Zenith 24T-2 Air Regulating Screw

[John3528]

I am refurbishing an elderly Zenith 24T-2 carburettor, Model F1372, and the Air Regulating Screw appears to be missing any form of finely tapering tip.  A replacement from Gower and Lee looks the part but does not fit being a slightly larger diameter.  G&L confirm that what they have supplied is the listed item and they have checked that it fits all the 24T-2 carburettors they have on the shelf.  Their conclusion is that my carb is an early version and that the thread was changed subsequently.  I have since determined that my thread is 2BA.    

Might anyone have a F1372 carb on the shelf and the time to spare to check whether it has a 2BA thread?  If it has, would you be prepared to photograph your screw so that I can observe its profile and attempt to re-create a new version from a 2BA brass screw?

Any help or suggestions most gratefully received.

John.

[Ian Williams]

I have just checked two F1372 for you, both are NOT 2BA and have a profile the same as your G&L replacement. I wonder if the steel 2BA is home made and has been forced into the carb damaging the thread??

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I should have added that my experience of Zenith carbs has been that they all have metric threads

[John3528]

Thank you so much Ian for taking all that trouble.  Although it is difficult to see in the photograph, the G&L replacement is a slightly larger diameter than 2BA and so will not even start in the hole.  This said, the TPI of both screws look to be about the same and the 2BA is indeed a rather loose fit in the hole so, maybe the threads in the hole have become corroded thus 'shrinking' it.

Having written off the G&L needle, thanks to your researches Ian I shall now look at it again because it sounds as though it should fit.  I shall also research different threads to see if there is anything very close to 2BA for which I can buy a tap.

Regards and thank you again.

P.S. Interesting point too about Zenith using metric threads.

[Ian Williams]

I think you will find its an M5 thread John, personally I would clean the thread out with a tap and use the G&L needle

[John3528]

Ian, you are a genius!

The pitch of a 2BA thread is 0.81mm and of a 5mm thread is 0.80mm, which explains why the 2BA screw fitted albeit a little loosely.  I only have a 5mm taper tap but it has cleaned up the first few threads such that the G&L needle now readily enters part way.  No obvious metal swarf emerged, just some grey powder suggesting that it is just severe corrosion on the threads.  A plug tap should clean the rest of the thread and I should be in business.

Thank you again for taking the trouble to look out your carburettors and for your very helpful advice.

John.

[Ian Williams]

I am pleased that you have a good result John, its always nice to hear when a suggestion has helped.