Zenith jet drill bits.

[700rpm]

Where can I find a chart that identifies the drill bit numbers that coincide with Model A Zenith-1 carburetor jets?

 

[dave in australia]

Years ago, Dykes printed a book on carburetors, covering pretty much all carburtors through the 20's and 30's. In it, for the zenith, was jet opening sizes in thous, and venturi diameters etc. Once you have he size for each jet, find he corresponding drill size in number or letter. I have a copy of the book, and can try and dig it out from storage and copy the pages on zeniths for you.

 

[Thodge]

I found the information here very helpful. Doesn’t list the actual drill bit number in all cases but does list orifice sizes and the formula for calculating Zenith jet sizes by number stamping.





https://www.fordgarage.com/pages/jetflowrates.htm

 

[Gary WA]

Where can I find a chart that identifies the drill bit numbers that coincide with Model A Zenith-1 carburetor jets?

here you go

 

[nkaminar]

The drill index will have the letter size or number size of the drill bits, but don't just take that as fact. Measure the shank diameter with a micrometer.

If you solder up and drill out the main jet to reduce it's size, remember to use a larger drill to dish the top of the jet. That will keep it from dribbling gasoline.

You can change the jet sizes to suit by drilling they out or soldering them up and re drilling them smaller. If you do this, keep an unmodified set of jets to fall back on.

 

[Chuck Sea/Tac]

The drill index will have the letter size or number size of the drill bits, but don't just take that as fact. Measure the shank diameter with a micrometer.

I agree 125%. They’re all over the place. I always mic to make sure it’s just a few thou bigger, when trying to get a little more flow.

 

[Bruce of MN]

How finicky do you have to be about hole roundness and precision? Do you need to drill undersize and use a reamer? An .035" reamer from McMaster Carr is $23 plus shipping.

 

[Gary WA]

How finicky do you have to be about hole roundness and precision? Do you need to drill undersize and use a reamer? An .035" reamer from McMaster Carr is $23 plus shipping.

No reamer

 

[Smokedtires]

I bought micro drill bits from a local hobby shop then used micrometers to verify drill bit size.

 

[Bob Bidonde]

In my opinion, the cost of the drills is better spent buying a flow tested set of jets.
No drill produces a hole that exactly matches its diameter. There is always a tolerance. A rule of thumb for a skilled technician / machinist is to expect a drill to produce a hole 0.003" to 0.005" over the drill's diameter. For more precision, drill under-size and then ream a hole to its final diameter. By reaming, it is reasonable to expect a final hole diameter tolerance of 0.0005" to 0.003". In the case of drilling carburetor jets, drill by hand using a pin vise.

Another consideration is that once drilled to a predetermined hole (orifice) size, the jets may not produce the necessary flow rates. Zenith recommended flow testing jets and adjusting the jet orifice sizes to achieve their recommended flow rates.

 

[Jim/GA]

The original Ford blueprints for the Main Jet and the Cap Jet call for a #60 drill and then quote diameter range of 0.0394-0.0409. They also say to flow test Cap Jet to confirm 180-190 cc/min of water with 37.25"+/-0.0625" head above jet seat, and 165-169 cc/min on the Main Jet. Both jets were to be stamped with the number "20" to indicate the hole size at the tip.

 

[johnbuckley]

whoops, sorry wrong post!

 

[rotorwrench]

Jet reamers are available but the sets are a bit on the expensive side. Manufactured flow jets are reamed. A drill is just to get it close to size.