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Need help with heater hose fitting in alum. manifold - '67 L78 Engine Project

L78fanatic

Well-Known Member
Senior Member
I'm attempting to tap my aftermarket aluminum intake manifold so the fitting for the heater hose will thread all the way into the manifold. The fitting for the heater hose as well as the temperature sending unit (in the top of the manifold) are suppose to be 1/2"-14 NPT as far as I know. Both threaded holes easily accept the heater hose fitting as well as the sending unit, however "in both cases" they will not thread all the way in, leaving too much thread area exposed to my liking.

So, I started today with the threaded hole for the heater hose fitting, and I'm tapping my way right along, little by little, but I don't think it looks right yet. It's difficult to tap this hole because I have to use a hand wrench, as the normal tapping T-handle will not maneuver this close to the aluminum manifold. The fitting in my pic is definitely further threaded in from where it was before I started tapping, so you can imagine how bad it looked.

I thought I'd stop and take a couple of pics for you all to see, and offer any advice you might have to make this easier. I suspect tapping the sending unit fitting will be easier to tap, but I haven't done it yet. I'm wondering if the manufacturer didn't use NPT threads but straight 1/2" threads?

You can see in the 2nd pic (if you zoom in) how much room is left from the end of the fitting to the inside face of the manifold....tapping it all the way in should make the fitting about flush with the face of the inside water channel.

Any thoughts appreciated,

John

IMG_0601.jpgIMG_0604.jpg
 
Go slow. I use a cutting paste. It helps for cutting, lubing and retaining most of the metal chunks. Some forget to use an NPT thread tap as its tapered and end up using a standard tap and dye set. When trying to match thread length to take up space like you, Ive either cut the fittings threads back or do like you are. The reason I say go slow is to not take out to much material. If the fitting is longer than the depth or thickness of the metal, once you break past that thin taper at the end of the fitting and it starts entering in to the cavity, the thicker part of the fitting taper by the hex MAY get pretty loose and if to much material is already out, you wont be able to get tit tight. I also dont go all the way down to the hex. I leave about an 1/8" of space.

One of the other reasons sometimes I choose to take the treads off the actual fitting end in an area like this is, i don't want the fitting to far in the water cavity to disrupt water flow.

Looks like its going great.
 
Thank you for that! I think I'll use my tap just a bit more (I use tapping oil) to see if I can get the fitting to show no more than 2 threads, and I can live with that. I know I don't have to use thread sealant but I like using Permatex Thread Sealant (I used that on my water pump fittings). I learned on those fittings: I bought a date correct rebuilt short water pump but the threads were not very good. It came out very nice.

I suppose I can try the same thing on the temp sending unit....that part submerges into the flow anyway, so I wouldn't think going a bit deeper would be hurt anything. As it is, only about 2-1/2 threads go into the threaded hole. If it was an old GM part I likely wouldn't have these issues, but overall this repop intake isn't too bad.
 
Just to follow up, I've been doing some research on my parts gathered for my engine build, and wouldn't you know it, the fitting shown in my photos is the wrong one! It's the fitting for the 3/4" heater hose; I had this fitting left over, because I didn't want to use it on the water pump (I used the extended style). Plus, the correct fitting is the one I have for the 5/8" hose, and it screws into the threads much better....go figure! I may still use the tap a bit more so the fitting will be flush with the inside chamber, but with a bit of effort it's almost there now! Glad I checked! DUH!
 
Great catch ! I have made that mistake. Not a real costly one these days with so many available fittings, but still, getting the correct parts is so worth it.
 
Just to follow up, I've been doing some research on my parts gathered for my engine build, and wouldn't you know it, the fitting shown in my photos is the wrong one! It's the fitting for the 3/4" heater hose; I had this fitting left over, because I didn't want to use it on the water pump (I used the extended style). Plus, the correct fitting is the one I have for the 5/8" hose, and it screws into the threads much better....go figure! I may still use the tap a bit more so the fitting will be flush with the inside chamber, but with a bit of effort it's almost there now! Glad I checked! DUH!
Curious if the 3/4 would cool better since the hole may be larger.
 
Curious if the 3/4 would cool better since the hole may be larger.
That's not a coolant line, but merely a line to the heater in the firewall. The stock size is 5/8". The return line from the heater to the water pump is 3/4". You can see the difference if you look at engine bay photos. The bypass line from the top of the water pump to the intake manifold is also 3/4".
 
Well, I tapped both the heater hose fitting and the temp sending unit hole, both 1/2-14 NPT threads. I tapped in far enough so the threads on both the heater hose fitting and the sending unit were either "flush" or within 1 thread of the opening in the channel in the intake manifold, so I think I'll stop there. Here's what they look like fitted (I have not used thread sealer yet). Thoughts?IMG_0610.jpgIMG_0607.jpgIMG_0608.jpgIMG_0609.jpg
 
I think they came out great !

Now would also be the time to tap any vacuum fittings in the rear intake runners if you need them.

Well, funny you mention the vacuum port. I have a single port 90 degree threaded fitting in there now with a robber boot covering the horizontal vacuum port.

I got the fitting threaded in but it was tight (i.e. hard to thread in). I’d prefer to simply put a plug in there for now but I’m not certain what thread size to use. It’s smaller than the 1/2”-14 NPT threads in the front.

Do you know what size threads are in that hole?
 
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