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Old 02-14-2014, 07:11 PM   #1
Walt Dupont--Me.
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Default crab- rotor clearance to-cap termanal

I've had a few of those crab rotors that would hit the terminals inside the cap. Trying to figure out how you can see through the cap and check the clearance. This stuff you bye today is real junk. Walt
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Old 02-14-2014, 07:38 PM   #2
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Default Re: crab- rotor clearance to-cap termanal

Hi Walt

Are you using the NAPA, (Echlin) FA 65 cap & FA 64 rotor ? Also, Fred Wilner at So South Obsolete recommends the gasket, which many of us forget about. Let is know how you make out.
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Old 02-14-2014, 07:42 PM   #3
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Default Re: crab- rotor clearance to-cap termanal

Hey Joe. Im missing something here. I still cain't figure out how you can see through the cap with it on the dist to see what the gap is. Walt
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Old 02-14-2014, 08:44 PM   #4
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Default Re: crab- rotor clearance to-cap termanal

You cant Walt,i just pass a file on the rotor tip a few strokes and try turning the toung slowly by hand , if it stops turning you know its hitting. The gap there is not that critical. OMWODT. LOL ken ct.
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Old 02-14-2014, 09:10 PM   #5
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Default Re: crab- rotor clearance to-cap termanal

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Originally Posted by ken ct View Post
You cant Walt,i just pass a file on the rotor tip a few strokes and try turning the toung slowly by hand , if it stops turning you know its hitting. The gap there is not that critical. OMWODT. LOL ken ct.
Okay Ken. Educate us old folks on OMWODT. Lawson
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Old 02-14-2014, 09:58 PM   #6
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Default Re: crab- rotor clearance to-cap termanal

We actually sacrificed a few repro caps and drilled inspection holes in key areas to measure gaps for consistency - it was amazing to see the variances of rotor to contacts around the clock - will try to find the records to report the findings.

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Hey Joe. Im missing something here. I still cain't figure out how you can see through the cap with it on the dist to see what the gap is. Walt
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Old 02-14-2014, 10:09 PM   #7
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Default Re: crab- rotor clearance to-cap termanal

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We actually sacrificed a few repro caps and drilled inspection holes in key areas to measure gaps for consistency - it was amazing to see the variances of rotor to contacts around the clock - will try to find the records to report the findings.
Yes, please do that. I busted one cap already. Lawson
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Old 02-14-2014, 10:31 PM   #8
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Default Re: crab- rotor clearance to-cap termanal

On the flip side of clearances, I've read somewhere that gaps too large can be closed up by applying solder to the caps contacts, from which the excess will be cut away by the rotation of the rotor.

And yes, Ken ct, wazziit mean: OMWODT? I think you just make up these texty thingies as you go.
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Old 02-14-2014, 11:31 PM   #9
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Default Re: crab- rotor clearance to-cap termanal

I'll take a stab. OMWODT = Only My Way Of Doing That. Ken am I even close?

I had to do a similar procedure on a helmet distributor. A least the rotor was shipped with a warning about needing to file the contacts to prevent breakage.
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Old 02-15-2014, 01:56 AM   #10
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Default Re: crab- rotor clearance to-cap termanal

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Okay Ken. Educate us old folks on OMWODT. Lawson
Honestly Lawson it was supposed to be a joke.but now I don't know what some of it stands for,my short term memory is taking a beating lately,if it comes to me i'll post it for you. ken ct. LOL Yea that was it [41 ford] thanks ken ct.
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Old 02-15-2014, 05:33 AM   #11
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Default Re: crab- rotor clearance to-cap termanal

How's this, turn up a disc that is top hat shaped in profile, the od of this fits snug in the cap picking up on the bit that the distributor rim fits, with the top of the hat inside the cap. This has a centre hole that you could slip a sharpie through to mark the center point. Whip the turned piece out, and measure from this sharpie Mark out to the contacts? As I type this, I think you could do the same using dividers from the cap mounting ring to the find the center of the cap.
These are just out loud thunks, not actual ways to do it, might spark a good idea from someone else?

If we cam plot the center, and go from there, we can trim the terminals in the cap easily for clearance, which is better than shortening the rotor. If the rotor is shortened to clear the close contacts, it will get further away from the others, this will create resistance on the further away ones.
Just ideas
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Old 02-15-2014, 09:19 AM   #12
Walt Dupont--Me.
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Default Re: crab- rotor clearance to-cap termanal

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Originally Posted by russcc View Post
Hi Walt

Are you using the NAPA, (Echlin) FA 65 cap & FA 64 rotor ? Also, Fred Wilner at So South Obsolete recommends the gasket, which many of us forget about. Let is know how you make out.
Russcc. I haven't bought a cap or rotor for yr's. but people have brought dist. to me and have got there stuff from some vendor Mac's ect. I can't set them up like Bubba but always managed make them run since I was a kid. But this stuff we're getting lately is ridiculous ( boy that's a big word for me) The rotor gap never bothered me, never thought about it until we started breaking rotors. Seems so there ought be a certain clearance like .010-015 or what ever. There has got to be a way to check that. (thanks to my wife for helping me spell) as you can see she's always near me. Walt
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Old 02-15-2014, 09:51 AM   #13
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Default Re: crab- rotor clearance to-cap termanal

Ford had a Spec. for the gap between the rotor and the cap. Not only for the 21A but also the other earlier distributors. It is published in the service bulletins about 46-48. Ford did not publish the diameter of the rotor turn circle, so the gap Spec. is more to inform mechanics what the gap between the parts should look like. I do not have the Bulletin with me but from memory the gap is very tight lets say 0.005-8-10. You can view the earlier distributor gaps, to find one with gaps around 0.030, the mechanic knows there is a problem.
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Old 02-15-2014, 10:02 AM   #14
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Default Re: crab- rotor clearance to-cap termanal

Easy to check before you break something by turning dist by hand BEFORE you install. OMO. ken ct.
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Old 02-15-2014, 10:48 AM   #15
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Default Re: crab- rotor clearance to-cap termanal

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Originally Posted by ken ct View Post
Easy to check before you break something by turning dist by hand BEFORE you install. OMO. ken ct.
Soooooooo......are you saying that if a rotor CLEARS a terminal by a theoretical .001" when turned by hand....that that rotor CAN'T grow a few THOUSANDTHS when it gets hot? DD
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Old 02-15-2014, 01:44 PM   #16
Walt Dupont--Me.
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Default Re: crab- rotor clearance to-cap termanal

What if it's got .030 or .040 clearance, how do you know? Walt
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Old 02-15-2014, 02:06 PM   #17
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Default Re: crab- rotor clearance to-cap termanal

With the 21A distributor you don't but with the earlier the gap can be viewed from the coil hole.
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Old 02-15-2014, 02:30 PM   #18
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Default Re: crab- rotor clearance to-cap termanal

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Originally Posted by Walt Dupont--Me. View Post
What if it's got .030 or .040 clearance, how do you know? Walt
Hey Walt,
I'm sure that everyone has considered this, but worth saying anyhow,eh.
IF the dizzy bushing/shaft clearance is moving/bad.. clearance is bound to be off..depending on how bad/much bush to shaft is..
That said, I run .025 to .026 gap in my four cylinder dizzy (with new bush), and always have, without any problem, i.e.- missing, arcing.
Of course, on my type setup, I can take cap off and make precise measurements (after putting on CVS .99c eyes).
If I couldn't see , as with crab/helmet, I'd set gap with unit off of engine by placing some matter (clay/wax/solder/whatever)on ends of rotor OR cap , spin it slowly...and keep at it until satisfied. And, yeah , I'd use .025 to .030 and be consistent at whatever gap I pick.
Note: obviously the points of contact MUST be clean to help prevent arcing.

Last edited by hardtimes; 02-15-2014 at 02:32 PM. Reason: speil chekc
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Old 02-15-2014, 02:51 PM   #19
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Default Re: crab- rotor clearance to-cap termanal

Since the inside clearance on the crab cap is based on diameter or radius from the center, a person could machine themselves a go/no-go tool that would just be a gauge to slide down inside the cap for crab types and see if it goes in straight into the recess where the cap fit the distributor housing with a minimum radius to fit between the electrodes on one end then the other end would be maximum radius. If minimum goes in smoothly and maximum doesn't then a person would know if it is in spec or not. If both go then it is either worn out or too much clearance from the get go. If neither go in, a person could use prussian blue or something to see if they can mark the electrodes. A person could make something up on a lathe with a bit of work. I wouldn't be surprised if the manufactureres didn't use such a tool to check them every so often. If they don't now, then they should start to. It's a pain to get new parts that are not made worth a crap and it seems the ignition parts are the worst these days.

Last edited by rotorwrench; 02-15-2014 at 03:00 PM.
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Old 02-15-2014, 03:21 PM   #20
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Default Re: crab- rotor clearance to-cap termanal

I use napa cap and rotor and they are the best out there. However i belt sand some material off each rotor.
rotor gap should be less than .025 , a little extra is very easiely made up with coil output, in this case we are not concerned with radio emissions etc......
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