heads too) has definitely been rebuilt and I'm guessing the owner installed it when it was done. Not too bad for an 85 ! Compression was also all 132-140 across all which was good to see.

 

[Lou C]

So from the pix yours came with the log style manifolds, which to my knowledge were not available in the aftermarket for the V6s, like they were for the V8s....
Here’s my center riser style conversion on my 4.3...Barr aftermarket manifolds and elbows and V/P 90* exhaust pipes and hoses. Measure the outside diameter of your Y pipe openings; should be 3.5”. This conversion uses 4” elbow exits and the 90* exhaust pipes adapt that to 3.5” on the end that mates with the Y pipe. The center riser style with 4” elbow exits add a bit of power because they are less restrictive than the original log or batwing style which had 3.5” exits.

 

[Lou C]

So to my knowledge both GLM and HGE make a conversion kit for the one piece batwings that were used on your engine for 1985, yours appears to be an '84, due to the log manifolds. Either will work if you want to just buy a whole kit and not have to piece things together. I'm in a salt water region so I wanted to use the best quality stuff I could, to avoid problems. Just make sure the rest of the boat is in good enough shape to make the cost worth it, either kit is 800-950 or so. Mine cost more because I used a Barr exhaust kit + had to buy the Volvo OE parts to make it all fit. Like about $1200 if I recall. But worth it to not have to worry about no longer available oddball OMC exhaust parts!

 

[Friendliest_catch]

Welp, I ordered the Sierra manifold kit that fits the 4.3 and the 3.8 , but for anyone wondering if it fits IT DOES NOT. The bolts line up great, but unless you fab up 3/4 inch spacers and install longer hydraulic motor mount bolts it will not clear the motor mount. It is a real shame because without them it would fit perfectly. I think im going to have to order another kit from Osco, glm or hge as many of you have mentioned. Mine do have two pieces as seen in the photos, so I would need both.

Couple this with the fact that I am completely redoing the rear section of my boats structure and I am having a grand time...

 

[Lou C]

some of these 2 piece manifolds for the 4.3 are thicker on the bottom, which is how the original OMC design was, but some of the aftermarket makes it thinner with more clearance.....
sierra:

OMC V6 EXHAUST MANIFOLD CONVERSION KIT REPLACES 1-PIECE MANIFOLDS 984221 984222 | eBay

Fits OMC sterndrives. V6 4.3L exhaust manifold kit. One-Piece exhaust manifolds. Two-Piece exhaust manifolds. Manifold Attaching Bolts. Two Manifolds. Replaces the No-Longer-Available. Two Exhaust Pipes.

www.ebay.com

Barr:

https://inetmarine.com/images/products/detail/image.591.png

HGE:

Marine Engine Service - Marine Cooling Systems, Manifolds, Heat Exchanger and oil coolers

Replacement of Volvo 3857656 Barr VO-1-3857656

www.manifoldwarehouse.com

see the HGE is a lot thicker on the bottom
I think the Barr is the smallest of all but hard to say without the pieces right in front of me.
On the Cobra this is not an issue because the Cobra has a conventional trim system no hydraulic engine mounts.
If you want I can measure from the center of the bolt hole to the bottom edge of the manifold on my Barr units.

 

[Friendliest_catch]

some of these 2 piece manifolds for the 4.3 are thicker on the bottom, which is how the original OMC design was, but some of the aftermarket makes it thinner with more clearance.....
sierra:

OMC V6 EXHAUST MANIFOLD CONVERSION KIT REPLACES 1-PIECE MANIFOLDS 984221 984222 | eBay

Fits OMC sterndrives. V6 4.3L exhaust manifold kit. One-Piece exhaust manifolds. Two-Piece exhaust manifolds. Manifold Attaching Bolts. Two Manifolds. Replaces the No-Longer-Available. Two Exhaust Pipes.

www.ebay.com

Barr:

https://inetmarine.com/images/products/detail/image.591.png

HGE:

Marine Engine Service - Marine Cooling Systems, Manifolds, Heat Exchanger and oil coolers

Replacement of Volvo 3857656 Barr VO-1-3857656

www.manifoldwarehouse.com

see the HGE is a lot thicker on the bottom
I think the Barr is the smallest of all but hard to say without the pieces right in front of me.
On the Cobra this is not an issue because the Cobra has a conventional trim system no hydraulic engine mounts.
If you want I can measure from the center of the bolt hole to the bottom edge of the manifold on my Barr units.

That would be awesome. I'm going to go in the garage and measure my clearances right now.

Already looking into nickel based mig wire to weld the single crack I have. Other side is in good condition. It's 110 dollars for a 2lb spool so not cheap . It is specifically made for welding cast iron and for use on high temperature exhaust systems though...


Photos below of clearances on the left side of engine..

 

[Friendliest_catch]

Purchased a mig wire specifically made for welding cast iron. Has a lot of good feedback in the miller forums and across the web. Holds up to cast iron exhaust manifolds on vehicles, so the water jacket should be no issue. Its expensive at 50 dollars a pound...but I think the best option is to try and stop drill, groove it, weld it, and paint it to see how it holds up. I also need to clean out the water jackets a bit from the scale accumulation and pick up some new gaskets.

I think next year Il invest in one of the other kits if I get some photos on here from another member that show that it will fit-I just need to move on with the project so I can enjoy this summer.

Im confident that the exhaust port is not cracked, its just that external section which is nice. Poured some acetone inside the jackets and looked for leaks in the exhaust ports and got nothing.

heres the wire. To be used with 75/25 argon c02

Crown Alloys .035" ROYAL 44-30 MIG Wire, 2# - RS44/1F

Crown Alloys Royal 44-30 2# .035 is a premium nickel alloy for MIG welding cast iron. Shop Baker's Gas and Welding now to get the best prices on the web.

bakersgas.com

 

[Lou C]

Closest I can see is mine are about 11/2" from the bottom edge to the center of the bolt head.
Those OMC original manifolds, it looks like they ground the fwd part of them to clear those hydraulic mounts!

I'm wondering if you tried the Barr manifolds but ground away just enough cast iron it might fit. They seem to be the thinnest on the bottom of all the aftermarket brands.

 

[Friendliest_catch]

Closest I can see is mine are about 11/2" from the bottom edge to the center of the bolt head.
Those OMC original manifolds, it looks like they ground the fwd part of them to clear those hydraulic mounts!

I'm wondering if you tried the Barr manifolds but ground away just enough cast iron it might fit. They seem to be the thinnest on the bottom of all the aftermarket brands.

I'm guessing you meant 11/32? I might try the barr manifolds if this weld doesn't work out like I want...I'm thinking the manifold gets to around 160 degrees which really shouldn't give it too much of an issue, but we will see.

I'm going to use a propane torch to heat the area to around 2-300 degrees before welding. The sheet I read said it isn't necessary but I think that can only help. Once its at temp I'm going to try and weld in small sections of 1.5 inches or so followed by a cooling cycle. Il use an infrared thermometer to keep the temperature around the same before continuing the weld. If it works well il post photos.

After done il wire brush the manifolds and recoat with high temp paint....what color? Maybe something with metal flake in it...something that makes the job standout!

 

[Lou C]

1 1/2”
So based on your pic even the Barrs would be to big to fit without grinding.