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Old 01-24-2015, 06:16 AM
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Default All About The Timing Pin

We have all heard about the timing pin slipping but few really know why it slips, so I'm going to show why.

In the following image, you can see the very basics of the timing pin. Everything else in the governor assembly is taken off. The flyweights mount on that shaft on top of the timing pin.



In this next pic, you can see the actual timing pin pulled off the pump. It is all one assembly, pressed together into one mold. I haven't analyzed it enough to see if it's all friction welded together or what.



You can see that it is just a hole in the center for it to mount on that shaft. No splines or anything special.



The actual shaft is just the back end of the cam. It is smooth and tapered. There are no keyways on it. It is that reason that the timing pin can slip, much as the timing on the pump can slip. Both the timing gear and the timing pin are afixed by static friction. This allows limitless tuning as there is no keyway to be limited to. But it also means if things get loose, things will slip. Some timing pins are practically welded on because of the method used to put it on, some are so loose it falls off with when you take the governor assembly off. There isn't a way to know which you have unless you take the pump apart and check. Since that isn't practical, it is why it is advisable to not use the timing pin at all.



Here you can see the fine adjustment of the pin, should you trust it is accurate. It lets the end user tweak it a few degrees so you can have a custom timing without having to pull the pump apart and reset the timing pin position on the cam.



So there you have it. That is why it slips. It isn't a definite scenario, but given that it happens unpredictably, it can be reason to suspect it if you have running issues and have set the timing with the pin.
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Old 01-24-2015, 12:16 PM
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Excellent write up Sir! Thank you for it. Now, What kind of custom timing can be had adjusting the timing in the rear of the pump?
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Old 01-24-2015, 01:13 PM
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Only a few degrees. Looking at the top pic and imagining 360* degree marks, its clear you can only adjust it a few degrees. So its pretty much just for fine tuning it. You aren't going to be able to adjust it from the 12* that some of these trucks are set at, to the 16* most people want. There are cheater methods I have explained in other threads, such as pinning it at stock timing then moving the engine the additional degrees to get the desired timing. If everything was stock (say 12* stock 12V) then you could pin the pump and the engine, then break the timing gear and turn the engine clockwise (from the front) another 4 crankshaft degrees, tighten the timing gear and you should be at 16*.

But as this thread was meant to show, it is all guesswork if you use that method. The spill port method is absolute. The more indirect the function, the more chance for flaws.
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1997-12V-NV4500-Dana 80/3.54/2wd-350k---21-27mpg
Removed AFC--No Plate--13.5* timing--BHAF--5" Exhaust--Valair Clutch
1994-Jeep Cherokee-5spd-3.07-4wd-202k---16-19mpg
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Old 01-24-2015, 08:02 PM
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I have never used the timing pin method. I have always measured the number one plunger and it seems to be about as close as I would need, but you are correct, the spill port method is the most accurate.
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Duct Tape can't fix stupid, BUT it can muffle the sound
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Old 01-24-2015, 11:33 PM
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I haven't got too deep into it, but I don't think the dial indicator method is accurate. It doesn't take into account the shim thicknesses under the barrel mounts that lift the barrel up. Some are 0.005, some are 0.100. I have thought about it for a long time and it just can't be right but everyone, including the service manual, uses it. Soo is the service manual just giving people an easy method that is close enough, or is there something I'm missing.

The dial indicator method is guessing that port closure will be at X lift, but if the barrels are raised because of thicker shims, then the plungers will have to go higher to close that port, hence why the method just cant be accurate. The pumps have shims of all kinds of thicknesses. Who knows. I can't think of anything that says I'm wrong, but obviously the trucking crowds use it. I'd like to study it more and figure out how it can be correct.

There are a couple different timing charts floating around the web. All with the same headers of pump CPL's, but different numbers. I'm assuming a few different people tested their pump and got different numbers because of the shims.
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1997-12V-NV4500-Dana 80/3.54/2wd-350k---21-27mpg
Removed AFC--No Plate--13.5* timing--BHAF--5" Exhaust--Valair Clutch
1994-Jeep Cherokee-5spd-3.07-4wd-202k---16-19mpg
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Old 01-25-2015, 11:18 AM
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The dial indicator should be fairly accurate. What you are actually measuring is the cam lobe lift in the pump. When using a dial indicator you start with the lobe not at the plunger. Then you zero the indicator to get the amount of lift. That takes the shim thickness out of the picture. Is it 100% accurate? No, I don't think it is, but it is pretty darn close.
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Old 01-25-2015, 07:39 PM
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Yes but the barrel moves up and down ever so slightly based on the shim. The dial indicator is in the barrel. So if the barrel is raised another 0.100, the spill port will be that much higher, and therefore the plunger will need to go that much more before it closes the spill port. None of that is factored into the tables. And 0.100 accounts for like 14*.
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1997-12V-NV4500-Dana 80/3.54/2wd-350k---21-27mpg
Removed AFC--No Plate--13.5* timing--BHAF--5" Exhaust--Valair Clutch
1994-Jeep Cherokee-5spd-3.07-4wd-202k---16-19mpg
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Old 01-25-2015, 08:34 PM
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But correct me if I am wrong, but since you zero the display on the measuring device all that you are doing is measuring total plunger lift. It wouldn't matter if the shim was .001 or .100.
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Duct Tape can't fix stupid, BUT it can muffle the sound
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Old 01-25-2015, 11:29 PM
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You are zeroing it and taking the plunger up to the desired number, but that desired number is derived from when the spill port closes and injection occurs. If the spill port is .100 higher than whoever made the chart, then its gonna be way off. The engine rotates another several degrees before it is covered.

The key is the barrel and plunger are NOT locked together. The barrel can be rotated and lifted with shims. Both calibrate it, and both are very different between pumps.

Think of a 2 stroke engine with the ports in the cylinder wall. The piston goes up and covers the port. Now imagine its a liner. So lets say you zero the piston out at BDC, then make the piston go up an inch, which we'll say is where the chart says to go for X timing. Now imagine the liner is on shims, and you have thicker shims.... now that one inch isnt enough, you have to go up the thickness of the shims to get the ports to seal. THAT is exactly the same scenario. So you can see where I think its completely flawed. It is using whatever shim they used to test it. Could be 100, could be .001.

Now I'm trying to find a flaw in my logic but I'm just not seeing it yet. I think people simply get lucky for the fact that the shims dont vary 100, but still, I am betting it could be up to 5* off, or more, potentially 10*.
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1997-12V-NV4500-Dana 80/3.54/2wd-350k---21-27mpg
Removed AFC--No Plate--13.5* timing--BHAF--5" Exhaust--Valair Clutch
1994-Jeep Cherokee-5spd-3.07-4wd-202k---16-19mpg
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Old 01-26-2015, 10:41 AM
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True, the barrel and plunger are not locked together, but the spill port is at the lowest position when the timing is set. As far as shims, each engine has a CPL number that is used for timing. For instance mine is 2202. From what I have been able to determine is that all engines with the CPL of 2202 have the same thickness shims and others such as a 1863 pump has different shims. I wonder if the shim thickness is what actually makes a pump 160hp, 180hp, 215hp, etc. That would explain why each pump has its own numbers to be used for timing. Now, I am not sure what differences there are in the cam for each pump,but I am sure there are some.
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1996 RAM "Custom Ground" 100 plate 3/4 forward, 300 injectors, 16.5 timing, Goerends Auto = 365HP, 833TQ Third Gen Rims, Aluminum Headache Rack
Duct Tape can't fix stupid, BUT it can muffle the sound
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