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Exhaust Manifold Hose Barbs |
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cmacbuck69 
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Joined: June-19-2013 Location: Jax, FL Status: Offline Points: 105 |
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 Topic: Exhaust Manifold Hose BarbsPosted: August-02-2016 at 2:26pm |
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I am replacing the plastic hose barbs from the raw water system into the top of the exhaust manifolds on my 83 PCM 351. I purchased the brass barbs shown in the photo below and the male end is just a hair larger than then originals and will not screw into the manifold. The male end is 3/4" NPT and I took them to the hardware store and they only screw into a 3/4 NPT female end like 1 thread. Has anyone had this issue before and any idea of a good resource for a different style barb that has worked for you?
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TRBenj
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Joined: June-29-2005 Location: NWCT Status: Offline Points: 21184 |
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Posted: August-02-2016 at 2:41pm |
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The manifold castings are the same, the brass barbs will work (I've converted some). Clean the threads with a wire brush and then install with a wrench.
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8122pbrainard
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Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Posted: August-02-2016 at 3:31pm |
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I'd chase the 3/4 NPT threads in the manifolds. Careful to not got too deep making them oversize.
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cmacbuck69
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Joined: June-19-2013 Location: Jax, FL Status: Offline Points: 105 |
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Posted: August-03-2016 at 1:01am |
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Well, it didn't feel right cranking the barbs in there with a wrench like that. They squeeled the whole way as it milled down the brass threads with every turn, but it worked and it's water tight. Thanks!
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DayTony
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Joined: June-30-2013 Location: Salem MA Status: Offline Points: 832 |
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Posted: August-03-2016 at 2:24am |
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exactly how much were you able to thread in? If you only got a few threads in than be careful,and I would be weary of the thread that got "milled" down.
You can buy the elbow you pictured on the right in cast steel, bronze, stainless and plastic at most marine suppliers. maybe not all of them at the same place though. that style will give more clearance to slide the thick hoses onto it. I always use a sealer on those I never run anything like that dry even a dab of oil is better than nothing at all on a thread like that. I use permatex aviation gasket sealer seems to work the best for that application. get iot at napa or autozone. |
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1988 Barefoot nautique-454
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cmacbuck69
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Joined: June-19-2013 Location: Jax, FL Status: Offline Points: 105 |
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Posted: August-04-2016 at 10:51pm |
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I got about 2/3 of the threads on, but can go further if it becomes a problem. I got a very slight drip out of one of them so will probably pull it and wrap with Teflon tape or maybe try the gasket sealer...
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8122pbrainard
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Posted: August-04-2016 at 11:07pm |
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I have a feeling the brass threads got goofed up with the "milling". You even mentioned that the treads didn't feel right. Rule of thumb with pipe threads is you want to be able to tighten them by hand 2 to 2&1/2 turns before wrenching. Get the pipe tap!!!
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cmacbuck69
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Joined: June-19-2013 Location: Jax, FL Status: Offline Points: 105 |
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Posted: August-04-2016 at 11:36pm |
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I agree, but I think the fitting was too larger to begin with. I tried to screw it into a female fitting at Home Depot and it wouldn't go in. I called Skidim and epower and they both suggested the threads were correct, but I disagree. I unscrewed the first new barb that I wrenched in and the threads after the fact matched the threads of the old fitting from the first pic and it screwed back into the manifold with no problem so the threads appear to be fine. They just aren't as sharp as they were when I received them. I thought the female end in the manifold was built up with corrosion over the years, but I cleaned the female threads with a Dremel mini wire wheel and they did not appear to be corroded... It has been one of those "simple modifications" that turned into a major pain in the arse!
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Duane in Indy
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Posted: August-05-2016 at 7:20am |
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The problem you are having now is minor compared to what you may have if you keep cranking that fitting in. Obviously a tapered pipe fitting. They are like driving a wedge into a piece of wood. Eventually something has to go and it will inevitably be the casting. It will split open.
Do like Pete said and beg, borrow, or steal a pipe tap and run it into the casting and cut the threads a little deeper. Then use a good sealer on the fitting and just snug it up. It will seal without having to put undo pressure on the casting. |
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Keep it as original as YOU want it
1978 Mustang (modified) |
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8122pbrainard
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Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Posted: August-05-2016 at 7:45am |
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"buy it now" 3/4 tap on ebay
Brass should not need any type of thread sealant with good threads since it's relatively soft. IA sealant won't hurt but I prefer a paste over tape. Rectorseal Tru-Blu is great on pipe threads. Chris, You mentioned the threads "appear to be fine" and "aren't as sharp as they were". You can't tell the condition looking at them without an optical comparator! A pipe thread is a spiral grove so if the threads are distorted, the spiral grove is a passageway for leakage. To stop the leak, I feel you are back to square 1 getting new elbows and chasing the manifolds as originally suggested. Sorry. |
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DayTony
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Posted: August-05-2016 at 8:51am |
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chances are your going to unthread that to put sealant on it and inside is going to be a tiny little brass looking spring.(thats your threads that were milled off)
 i'm no plumber but i think that the only NPT threads that call for a no sealant situation is when you are talking pvc to pvc or plastic. regardless sealant is a good safety measure. I agree with everyone, get the tap and just run it in there. A little tip though, don't cut any new threads yet. just clean up whats there and try the fitment. run it in until it is just snug then pull it out and blow off the threads. I bet thats all that needs to be done. one weird thread will mess up the whole thing. |
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1988 Barefoot nautique-454
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Duane in Indy
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Posted: August-05-2016 at 9:41am |
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Without checking the fittings on a 3/4 NPT pipe gage there is no way to know if the fitting has the right pitch diameter. The same goes for the casting. Yes it is spelled "gage", I worked as a Tool and Die Maker in gages for 30+ years.
Those little China men don't always get things just right!! I use "Locktite" brand teflon pipe sealer on everything pipe. Teflon tape would work, as any that gets loose will just flush out the exhaust. Never on fuel fittings or oil fittings. Just my $.02 |
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Keep it as original as YOU want it
1978 Mustang (modified) |
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cmacbuck69 wrote: