1940 V8 Helmet Distributor Rotor to Inner Cap Gap

dorvedahl47pu

dorvedahl47pu

Senior Member
Hello out there. Does anyone know what the gap between the rotor posts and the inner cap posts is supposed to be for a 1940 Ford V8 with helmet distributor? Thanks in advance.
 
There must be a Ford spec, but I can’t answer the question. I have heard of gaps way too large, and non-existent gaps that self destruct. After-market caps, due to those defects, should be checked by hand turning the rotor. It has been suggested that a large gap can be closed with solder, the passing of the rotor then cutting a new path through the new solder. I cannot confirm or deny, never having used aftermarket caps.
 
The gap I go for is somewhere between just clearing to notably smaller than the spark plug gap. I check the rotor to inner cap gaps whenever I am assembling a distributor, or swapping a distributor for someone else. With the coil off the distributor and the distributor on the bench you can look down inside the distributor and see the gap. A small mirror on a stick helps. I find I need to file down at least one post on an inner cap almost every time I assemble a new set of parts. One memorable time I had to file down one of the rotor posts since it hit all four of its inner cap posts. I filed that rotor post down until it cleared the shortest inner cap post, then filed the remaining three inner cap posts to match. Not difficult, but it sure is tedious.
 
Spec can be found in the Service Bulletins for April 18, 1948 the 68-12201 rotor gap is max 0.0125" min 0.0065"
 
Thanks for all the input. I guess I'll be taking my distributor off again and fixing my large gap. I just had a distributor rebuilt and bought NOS inner caps from the guy who rebuilt the distributor. Gap was way bigger than spec...probably worse than it was with the old inner Chinese caps. Oh well...I tried. LOL
 
dorvedahl47pu said:
Thanks for all the input. I guess I'll be taking my distributor off again and fixing my large gap. I just had a distributor rebuilt and bought NOS inner caps from the guy who rebuilt the distributor. Gap was way bigger than spec...probably worse than it was with the old inner Chinese caps. Oh well...I tried. LOL
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How is the rotor?
 
Great question. I'm not sure to be honest. I was told it was "nearly new" by the guy who rebuilt it. I will check....
 
dorvedahl47pu said:
Thanks for all the input. I guess I'll be taking my distributor off again and fixing my large gap. I just had a distributor rebuilt and bought NOS inner caps from the guy who rebuilt the distributor. Gap was way bigger than spec...probably worse than it was with the old inner Chinese caps. Oh well...I tried. LOL
Click to expand...

What is "way bigger"?
Even up to .020", the engine should fire quite well if you have a good coil.
 
The larger the gaps the higher the firing voltage will have to be — a large rotor gap would help with firing fouled plugs but also stresses the insulation more.
In an old radio installation book it said no more than.005 to reduce interference.
 
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