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Stripped / stretched vacuum thread on DCOE install?


sam1904
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Hi all.... me again,
 
While installing my DCOE’s with Halboyles linkage and a Korman manifold I think I have made a (potentially large) error...
 
When I installed the brass pipe thread vacuum fitting off of #4 I think I over tightened it when I removed it once and reinstalled. Teflon tape may have sealed my fate? Now when I tighten I can essentially bottom it out with just ‘finger torque’....
 
Then when I fired the car up for the first time the rear two cylinders where pulling 1/2 as much air on my squirrel cage and it sounded like a very strong misfire. 
 
Combining these two facts I am concerned that I have a large vacuum leak around the brass fitting and hence on the rear carb as I have stretched the aluminum threads??? Sound feasible??
 
When I photograph the threads they still have form so I am hoping they are not stripped?
 
I was thinking of adding some permeatex and reinstalling before pulling the whole thing off again?
 
Any other suggestions?
 
Thanks
Sam

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You could try using propane or map gas to check for a vacuum leak, while the engine is running use an unlit torch to put gas around the fitting and if the idle speed increases you have a vacuum leak. But it also could be a bad oneway valve sucking air from the brake booster, so you could pinch the hose off or cap the fitting to rule that out. If it is leaking vacuum you could re-thread it to a bigger size or use a helicoil.

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I have to say, I just installed the IE manifold w/ my DCOE 40s and was very puzzled by the way this fitting didn't want to fit very well. I was worried it was going to bottom out and not seal. My solution was a 2" long 1/4 NPT male-male extension into a T. I was able to thread it in until it seated quite firmly. I actually emailed IE who initially told me the fitting was 1/4 NPT, then emailed back saying it was actually 3/8 NPT. It is in fact 1/4 NPT (I think). I'm wondering if there were some issues w/ tapping @ IE. I also preferred the way the T allowed me to point the vacuum line at the brake booster versus straight back at the firewall.

 

Good luck. Let us know how things turn out.

 

 

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Edited by man_mark_7
fixed typo
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Thanks. This is actually a Korman manifold and I am pretty sure I have the right thread as it engages well. Unfortunately the more I messed with it and reinstalled it the deeper it went which is why I think I may have exerted too much force and the taper has essentially stretched the aluminum thread. I am assuming this is the case and not the fact that the male threads have become diminished?

 

I also had the issue of meeting making the turn to the booster so I assembled a plastic elbow, I wonder if this is potentially not another vacuum leak sauce and I may see if I can source of brass variant. 
 

I do like your idea of just assembling a T and will try tomorrow, hopefully I also find something that has a slightly larger male taper and as such also helps solve the problem!

 

 

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45 minutes ago, sam1904 said:

Thanks. This is actually a Korman manifold and I am pretty sure I have the right thread as it engages well. Unfortunately the more I messed with it and reinstalled it the deeper it went which is why I think I may have exerted too much force and the taper has essentially stretched the aluminum thread. I am assuming this is the case and not the fact that the male threads have become diminished?

 

I also had the issue of meeting making the turn to the booster so I assembled a plastic elbow, I wonder if this is potentially not another vacuum leak sauce and I may see if I can source of brass variant. 
 

I do like your idea of just assembling a T and will try tomorrow, hopefully I also find something that has a slightly larger male taper and as such also helps solve the problem!

 

 

Oh right - you said Korman in the first line of your post if I had paid attention. They look remarkably similar. On the IE manifold the ports don't connect with each other. I can't imagine that they would connect on the Korman either so i don't know why both of your rear cylinders would be pulling less. If the vacuum leak is from the brake booster line only the very rearmost cylinder should be showing less vacuum.

 

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  • 2 weeks later...
12 hours ago, NickVyse said:

 

yep

I assume the "weld" in jbweld means you'll never get that sucker out if needed?

I thought you said it was just #4 running lean.

Did you sort out the thread size issue?

I'd want to be sure that was my problem. I still think the carb to manifold looks odd. I'm pretty sure my gaskets are thinner.

 I'd do the propane test around each of  those likely leak areas.

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5 hours ago, Hans said:

I assume the "weld" in jbweld means you'll never get that sucker out if needed?

I thought you said it was just #4 running lean.

Did you sort out the thread size issue?

I'd want to be sure that was my problem. I still think the carb to manifold looks odd. I'm pretty sure my gaskets are thinner.

 I'd do the propane test around each of  those likely leak areas.

 

Thanks, Some thoughts.

 

I did find an interference (with help!) where the cold start casting was hitting the manifold and this was preventing the rear carb seating fully on the gaskets, and especially #4. After removing the interference the flow rates through 3 and 4 came up but 4 is still around 10-15% lower.

 

I have sprayed starter fluid at any possible leak sources and there is a (minor) change in tone, with that said all of those leak sources are also within 12” of the open stacks so the effect could just be the starter fluid getting sucked in that way…

 

I am still concerned about the threaded connection from the ¼ pipe nipple into the rear of the manifold. It is absolutely a ¼” Pipe thread. The issue is that as with every tapered pipe thread, especially those where the female thread is a softer material (Aluminum vs. brass in this case) there is the opportunity to ‘stretch the thread’ through either repeated installations or in my case over tightening. In this case when first installed the nipple would seat with 1-2 wraps of Teflon with 2-3 threads still proud of the casting. At this point the same nipple (and another the same) will thread fully (e.g. 3-4 threads deeper) into the female thread even with far more Teflon tape. As such the average OD of the female taper is now larger allowing the tapered male thread to install more deeply. It also feels far looser and has more “angular” play during installation…

 

You are correct that JB-Weld would be a permanent installation. However at this point it is a little bit of a moot point. If there is thread damage (which there is – even if not leaking) the manifold is at a point where if this fitting comes out again something different is going to have to be done… JB Weld would be the last option I feel…

 

Before I go any further I have a 4 channel manometer arriving this week and am going to take a more detailed look at vacuum over each butterfly as my 40DCOE151 DOES have vacuum take offs… (Carbtune Pro Xmas present from my in laws! – always marry the daughter of a mechanic 😊 )

 

Will report back!

 

Sam

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