Cap and Comp Jet Helicoil repair

TinCup

TinCup

Well-known member
Both of these are 5mm 75 pitch threads. Checked local auto parts and no luck there. Trying to repair Zenith 3 bottom that I found in a box at an antique store. All I can find is M5 .08.
 
Are you sure it is 5mm X 0.75 pitch? That is an odd thread. Metric coarse is 5 X 0.8 mm and metric fine is 5 X 0.5mm. A pitch of 0.75mm is very close to 34 tpi, again odd!
 
Synchro909 said:
Are you sure it is 5mm X 0.75 pitch? That is an odd thread. Metric coarse is 5 X 0.8 mm and metric fine is 5 X 0.5mm. A pitch of 0.75mm is very close to 34 tpi, again odd!
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Yes Zenith French Made!
 

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TinCup said:
Both of these are 5mm 75 pitch threads. Checked local auto parts and no luck there. Trying to repair Zenith 3 bottom that I found in a box at an antique store. All I can find is M5 .08.
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Sent email may be able to help you out,
 
https://www.snydersantiqueauto.com/...ory=Products/model-a/carburetor/gaskets-parts


https://en.wikipedia.org/wiki/Zenith_Carburetor_Company
They are an American Co. The 2 jet design used was developed by the guy in the above posting and with more research would probable find that the two jet design was used under license, payed to the designer in France, so used some metric parts. The carburetors were made in the United States not France and used the same parts that Holley and Ford used in their carburetors.
 
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Hmmm, Made in France - strange threads! I wonder what threads I will find in a Japanese copy I have. I'd like to get it going and I've asked before if anybody know of them and did Zenith parts fit. I received no responses so I guess I'm on my own. It looks to my (non expert) eye that it was a double venturi type. Do they even make the uniquely double venturi parts for those now?
BTW, it is branded KTK.
 
I use the tap all the time to chase threads I have used it for years. The challenge is the threads are worn away and even cleaning up what's there isn't enough to hold the jets.
All I can find is a 5mm .08 helicoil and not .75.
 
What about making some thin wall inserts using US thread but drilled and tapped for the 5m.75 thread.
 
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Finally heard back for Steve at Bert's at as I was afraid they did not have a resource for the 5mm .75 helicoil for the repair. Because there is no way access the passage for the secondary well you cannot simply fill and retap. I had another old Zenith bottom that I tested using a 5mm .8 helicoil and it worked. It holds the jet and seals with no leak. Tends to chew up the threads a bit on the jet but is much better solution than JB Weld.
 
The secondary well is a different size then the 5M.75 jets. So you need more then just the jets thread repaired to refurb this carb?
 
redmodelt said:
The secondary well is a different size then the 5M.75 jets. So you need more then just the jets thread repaired to refurb this carb?
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The secondary well threads are not relevant here. Only the passage to the comp jet which cannot be reached if one was to fill the area with brass and cut new 5 mm threads.
 
Take the jets to a plating shop and have a copper or nickel flash put on the fittings. This will easily add .0002" to the surface of the threads. Make sure you put wax in the orifice of the fitting otherwise it will get plugged.
 
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