The Ford Barn

Russell Reay · 05-27-2025, 02:23 PM

The rotor gap is supposed to be 0.025". I have two distributor bodies and two rotors. In all combinations, the gap at the contact for plug 4 is pretty close, but all the others are much greater. The gap for contact no.1 is the greatest at 0.050"+. The engine starts and runs fine, but doesn't really purr. This is my first A, so I am not too sure what to expect. Both bodies fit squarely to the dist. with no wobble, suggesting that the shaft is not perfectly square to the base. I have never removed the distributor, but I know it has a modern plate. Is this a matter in need of correction, or would I sacrifice good in pursuit of perfection?

nkaminar · 05-27-2025, 07:08 PM

In another discussion about the rotor gap, one responder suggested filing down the contacts to obtain an equal gap. The tab on the rotor can then be bent to get a small gap. This assumes that the distributor and housings are all OK. The distributor shaft, where the cam resides, should not have any perceptible sideways movement. The housing should fit snugly on the distributor.

The car will run better with a small gap. I run about 0.015. But make sure that everything is correct so that the gap cannot close up to contact the contacts.

jb-ob · 05-27-2025, 07:32 PM

Try another rotor as some cheap repos don't have the same cam lobes.

Y-Blockhead · 05-27-2025, 10:56 PM

Quote:

Originally Posted by jb-ob

Try another rotor as some cheap repos don't have the same cam lobes.

I'm not sure I understand this. Cam lobes on the rotor? Please explain.

jb-ob · 05-28-2025, 05:32 AM

You are correct y-block.
My experience has been with distributor cam center hole drilled off center and distributor shafts machined poorly putting the cam off center. These both need to be checked before grinding & bending parts.
Thanks for the 'correction'.

nkaminar · 05-28-2025, 05:55 AM

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There are two things at play here. The rotor gap is the gap between the rotor and the "contacts" on the housing. A cam that is not drilled properly will vary the points gap at each position. I think the original poster is talking about the gap between the rotor and the "contacts." Contacts here is in quotes because there is no real contact. The spark jumps between the rotor and the protrusions on the inside of the housing.

The gap between the rotor and the "contacts" is determined by the housing and by excessive wear in the distributor shaft bearings. Not by the points cam being drilled off center.

Any gap along the path between the coil and the plugs will add some resistance to the high voltage necessary to jump the gap at the plug. Modern cars have eliminated the gap at the rotor by having individual coils for each plug.

Bob Bidonde · 05-28-2025, 06:35 AM

These slides may be of interest to you.

Big hammer · 05-28-2025, 07:36 AM

If in you would google,” spark intensifier for internal combustion engine” , you’ll find that a small gap before your spark plugs isn’t a bad thing ! We where having a hard time getting a small engine to start and stay running, someone pulled the plug wire off of the plug and left it a little loose, it started and after a few minutes it was running just fine ! Spark intensifier’s do work !

05-27-2025, 02:23 PM	#1
Russell Reay Senior Member Join Date: May 2021 Location: NH/ VT Posts: 460	Rotor gap The rotor gap is supposed to be 0.025". I have two distributor bodies and two rotors. In all combinations, the gap at the contact for plug 4 is pretty close, but all the others are much greater. The gap for contact no.1 is the greatest at 0.050"+. The engine starts and runs fine, but doesn't really purr. This is my first A, so I am not too sure what to expect. Both bodies fit squarely to the dist. with no wobble, suggesting that the shaft is not perfectly square to the base. I have never removed the distributor, but I know it has a modern plate. Is this a matter in need of correction, or would I sacrifice good in pursuit of perfection?

05-27-2025, 07:08 PM	#2
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,863	Re: Rotor gap In another discussion about the rotor gap, one responder suggested filing down the contacts to obtain an equal gap. The tab on the rotor can then be bent to get a small gap. This assumes that the distributor and housings are all OK. The distributor shaft, where the cam resides, should not have any perceptible sideways movement. The housing should fit snugly on the distributor. The car will run better with a small gap. I run about 0.015. But make sure that everything is correct so that the gap cannot close up to contact the contacts. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.

05-27-2025, 07:32 PM	#3
jb-ob Senior Member Join Date: Jun 2010 Posts: 787	Re: Rotor gap Try another rotor as some cheap repos don't have the same cam lobes.

05-28-2025, 05:32 AM	#5
jb-ob Senior Member Join Date: Jun 2010 Posts: 787	Re: Rotor gap You are correct y-block. My experience has been with distributor cam center hole drilled off center and distributor shafts machined poorly putting the cam off center. These both need to be checked before grinding & bending parts. Thanks for the 'correction'.

05-28-2025, 05:55 AM	#6
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,863	Re: Rotor gap Sponsored Links (Register now to hide all advertisements) There are two things at play here. The rotor gap is the gap between the rotor and the "contacts" on the housing. A cam that is not drilled properly will vary the points gap at each position. I think the original poster is talking about the gap between the rotor and the "contacts." Contacts here is in quotes because there is no real contact. The spark jumps between the rotor and the protrusions on the inside of the housing. The gap between the rotor and the "contacts" is determined by the housing and by excessive wear in the distributor shaft bearings. Not by the points cam being drilled off center. Any gap along the path between the coil and the plugs will add some resistance to the high voltage necessary to jump the gap at the plug. Modern cars have eliminated the gap at the rotor by having individual coils for each plug. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past. Last edited by nkaminar; 05-28-2025 at 06:04 AM.

05-28-2025, 07:36 AM	#8
Big hammer Senior Member Join Date: Jun 2014 Location: Land of Lincoln Posts: 3,430	Re: Rotor gap If in you would google,” spark intensifier for internal combustion engine” , you’ll find that a small gap before your spark plugs isn’t a bad thing ! We where having a hard time getting a small engine to start and stay running, someone pulled the plug wire off of the plug and left it a little loose, it started and after a few minutes it was running just fine ! Spark intensifier’s do work ! __________________ Don't force it with a little hammer tap, tap, tap get a bigger hammer tap done