An Injector Pump Shim Question

[Henro]

I know if you change the injector pump, you need to use the exact shims that were under the original pump.

Injector pumps must be made to specific specifications. So it seems like the mating surface on the block must be position at a specific place, so the injector pump will work properly in all cases.

But engine blocks are castings and machined as needed after leaving the mold.

This apparently introduces variability when the blocks are machined as required, whatever that might be.

So...It seems that to physically position the injector pump on the block, after machining, shims are used between the block and the pump.

My question is: How is the required shim total thickness determined?

 

[JohnDB]

Using the injection timing mark on the flywheel, I reckon.

 

[number two]

Having problems?
Or just curious?
I am as well!

 

[North Idaho Wolfman]

My question is: How is the required shim total thickness determined?

If you have the original shims you would just add the amount you mill off the block.
If you don't have the original shims or if you think someone has changed them you need to do a spill test.
You use the crank timing marks to determine what is needed.

It's much better if you know the original shim /s size
And the shims come in many different thicknesses, also do not ever use a thick sealer like RTV as that changes the dimensions.

 

[Henro]

Having problems?
Or just curious?
I am as well!

Just curiosity. A weakness I’ve been suffering from more as I get older…

I was just wondering how the manufacturing process went as far as machining the block surface and then determining the spacing that was needed (if any) between the finished block surface where the injector pump is mounted and the finished surface of the injector pump that mates with it.

This is for the older style totally mechanical engines. It seems that you might start with the position of the piston in a cylinder, and decide at what point you want to start injecting the fuel into the combustion chamber.

Because it’s a mechanical system physical change in the position of the physical components can change how the system works.

So I’m guessing after engine blocks are machined at the factory, at some point they probably have a method where they place a standard of some type on the engine block (which simulates the injector pump) and then determine piston position when that standard starts pumping.

This is all speculation because I don’t know. But I can see how they could have a chart that would relate spacing between the two surfaces with the position of the piston in the cylinder. And they could relatively easily determine the amount of shims needed to set engine timing properly according to their design.

In other words, they would know that 0.1 mm shim would change the timing by 1/X degrees.

I would further guess that due to the variability in the manufacturing process, they probably target to always need a shim of certain thickness, and the rare engine that has no shim between the injector pump and the block is probably one that almost missed the manufacturing tolerance window.

How badly did I miss the target dreaming up an answer to my own question?

 

[JohnDB]

@Henro I believe that you are missing the point about the injector timing mark on the flywheel. There's a matching pointer cast into the bell housing. The flywheel is indexed to the crankshaft. So the said timing marks will line up at the precise number of degrees BTDC that Kubota wants the injection to initiate. The spill test Wolfman mentioned shows when the injection initiates, and if the spill happens other than when the pointers are aligned, then that indicates more or fewer shims to be added.

Think about timing mark on the flywheel or crankshaft pulley of a petrol/gas engine for setting spark BTDC. Similar idea.

 

[Henro]

@Henro I believe that you are missing the point about the injector timing mark on the flywheel. There's a matching pointer cast into the bell housing. The flywheel is indexed to the crankshaft. So the said timing marks will line up at the precise number of degrees BTDC that Kubota wants the injection to initiate. The spill test Wolfman mentioned shows when the injection initiates, and if the spill happens other than when the pointers are aligned, then that indicates more or fewer shims to be added.

Think about timing mark on the flywheel or crankshaft pulley of a petrol/gas engine for setting spark BTDC. Similar idea.

Oh, no, I’m not asking about how to make an engine work that has been working.

What I’d like to understand is how they get the engine to work to begin with,
since different engines have a different amount of shims under the injector pump. I’m talking about the exact same engines.

Thinking about it, I came up with one way that they might do this, but I don’t know if it’s on target or totally off base.

I didn’t mean to sound like I was asking “if you lose the shims under the injector pump, how do you figure out what to put in place of the ones you lost.” Although that question really relates to my question.

I guess my question relates to the manufacturing process rather than the normal maintenance/repair process. Or maybe not?

 

[Russell King]

I am not 100% sure how it is done at the factory but obviously there is a direct relationship between the distance from the machined face of the block and the injection pump actuation mechanism. There is seldom a perfect distance machined due to cost and time. I assume the distance on the injection pump (face to actuator) may be adjusted to perfection during assembly through some adjustment screw or internal shims.

But there is a perfect and known distance that is required for the timing to be within needed specifications. So they would (probably) record the distance on the block during machining or assembly and the add the required shimming to reach the required distance.

I assume that some engineers have looked at all of the tolerance stack ups and set the distance so it could never exceed the required distance on the block so there are (probably) rare instances when zero shimming is required.

At least that is how I would have thought about setting up the machining steps to get a good result with least costs and times.

 

[North Idaho Wolfman]

Considering the amount of engines that Kubota produces they more then likely have a very precise way to measure the amount of shims needed to get to the proper height off the cam and do it with in a split second.
And that height has no bearing on the piston location at any point.

 

[Henro]

Considering the amount of engines that Kubota produces they more then likely have a very precise way to measure the amount of shims needed to get to the proper height off the cam and do it with in a split second.
And that height has no bearing on the piston location at any point.

Not sure about that…

I view the engine as a mechanical device with all the rotating pieces interlocked in some fashion, either through gearing or chains. So the position of one element actually does relate to that of another.

The camshaft has to relate to piston position because you want the valves to open at the right time so there is no crash between the piston and the valves, and obviously for other reasons.

My background electrical so obviously I can miss things easier if they are mechanical rather than electrical, not that I can’t miss electrical things pretty easily either now as I get older! LOL

Certainly, the position of maximum lift at the camshaft lobe tip would have a relationship with piston position, so that very well could be used maybe. But the variable that needs to be accounted for is the relationship between the mounting surface on the block, and the injector pump mating surface, so that the lobe on the camshaft accurately causes the injector pump to pump at the proper time.

Assumption is that engine blocks are made to a certain specification, and injector pumps are made to a certain specification. So the one variable is the position between the pump and the engine block. They are essentially two independent mechanical systems working together from what I can see. The shims are used to calibrate the interaction of these two mechanical devices. That’s simply my guess.

I don’t have a clue on how the injector pump switches between cylinders. Perhaps that will be my next question….

 

[GreensvilleJay]

probably some fancy ,magical 'block' that gets bolted on, turn crank to TDC (or 'the mark' ) and magically some display says 'add xx thou in shims'.

along the same thought ..... is HOW 'wheel balancing' machines tell you 'xx oz @ yy degrees'.... again a 'magical' device.

 

[North Idaho Wolfman]

Kubota engines are very very exactly cast, machined and built.

During first assembly:
The distance between these two points (pic below) will tell you how thick the shim needs to be.
It has Zero to do with the height or position of the piston.

During field service work:
It too hard to get to or measure this dimension so they tell you how to do it via timing marks.
And the timing marks for this purpose do not have anything to do with the height of, or the position of the piston.
It has to do with the position of the fuel cam that pushes the injection pump.

 

[North Idaho Wolfman]

I don’t have a clue on how the injector pump switches between cylinders. Perhaps that will be my next question….

Because there is one pump that runs one injector.

The yellow is a pump, the whole thing is a pump assembly.

 

[William1]

I suspect they simply use a few dial indicators. One to determine piston location and the other, injection pump cam lobe to surface. Piston at a certain point (probably BTDC) and then the corresponding value at the cam gives them another number and math tells them how much thickness to add. Or it could be a precision measuring block and feeler gauges (more likely).

 

[Henro]

Before coming back to this thread, I figured why not ask my AI buddy?

I am smart enough to know not to trust anything AI tells you, least right now at this point in time. But I also know that AI often gets the answer right. But not always, and when you don’t know the answer, how do you determine the validity of the AI answer?

So anyway, the AI answer was rather long, so I just cut and pasted below the answer that related to the manufacturing environment:

In Manufacturing Settings

• High-precision timing fixtures and dial indicators are used.
• Engine-specific tables or software guide assembly technicians on starting shim thickness.
• Then, the actual injection timing is verified with either:
  • Dial indicator plunger lift
  • Spill timing method
  • Laser or electronic crank-angle sensors (on newer engines)

The spill timing method which the Wolfman mentions is listed. My intuition was that the high precision timing fixtures, and dial indicators were likely to be used. But as I said, I don’t know, that’s why the question was asked…

 

[hagrid]

The answer is metrology.