The Ford Barn

gwistrup · 03-17-2023, 01:29 PM

I purchased a 1940 Tudor sedan a few months ago and in the process of sorting it out, I found that the ignition coil resistor has been eliminated. Both wires are connected together on the same terminal on the resistor under the dash. I also found that the car does not have its original engine in it. The casting number on the heads is 81T and the number stamped on the rear of the head is 81T-0050. I believe this is a Mercury or truck engine? The intake manifold boss in between the heads is raised about 1/16". Does that also mean the engine is newer than 1940? The engine has a Crab style distributor on it with an external coil located on the left cylinder head. I have two questions: Should the crab style distributor still use a resistor and what is the resistance of the resistor under the dash so I can see if it is any good? Thank you, Gil

koates · 03-17-2023, 06:53 PM

I guess you still have a 6 volt system and the ignition coil is the original bakelite FORD type which runs on about 4.5 volts through the ignition resistor. The resistance of the resistor should be about .4 ohms COLD and could be about .6 ohms when HOT. If you have a non genuine round canister coil fitted then you need to measure the primary winding resistance in ohms. The combined resistance of the coil primary winding and the ignition resistor should be approx 1.5 ohms only on an original FORD bakelite type coil. Current draw in the ORIGINAL FORD coil primary circuit with the engine stopped (dist timing points closed) is approx 4 to 5 amps and with engine running is approx 2 amps depending on engine RPM. You can see how important it is to have the correct ignition coil. Too much current flowing in the coil primary circuit will reduce the life of the distributor points. Regards, Kevin.

Kube · 03-17-2023, 08:31 PM

You have a '41 or later block. The so called "raised deck" blocks were built beginning August 1940; too late to have been installed in a '40 upon an assembly line. You really can't count on the heads to identify the engine. Lord knows, they could have been changed one or more times.
Pulling a head would go a long way to identify what you really have.
With that crab type distributor and NO adapter plate, it's most likely '42 - '48. But still, the cam could have been changed during the past.
Are there any casting numbers on the bell housing?

JayChicago · 03-17-2023, 09:58 PM

Quote:

Originally Posted by gwistrup

with an external coil located on the left cylinder head.

Chances are that replacement coil is why the resistor was by-passed. If the aftermarket can type coil is about 1.5 ohms in itself, it doesn't need the resistor to limit current.

gwistrup · 03-18-2023, 09:06 AM

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You are correct that I still have a 6 volt system. Thanks for the information about the primary coil resistance. I will look for casting numbers on the bell housing when I remove the floor wood. Can the original diver's helmet distributor and coil be installed on this block to make it look more original?

03-17-2023, 01:29 PM	#1
gwistrup Junior Member Join Date: Jan 2023 Posts: 25	Ignition coil resistor on Mercury or Truck engine? I purchased a 1940 Tudor sedan a few months ago and in the process of sorting it out, I found that the ignition coil resistor has been eliminated. Both wires are connected together on the same terminal on the resistor under the dash. I also found that the car does not have its original engine in it. The casting number on the heads is 81T and the number stamped on the rear of the head is 81T-0050. I believe this is a Mercury or truck engine? The intake manifold boss in between the heads is raised about 1/16". Does that also mean the engine is newer than 1940? The engine has a Crab style distributor on it with an external coil located on the left cylinder head. I have two questions: Should the crab style distributor still use a resistor and what is the resistance of the resistor under the dash so I can see if it is any good? Thank you, Gil

03-17-2023, 06:53 PM	#2
koates Senior Member Join Date: May 2010 Location: Melbourne Australia. Posts: 2,079	Re: Ignition coil resistor on Mercury or Truck engine? I guess you still have a 6 volt system and the ignition coil is the original bakelite FORD type which runs on about 4.5 volts through the ignition resistor. The resistance of the resistor should be about .4 ohms COLD and could be about .6 ohms when HOT. If you have a non genuine round canister coil fitted then you need to measure the primary winding resistance in ohms. The combined resistance of the coil primary winding and the ignition resistor should be approx 1.5 ohms only on an original FORD bakelite type coil. Current draw in the ORIGINAL FORD coil primary circuit with the engine stopped (dist timing points closed) is approx 4 to 5 amps and with engine running is approx 2 amps depending on engine RPM. You can see how important it is to have the correct ignition coil. Too much current flowing in the coil primary circuit will reduce the life of the distributor points. Regards, Kevin.

03-17-2023, 08:31 PM	#3
Kube Senior Member Join Date: May 2010 Location: Wisconsin, USA Posts: 9,008	Re: Ignition coil resistor on Mercury or Truck engine? You have a '41 or later block. The so called "raised deck" blocks were built beginning August 1940; too late to have been installed in a '40 upon an assembly line. You really can't count on the heads to identify the engine. Lord knows, they could have been changed one or more times. Pulling a head would go a long way to identify what you really have. With that crab type distributor and NO adapter plate, it's most likely '42 - '48. But still, the cam could have been changed during the past. Are there any casting numbers on the bell housing? __________________ "I can explain it for you. However, I can't understand it for you".

03-18-2023, 09:06 AM	#5
gwistrup Junior Member Join Date: Jan 2023 Posts: 25	Re: Ignition coil resistor on Mercury or Truck engine? Sponsored Links (Register now to hide all advertisements) You are correct that I still have a 6 volt system. Thanks for the information about the primary coil resistance. I will look for casting numbers on the bell housing when I remove the floor wood. Can the original diver's helmet distributor and coil be installed on this block to make it look more original?