Technical origins of "long tip" and "short tip" distributor rotors?

[Ken A.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

You need to use a stud in the manifold & an old stamped steel GM clamp from the early 60's. Do NOT use the wire style clamp-they will slip.

[Michael H.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

This is heart attack serious Joe. I am in a rebuild of the motor right now getting ready for Monterey in three weeks. What i know is that i set the total advance at 38 degrees at 4000 RPM and went out for a race and came back and it was now at 52 degrees. This happened three times before i got smart enough to put the car on the trailer and go home. Any ideas what could be happening here. The distributor is a GM L-88 magnetic trigger that drives an MSD 6 system. Never had this problem with the FI distributor which as you know is pinned at the base. I haven't solved the cable breaking problem on the FI so I am going carburetor until i figure this out.

Jerry,

Possible that the distributor/shaft is bottoming on the oil pump?

If the block is decked, the heads are shaved and the manifold is machined to compensate for the other two, plus the distributor shimmed to eliminate most clearance, often times the distributor bottoms out on the oil pump shaft instead of distributor housing to manifold.
You can check for this by installing the distributor without a gasket. The distributor housing should make contact with the manifold without a gap.

If there is a gap, you will have to machine a bit of material off the oil pump shaft.

This is a VERY common problem in race motors.

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Thank you Michael, All of the above have been done to this motor so I think you may have put your finger on the problem. I love this board.

Jerry,

Possible that the distributor/shaft is bottoming on the oil pump?

If the block is decked, the heads are shaved and the manifold is machined to compensate for the other two, plus the distributor shimmed to eliminate most clearance, often times the distributor bottoms out on the oil pump shaft instead of distributor housing to manifold.
You can check for this by installing the distributor without a gasket. The distributor housing should make contact with the manifold without a gap.

If there is a gap, you will have to machine a bit of material off the oil pump shaft.

This is a VERY common problem in race motors.

[Michael H.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Thank you Michael, All of the above have been done to this motor so I think you may have put your finger on the problem. I love this board.

If that's the case, no amount of clamping force will prevent the distributor housing from moving around.

Are you using the over the counter 263? ball bearing distributor? That's a good unit.

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Yep, 263, I just haven't used a carburetor setup in the last few years and the block and intake have both had work and I used to use an MSD distributor setup for an electric tack and dry sump, but I have a mechanical tack now for "vintagy' look so i put my old GM distributor back in. That was costly! Racing is very demanding, no mistakes allowed.

[Joe C.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

My block was decked some .050" and the aluminum heads have different config than the 461's, to the extent that I elongated all the bolt holes in my LT1 intake manifold, and added aluminum around the port floors in order to get the port rooves to line up without leaking out the port floors into the lifter valley. I did NOT, however, cut the rails on the intake, but the head config makes it sit lower into the lifter valley, according to the port misalignment (which was not present when the milled 461's were installed). In any event, the distributor does not bottom into the pump shaft, so each case will be different and yours may or may not be the same. I am NOT using a standard pump shaft, but a Moroso tool steel shaft without the plastic locking sleeve, which may change the equation even further. I have used the "correct" formed wire hold-down with no problems whatsoever, but I DO make the bolt Gorilla Tight, by habit. I still don't see how the housing got turned CCW. What did the vac advance can contact to stop it from turning further? The #8 intake runner, or the coil bracket?

[Joe R.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hi Joe,

Funny that you built a test fixture to duplicate the Sun. I was thinking along the same lines, but did not mention it as I have not thoroughly though it out yet. And besides, Gene is close by.

I would think that a heater motor would not have enough power to turn the distributor. Especially since you would have to gear it up to turn at least 3500RPMs. I saw the motor and speed control as a stumbling block for the design. For that matter, I don't know how fast a Sun machine can spin.

-Dan-

Hi Dan:

My setup with a 12V heater motor spins the distributor to about 2500 RPM, equivalent to an engine speed of 5000 RPM. Since my mechanical advance is all-in by 3500 RPM engine speed, the present fixture has not been a problem for my needs. However, your proposed experiments call for spinning the distributor to an equivalent engine speed of 7000 or more to test the limits of the spark generation. So, a different setup would be needed.

I experimented with a variety of power supplies and speed controls. With a Restorer article in mind, I used a 12V power supply and a 12V PWM speed controller, since these do not require the user to work with hazardous voltage. However, I got smoother speed control using a 120 VRMS variac followed by a rectifier/filter. That also allows the option of driving the motor with a little more than 12V to get higher speed if desired.

I'm off tomorrow on the Road Tour, but when I get back I can share more details if you want to try and build one of these.

[Dan D.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hey Joe,

Enjoy the road tour and the convention. Would be interested in seeing a pic of your fixture.

I will have to think about this some. May be a good winter project. -Dan-

[Dan D.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

This thread is going in 2 different directions with us and Jerry. Would probably be best if we start a new dedicated thread for just sparks and distributors.

-Dan-

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe i don't have the luxury of letting this go. I'm trying to get motor ready for two races in Monterey california and i'm in Colorado. Lots of money and time committed here, i need to be sure the problem is correctly analyzed. Any way to verify which direction the BASE of the distributor will move if the holdown is allowing slip?

14:1...........what are you using for a cam?
Strange that the timing went advanced, since the rotation of the shaft should drag it retarded if the hold-down clip came loose.

[Joe C.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe i don't have the luxury of letting this go. I'm trying to get motor ready for two races in Monterey california and i'm in Colorado. Lots of money and time committed here, i need to be sure the problem is correctly analyzed. Any way to verify which direction the BASE of the distributor will move if the holdown is allowing slip?

Again, I would think that since the shaft moves CW, that if the base was loose, that it would have a tendency to move CW as well, in which case, the timing would become retarded. The only way this tendency might change is if the flyweights and/or the vacuum advance mechanism had some sort of effect, which seems highly unlikely to me.

There should be no reason for it to come loose, and you should do the check that was suggested earlier. If it's bottoming, there are spacers available for this, or you can add more gaskets under the distributor.

You can use paint or chalk marks on the housing and adjacent parts to verify movement, using any kind of alignment scheme you can devise. I sometimes run a straightedge across the dist cap thru the centerline and project a point onto the valve cover or any other part of the engine nearby.

Better still, unless and until you get this fixed before the races in Monterey, you should wire the distributor in place temporarily. Use some stainless steel safety wire in much the same way bolts are locked into place. Easier still is to get some simple bailing wire and take a few turns around the tach drive leaving two pigtails long enough to twist around two fixed objects on opposite sides of the tach drive. For instance, you could fasten one end to the master cylinder or drivers side ignition wire bracket and the other end to the coil bracket.

[Terry M.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Hey Joe,

Enjoy the road tour and the convention. Would be interested in seeing a pic of your fixture.

I will have to think about this some. May be a good winter project. -Dan-

Dan,
We. didn't build this in school. It came from either SUN or Snap-on -- it is red. I'll get some photos and send them to you after convention

[George J.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Joe i don't have the luxury of letting this go. I'm trying to get motor ready for two races in Monterey california and i'm in Colorado. Lots of money and time committed here, i need to be sure the problem is correctly analyzed. Any way to verify which direction the BASE of the distributor will move if the holdown is allowing slip?

Jerry, If you only have a couple of weeks, it would not seem prudent to change from the FI unit to a carburetor. I'd just limit the rpm's to solve your cable issue. I'll be stopping by to see you in the pits, by the way. Good luck!

[Jerry G.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

George, I agree. I hate removing the unit. it is so stable and responsive. BUT i limited my RPMs at Portland race two weeks ago to 6500 RPM and it broke the cable after 1 1/2 laps. The sustained long pulls down the straights is what breaks them. i have tried everything I and folks who have more experience than me have suggested and nothing has cured problem. I'm tried of fighting it for now. i'll pick back up the battle this winter. Look forward to seeing you there. Jerry

[George J.]

Re: Technical origins of "long tip" and "short tip" distributor rotors?

Jerry, that's too bad. I wonder if the distributor moving is causing this problem? Anyway, the important thing is that you show up and run. See you there.