Welded up dist slots/FBO plate?

I did a distributor about 25 years ago with welding up the slots but cant remember what side I welded up.. Is it the inside of slot or the outside of slot?? I seen on abodies that there was a thread and it had welded up the inside. By looking at pictures of the FBO plate it looks like that it limits timing by taking up the space on the outside of the slot, Make any sense? LOL Ive got the measurements for welding of the slots/timing. So what are some opinions. Inside slot, outside slot or just getting a FBO plate? THANKS

Outside. In theory it doesn't matter, though if you weld up the inside you have added advance you remove by rotating the distributor. The only issue would be vacuum cannister clearance. Example Cross Ram Manifold. Welding outside would put orientation in stock location and then limit how much mechanical adv.

i bought two of the FBO plates and the stated advance numbers on the plates are far from accurate. welding up an existing plate is a better option.

lewtot184

So I presume you welded your on the outside of slot?

outside

I do the inside to preload the springs more.

If you want the advance fast, do outside, slower do inside

on a side note there was something awhile back & the guy said the center hole on the FBO plate is too big (dont remember the rest of it)

I would weld the inside, the geometry of factory Chrysler distributors is such that the outside of the travel combined with a heavy spring helps get a very slow progressive curve, which is actually what you want (1-2 degrees / 1000 RPM).

If you're going to set it all in at 2000 or 2500 it won't really matter much though. But still will pre-load a super light spring (like the Mr Gasket/Trans Dapt/Mopar springs which are all the same) more to keep you steadier at idle.

And if you do a bad job welding and it falls out you won't end up with too much advance.

Right now these advance slots are setting at .470. This distributor came with the rebuilt engine. I can tell it was cheaply made. Whoever put the distributor together somehow managed to get both roll pins in the reluctor pin holes. LOL Was a pain getting reluctor off. It raises another question as to what hole should the roll pin be located in. the hole that is even with tooth or the hole that is off a little from the tooth. bb engine. My personal engine it is in the offset hole. a buddy of mine has 2 distributors and one is even with tooth and the other offset, Lol

I recently installed the FBO plate,very easy. How accurate? Haven't tested yet. In the past I've also welded them. How accurate is that without a distributor machine? Try doing the math to figure it out. I would say its way more inaccurate to weld. It's trial and error, weld/grind. At least with the plate you can just move it. I'm assuming FBO researched the math before it was made.
Doug

one distributor degree is .0154" of travel. very easy to verify with number drills. the FBO pates I have are using .010" of travel for each distributor degree. so, the numbers on the FBO plates I have don't match up with the mopar stuff. can you still use the plate? sure, just verify with the number drills what the travel is and having a timing tape or degrees marked on the dampner can get you where you need to be. to me the FBO plates are an unnecessary hassle compared with modifying a stock plate.

To limit the mechanical advance mechanism's travel, you weld the end of the slots that the advance arms contact when it advances. I found that chainsaw files are the perfect size to file and shape the slot with after welding. It's a bit of trial and error, but you must use a dial back timing light to see the effects your welding and filing have resulted in.

if the slots are being welded up a person needs to keep in mind that .0154" is one distributor degree. the pin on the centrifugal weight is real close to .25". so, if you want 9 distributor degrees for example all you need is a 9/64" drill bit to fit between the open section of the slot and the centrifugal weight pin. very simple, no special timing lights or anything needed. it's always best to set total timing in a performance application but by simply doing the math initial timing settings will get you pretty close to the desired total. I've been doing this for decades on mopar distributors; it's not rocket science.

Thanks Lew. Your method makes it easy.
Doug

Me too.

While choosing which side to weldup based on spring advance desired maybe a method, isn't just as easy to curve it and if you want preload, bend the spring tab or turn the offset stud on a Prestolite? What if your going from a 15 to an 8, do you really want to preload the spring that much?

I weld up the outside due to that being easier for me to Tig or Mig weld with it held in my vise
I do use both Mr. Gasket 925B springs also

welding the outside of the slot duplicates what the factory did to reduce distributor degrees. truth is either side will work but the curve will be a little slower welding the inside. the FBO plate slots are shortened on the outside.

It's always worth checking the advance curve first!
You can do it on the engine with timing tape.

Measuring the slots will usually give a close idea, but there are other variables.
In general, with 'hot' cam (ie more overlap) and same compression, the engine wants more initial timing. So shortening the inside of the slot compensates for that. Bonus of doing it that way is, the timing will stay in the right range to work with vacuum advance. The real win at the track doing that way is the engine never sees any loss of tiiming in the upper rpms due to slew rate. The extreme example of that is the DC/MP race distributors. (aka 'tach drive')

Here's an illustration of how welding effects the advance.
Its the acceptable timing for a smogged '68 set at the suggested initial. But it could be any distributor with a longer advance than will work with the initial.
The heavy black line shows what happens when the slots are shortened on the inside.
It's drawn at 12.5* because that was the recommended initial timing for non-CAP version of the same engine in 1967.

If the above distributor was on a 'hot' engine, it might be fine that the advance doesn't start until 800 or 900 rpm.

But if that's too late, then slightly reduce the tension of the light spring. That's done by bending the tab or rotating the perch.
With less tension, there will be slightly less force holding the weight and it will move out at lower rpm.

In contrast, here's what can happen when the advance is shortened by limiting how far out the weights can move.
Initial is set at 12.5* 700 rpm.
This will be a problem for street use or other applications where vacuum advance is wanted.
(Obviously there's no slew with points. Pointing out this disadvantage of having an advance 'all in' early when used with electronic ignitions.)

You lost me. You are throwing a lot of data out without the analytics. Your also saying the distributor never moves and you set initial by welding inside. Assuming everything else in the distributor is perfect for your new set up of higher initial and less mech adv. Which it is not. I also think some of your assumptions about vacuum are wrong. Was this just a linear computer equation you used, or a generated graph from actual experimentation?

I just measured several Prestolite dual point cams. Everything from the Hemi Cap 15deg distributor adv (0 initial), to a tach drive race cam, and several intermediate ones. The inside of the slot is at the exact same point at about 7/16" from collar. The difference is the length moving outward. I do not have as many Chrysler cams loose, but did check two 8s versus a 11 and again the inside distance from collar was the same. In this case 3/8".

Having studied the differences in shaft tip clocking, and cam stop flat location, it was all about housing orientation to ensure vacuum cannister, oiling port access was not obstructed by manifold, fuel lines, etc... Watching a distributor being fitted on a hemi with cross ram manifold with vacuum advance (street motor). You had limited amount of rotation for setting initial timing. In fact one Chrysler type would not work. When I inspected why, the tip of shaft was clocked slightly different around 10 deg (Chinese knock off). Would not have mattered in a 440 or stock setup. This prevented getting intitial high enough because vacuum cannister hit manifold.

As said in beginning you can do this either way, but that does not mean your tuned right. Weld up to limit mech adv. Set initial by housing rotation, set advance curve by spring choice and tension (adjustable).

Sure, the weights cut a curve, but the rotation is small compared to 360 degree circle. So it can be treated as linear. Well within a tolerance of the mechanical play, bearing clearance, spring rate etc.....

regarding the accuracy of the factory plate/cams I've found no more than an half of a distributor degree difference in actual degrees vs what's stamped. for instance a 9 degree plate may be 8.5-9 degrees. never enough to be any kind of "make it or break it". I keep it fairly simple but i'm fortunate enough to have a variety of factory parts and springs. some 5th grade math and a timing tape are your best friends.

I'll see if I can clarify.
The other way. That's the analytics with an example that was handy for me to upload. LOL.

Seriously. For the purpose of showing the effect on the timing, yes that is what this first graph with the black line shows.

We could just look at it as advance. That's often harder for people to picture how that effects the timing curve.
This example is about changing the advance so more initial timing can be used without changing the middle and top end timing.

The next graphs were illustrating how the curve would shift if the initial timing desired (12*) was set at 700rpm.



There is no equations here at all. Those are the timing and advance specifications from the 1968 Plymouth Service Manual. Anything within those lines would have been acceptable. I'm assuming the top or maximum timing for the examples where I sketch in the result of a change. Its not the real curve. Its just four points connected so its easier to see.


That's interesting. I thought I'd seen some variation in distance from collar with the Chrysler cams, but not much. I don't have any points cams to check. Am verious as to the pre-68 non-CAP vs the later ones for the same engines.
One thing certainly seen is angling of the slots in some of the longer advances.


Yes.I agree. If you have a selection of springs, or can wind your own, then it doesn't matter. But the heavy long looped spring, which can be really important, is really difficult to find if you don't have a collection already. So that's why for most folks, the inside is easier approach.

Now you lost me here. LOL. If you're describing the arc of the weights, and the changing angle of the spring, the design is pretty good because it helps to compensate for the centripital force. Some of the aftermarket designs really fail to do that.

Agree that it would be best for the OP, Challenger, to measure timing from idle to as high as he feels comfortable.

My comments about the vacuum advance were about effect of not slowing the rate mechanical advance before 2000 rpm.
Its not hard to reclock distributor to change where the vacuum canister points on a small block. No disagreement that is something to consider.
No idea what engine Challenger is working with.

When vacuum advance is used with a mechanical timing curve like shown below, the engine will ping under light to moderate throttle in the 2000 - 3000 rpm range.
Been there done that.

If the counter weight was locked not to rotate, the pin on the weight that sits in the slot rotates in a small circular motion if viewed from above. As the weight moves outward that pin moves in a curved motion to the new position (not linearly) and makes a larger dia circle in motion. At full advance it is tracing the largest circle. The force that pin puts on the cam to make it rotate is not perpendicular to the slot, rather angular as it curves outward. Lower force applied in the direction of rotation. A cam with an angular slot actually changes that force to perpendicular against the cam slot. Which means it takes less force to move the cam, which means the springs can be heavier and still get the same angular rotation at the same RPM as straight slot which has a lighter spring. If the spring is the same on both, the angular cam will rotate more at the same RPM and get advance in faster.

So besides effecting how much rotation is allowed because of the angular slot, it also effects the force on the cam.

I was confused on your slew rate comment.

We are making this more complicated then needed, and clearly folks think one is better then the other.

I never thought some one was doing this welding up to tune all aspects in one shot. Set initial timing, effect spring rate, limit mechanical. But I get it.

i have welded slots for years. it's easy if one clamps a piece of flat copper under the plate before welding. the weld doesn't stick to the copper, so after a quick clean up of the slots to match the length you want, you are good to go.
i did buy one of those plates, but have yet to try it.
my biggest hurdle in factory distributor modification, is getting the shaft retaining clip out from under the rotor seat.
i finally made a tool for that by welding a couple of pieces of tiny tubing that fit the wire size of the retaining clip onto a pair of small, seldom used pliers. that works great !

I use a thin bladed pair of needle nose pliers to remove them by pulling them straight out, I do have to compress them back so they will work again

2 small tip flat screw drivers and spread clip while gentley lifting cam up. Or one small long needle nose and flat tip. Grab one end, and hold other with flat tip.