Model Diesel: 32mm bore, 38mm stroke, indirect injection

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petertha

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I don't have a lot of lapping experience but I found the ones offered by Acro hard to beat. Some of my home brew inventions that were influenced by others I've seen in model engineering & gunsmith articles. They worked & probably viable for custom sizes. But for more routine sizes, it was pretty hard to beat Acro considering cost & effort. One thing I'll mention, the smaller ones have proportionately smaller slits & (IMO) if they are not cut somewhat precisely & consistently (meaning radial spacing) then they don't barrel expand quite optimally. It tok me a while to figure this out.

FWIW I did my radial engine CI liners which were about 25mm nominal bore but, the target was within 0.0001" (tenth) down the length because I was sizing to existing rings. Slow, laborious process but I think ultimately achieved. I've used smaller sized Acrolaps for other parts too, they work just as well. And you can buy the brass barrels (lap) separately which is another consideration when switching grit medium on the same hole or a series of parts.

 

Lloyd-ss

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This might be old news to many of you, but I like to use Craytex sticks for final polishing. The center punch and aluminum rod were both polished on a lathe for about 30 seconds. The stuff is great for final polishing. It removes an unmeasurable amount of metal, but will make the part shine.
Abrasive is already embedded in rubber. I have been using the stick in the pic for over 5 years.
 

Lloyd-ss

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Sorry, forgot the pic.
 

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petertha

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For a lower cost solution, I use these rubber abrasives in my Dremel all the time. They are very inexpensive & come in a range of grits. I just hold the disc wheel, without the arbor of course. Or use the cylinder/point style, but use one of those jeweler collet handles normally used for needle files.
 

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Nerd1000

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Ok, it took me a few tries but I think I can declare success in the lapping of a plunger and barrel!
IMG_20221003_164530418.jpg

Here's the ID and OD laps I used.
IMG_20221003_164640802.jpg

And finally, the plunger and barrel assembled with the delivery valve for my yet to be built injector tester. The black rings are delrin gaskets and the conical nose is intended to connect to a HP fuel line via a flare and nut.

IMG_20221003_194602669.jpg

The plunger seals well enough that it springs back into the bore under air pressure if withdrawn while the spill port is blocked, and I cannot move it beyond the spill port when the barrel is filled with oil. Further testing will be needed to be sure it works at higher pressures.

A few thoughts on the process:
It's a good idea to leave yourself a healthy amount of stock to lap away, 0.02-0.03mm. This gives you room to clean up toolmarks and taper in the part. The lap will tend to climb up the part (due to spiral toolmarks) during roughing and you need to be prepared for this as it can easily become stuck if it climbs a taper.

Abrasives wise, I used 600 grit SiC for the initial phase, followed by 1500 grit alumina for finishing. In both cases I used WD40 as lubricant.
 
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As WD40 40 is a water dispersant not a particularly good lubricant it may have been a good choice? The paraffin content would have helped clear debris and swarf from the job, as it works by lowering surface tension - I think? - to clean surfaces.
Maybe for the wrong reason you did a good thing?
It reminds me by feel between finger and thumb, of the Delapena honing fluid I used in the workshop... Dries the skin, because it lacks oils.
WD40 contains a lot of white spirit and clever additives. But is NOT a lubricant. They do not specify lubricity on their data sheet.
Any experts add to that?
K2
 

minh-thanh

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Nerd1000 !​

That is good news .
Try to do the best you can and the engine will run!
Tool lapping for nozzle - that's how I do it, you can do it however you want as long as the injector is sealed

20221004_104123.jpg

 

petertha

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My own experience is that WD-40 is good at removing / washing away lapping compound from parts/laps, at least when using the commercial oil/grease based compounds. Some commercial paint thinners work & others don't so I wont volunteer trade names. I think a machine oil might be a better choice to lubricate the lapping process.
 
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