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

[minh-thanh]

My mistake, : near the hole
Lapping the cylinder is really good and you can try it with drill shank
If they can create good enough pressure and then you make a piston that fits the cylinder

 

[Lloyd-ss]

This might be a totally useless suggestion, but I have had some success with, instead of using a hardened steel barrel, using a brass barrel, reamed slightly undersize and then repeatedly forcing a correct sized dowel pin in and out of the brass to expand the hole and burnish the finish. It works best if the plunger is also made from one of the dowel pins from the same box of pins. The brass should have enough durability for a model engine, yes?

 

[Nerd1000]

This might be a totally useless suggestion, but I have had some success with, instead of using a hardened steel barrel, using a brass barrel, reamed slightly undersize and then repeatedly forcing a correct sized dowel pin in and out of the brass to expand the hole and burnish the finish. It works best if the plunger is also made from one of the dowel pins from the same box of pins. The brass should have enough durability for a model engine, yes?

Interesting idea. I think the main challenge with it will be the quality of the dowel pins, I know a lot of them are somewhat 'tri-lobed' due to flaws in the centerless grinding.

 

[ruben]

What is the bore and stroke of the fuel pump

 

[Nerd1000]

What is the bore and stroke of the fuel pump

No pump now. Due to moving overseas I can no longer work on the engine, I did make a new cylinder head for running on petrol with a spark but I doubt I'll have time to fit a carb and ignition system before leaving, and I cannot take the project with me. So it will be shelved for 2 years until I return.

 

[Steamchick]

Bon Voyage Nerd,
You will be missed, unless you can continue to contribute with the brain you cannot leave at home.
Guten Tag!
K2

 

[Nerd1000]

Bon Voyage Nerd,
You will be missed, unless you can continue to contribute with the brain you cannot leave at home.
Guten Tag!
K2

Hah, you won't get rid of me that easily.

 

[Larry G.]

If you can source or fabricate an accurately round pin for the piston, ball broaching, ballizing, or ballsizing will produce an accurately sized, accurately round, exquisitely finished cylinder. Forcing a hardened ball through the slightly undersized hole will do all those things.
Credible information is provided in the article linked here;

https://www.precisionballs.com/Ball_Sizing.php

Don't wait 'til you get a round tuit, do it NOW..

Best regards,
Larry

 

[Nerd1000]

If you can source or fabricate an accurately round pin for the piston, ball broaching, ballizing, or ballsizing will produce an accurately sized, accurately round, exquisitely finished cylinder. Forcing a hardened ball through the slightly undersized hole will do all those things.
Credible information is provided in the article linked here;

https://www.precisionballs.com/Ball_Sizing.php

Don't wait 'til you get a round tuit, do it NOW..

Best regards,
Larry

Only problem is you need a press to do it!

By the way, this method works very well indeed on oilite bushes because their porous structure is naturally a little compressible.

 

[Steamchick]

Have you got a decent engineers' vice? - It's a good screw press for me!
K2

 

[ruben]

This is one of the good questions, how far does the fuel spray travel in air at 30+ Bar as against in open air, will it reach the piston? I haven’t had the head off for a while but last time there was no obvious washing of the piston bowl, just signs of contact between the piston and the valves (which is why I took the head off to insert a 0.5mm shim gasket).

What is the difference between the diameter of the piston and the inner diameter of the cylinder?

 

[Roger B]

I don't actually know, I don't have sufficiently accurate measuring equipment. I turn a plug gauge to the planned diameter and use this to check the bore whilst lapping. The bore is lapped using an Acrolap, https://acrolaps.com/index.htm , and 40 micron diamond paste. The piston is then turned to size using a micrometer to transfer the diameter of the plug gauge. I am aiming for the closest possible running fit. The piston and cylinder are both cast iron to minimise the effect of thermal expansion. This fit is too close for an aluminium piston. I also use the plug gauge when setting up the piston ring fixture.

 

[Steamchick]

Sounds like the gap is "Greater than 40 micron" (grinding particle size), but less than 0.001" (smallest regular size that most modellers can measure)... at a crude guess! (which is ridiculous as 40 micron is 0.0016inch - to those that understand).
Away from such nonsense, is the Acrolap a carborundum stone holder? - or a simple "ground steel precision bar" (or whatever) holder for using with diamond lapping compound? The picture looks nice... Probably more precise than my home-made lapping bar with a carborundum stone slip.
Can you insert a human hair in the gap between piston and bore? - I doubt it.
Or the finest copper wire you can find as a single strand in and bit of scrap wire? - maybe screening off an old HiFi wire or something? My Dad uses the thinnest foil that was suck to tissue paper in the end of his cigarette packet back in the 60s, but he packed-in smoking in the 1970s, and packaging had changed by then anyway. That foil was only 0.0015in thick.
Whatever you can insert, can be measured with your micrometer, as far is its smallest reading.
Cheers,

K2

 

[Roger B]

The Acrolap is a split brass lap with a tapered expander which is designed to reduce the chance of bell mouthing the bore. I have used them successfully on a number of engines. I use the smaller needle laps for my fuel injection components.

At the end of the day getting sensible measurements in a amateur’s workshop is unlikely. Temperature control and something like a pneumatic bore gauge would be required. The piston is simply made to fit the bore as closely as possible.

https://www.marposs.com/eng/product/pneumatic-gauges-for-internal-diameters

For an aluminium piston in a cast iron bore I would try for a clearance of around 0.05 mm. My 25 cc petrol engine seized several times in the early days due to a lack of clearance.

I think that a hair would certainly not fit, one was enough to jam the piston of my SU style carburetor.