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How to make very tight cylider / piston clearances ?

Posted: Thu Oct 15, 2020 10:24 pm
by xtal_01
OK ... I am a semi retired machinist (first post)

In the "real world" almost everything I worked on was large ... sometimes very large. You could park 4 cars on the bed of the vertical lathe we had (this was at Westinghouse Generator and Turbine in Hamilton Ontario).

When required to do accurate work, I had access to extremely expensive equipment ($250K wire EDM at Westinghouse Nuclear in SC).

So now I am setting up a new home shop (have 3500 sq ft of concrete poured) ... I have the usual in storage ... a couple of older vertical mills, lathes, band saw, welders .... I hope to get a few more once the shop is up (upgraded lathe ... shaper ... horizontal mill ....)

I have been thinking about projects once I get the shop finished.

One that came to mind was a small 2 stroke diesel (model airplane type ... like a Mills).

I am still looking for a full set of plans but the one thing I did find was an original drawing of the cylinder / piston.

According to an article I found: Of more interest is the dimensions and tolerances specified. For example, the bore was to be 0.497 ±0.0005" before nitriding (hardening) and grinding, after which it was to be 0.500" ±0.0001". The skirt, which needs to be a close fit in the crankcase to prevent leakage between transfer and exhaust, was to be ground to 0.5993", plus 0.0005", minus 0.0000"! These are fine tolerances for a mass produced model engine and a credit to Mills quality.

I know people make copies of this engine at home ... how? Do you guys have cylindrical grinders (capable of ID and OD) or ????

I am not even sure how you would measure this without an air gauge .... and maybe on a one off that is not necessary as long as you have a "good" fit.

At work I would have cheated ... turned the outside .. put a hole in the center ... hardened it .. put it on the wire EDM an told it to cut a hole.

Maybe some of you have built small diesel engines and can tell me what you did ... maybe the article exaggerated how tight the clearances need to be?

Any suggestions would be great.

Thanks so much .... Mike

PS ... for the past few years, most of my projects have been more fabrication than machining. My wife was paralyzed in a diving accident from the shoulders down. She needs care every 4 hours so I converted an old RV, cutting a new door, building my own lift (they wanted more for a lift than I payed for a 10 year old RV), gutting the interior and putting in hospital beds. We have just finished building an accessible house. It took 7 years but I did as much as I could myself. They quoted us almost $40K for an elevator. I built one using he mast of an old forklift ... $2K. Here are pictures of he RV and the lift (no car yet ... pictures of first test run).

Re: How to make very tight cylider / piston clearances ?

Posted: Fri Oct 16, 2020 7:30 am
by tornitore45
There is tons of written articles on line on this subject, you may start browsing these web sites Ron Chernich is no longer with us, but the website is still running and has great instructional value.

The bottom line on achieving cylinder compression is to hone the cylinder with minuscule taper. A comfortable fit at the bottom with a slight pinch at the top. Low friction where the pressure is low and tight fit when the pressure is high.
You hone by entering the bottom and manage your stroking by giving more time at the bottom than at the top until your piston has the right feel as it moves progressively tightening up.

2 stroke diesel are the most difficult compared to all other configurations.

Re: How to make very tight cylider / piston clearances ?

Posted: Fri Oct 16, 2020 11:22 am
by SteveM

I applaud your ingenuity.

The camper door looks like it's OEM.

I would never in a million years have thought of using a forklift. Great idea.

I'm appalled at how much accessibility equipment costs.


Re: How to make very tight cylider / piston clearances ?

Posted: Fri Oct 16, 2020 3:12 pm
by xtal_01
Mauro ..... Awesome! Just checked out Adrian's site. Found an article by Ken Croft with exactly the information I was looking for. I had assumed the taper just because I had read Cox 049 glow engines had this taper. Thanks so much!

Steve .... my dad always told me a man is limited by his pocket book and his imagination. I never had a large amount of money so I needed to use my imagination. I have always been given credit for thinking out of the box.

I was just going to get an old lift out of a van and put on a swinging door. I liked the idea of a door that acted as roof shielding the lift and rider from the weather. I also hated the idea of a lift taking up so much room inside the RV. The one I designed folds and then slides into the basement of the RV (where the spare tire was ... made a holder and moved it to the back of the RV above the bumper). The door came out pretty good. I cut the panel out carefully ... built a frame and used it so it matched. Made SS trim for the door. Used automotive type weather strip so I don't get any wind noise.

I am still playing with the elevator. It works great and my wife uses it every day. I did things like put velocity fuses in the line so if you get too much flow (like a broken line) the valve shuts so it can't drop. I till need to interlock the door so no one can go under it while it is up (with only two of us in the house no big deal but I like things done right). I also don't like the bump when it starts up or down. It has a mechanical valve that slowly engages but still not 100%. I just bought a proportional flow valve with a 4-20 mA input. With that in place I will have a lot more control ... slow at first ... fast int he middle of the travel ... slow down before stopping.

Yes, when ever you put "medical" on anything, the price seems to skyrocket. I can't tell you the number of things I have fixed or modified for people.

Thanks again!!!