Piston/cylinder-lapped fit--no rings

[Brian Rupnow]

Time to ask for some help now. I plan on building a model i.c. engine, 1" bore x 1" stroke. Cast iron piston and cast iron cylinder liner. I would like to do this without any piston rings. I have a lathe and a mill to work with. I have always used Viton o-ring on my previous engines, because they work so well and don't require a great knowledge of "ring making)--(at which I have proven to be a dismal failure). My new engine is going to be a two stroke with ports in the cylinder walls, and I know that a Viton ring would bulge out into the ports and die very quickly. On all of my previous engines, with either a cast iron cylinder or a cast iron liner, I have drilled and reamed the cylinder to 1" bore with a 1" reamer. then I apply a bit of light oil and run my 3 stone brake cylinder hone thru it a few times to knock down any high spots and to texture the wall a bit. Then I turn a round piece of aluminum to 0.002" smaller diameter than the bore size, coat it with 600 grit lapping compound in an oil base, and work the round aluminum thru the cylinder with a twisting motion until it gets free enough to chuck the aluminum in the lathe and hold the cylinder in my hand while running the lathe at a low rpm. I work the cylinder back and forth on the aluminum (which is longer than the cylinder barrel by about 1" plus chucking length of another inch). This is scary business, and you have to be prepared to immediately let go of the cylinder if it "grabs" and let it rotate with the lap until you can shut the lathe off and work it free by hand again. After about a minute of lapping, I'm done with the cylinder. I then turn the piston to about .002 less than the finished bore size, coat it with 600 grit compound, and repeat the process, sliding the piston (with a "handle" attached to the wrist pin) in and out of the cylinder over it's full length with the same twisting motion.--this knocks any high spots off the piston and gives a very nice sliding fit into the cylinder. HOWEVER---I know that this is not sufficient to give a fit that will create good compression without rings on the piston!!! I have read about internal expanding laps, external adjustable laps (which I have neither of) and 1000 grit lapping paste. I do know that Chuck Fellows, who is a remarkable machinist tried to make an engine with a ringless piston a year or two ago, and was unable to get the engine to run, because of compression issues.--So--I am asking for help, encouragement, knowledge about what I wish to do---and I don't want any suggestions that I give it up and learn to make proper rings please.---Brian

 

[bb218]

I am sure with something like a Sunnen hone you can get bore straight and round enough. Precision grind a piston to fit to get good compression. What I doubt is that it would run very long before it would seize up, your clearances would need to be in the .0005 or less range. That tight a fit leaves no room for burnt oil or other dirt, plus everything would need to expand at the same rate when it heated up.

Mike

 

[Brian Rupnow]

Mike---I don't have a Sunnen hone.----However, I did talk to me local machine shop, and they do. I don't have the machinery to "precision grind" a piston either. As far as "seizing up", well, you may be right. The engine would be running two stroke oil mixed with the gas, and it wouldn't be unheard of for me to mount an auxiliary drip lubricator to the cylinder. (The original plan has that). With a piston and cylinder of the same cast iron, I would expect them to expand at the same rate. Thank you for your answer.---brian

 

[Charles Lamont]

The ability to make and use laps is a weapon that a good machinist should add to their armoury. I have to admit I came to it quite late, and found it easier than I had feared.

For iron, copper makes a good lapping surface. You want something soft enough that the abrasive embeds in the surface of the lap. You should not be relying on it being awash with masses of slurry, and the lap needs to be adjustable. I had a quick look on YouTube. There does not seem to be a lot, but this is OK:

[ame]https://www.youtube.com/watch?v=H2SeLcbcYkE[/ame]

 

[Brian Rupnow]

Thank you Charles---that was a really informative video.---Brian

 

[GailInNM]

Brian,
I have lapped over 30 cylinders with bores ranging from 5/16 of an inch to three quarters of an inch so yours is a little larger than mine.I have also lapped numerous steam engine piston valves, mostly in the 5/32 inch range.

In the early days I made some of my own laps, but for many years now I have used nothing but Acro laps. They are inexpensive and for our purposes are really a once in a lifetime purchase for each size. As they are sold worldwide your industrial supplier probably has them as they are used for precision work in many machine shops. A 1 inch diameter through hole barrel lap has an MSRP of US$ 16.50. Replacement barrels are about $10 each and if you use two or more different grits of compound you need a barrel for each grit. Because the complete lap is not much more expensive than the barrel, I just buy two laps so I don't have to fuss around with changing the barrels.

If you do a good job of boring you really only need one lap and one grit however. For cast-iron I use a solid carbide boring bar and only use one grit to rough and finish.

I used diamond lapping compound for all my steel and iron work. If you're using a brass lap, like the Acro, embedding of the diamond in the cylinder is no worse than if you are using silicon carbide lapping compound. I don't subscribe to the "diamonds are forever" view that I see on the Internet. With any lapping compound you have to scrub the cylinder well before measuring or inserting any hard material in the cylinder that might cause abrasive particles to embed in the cylinder walls. Diamond lapping compound is also inexpensive with a 5 g tube of light loaded compound well under $10. 5 g doesn't sound like much but it is a lifetime supply for the hobbyist. Buying diamond compound can be confusing as the different manufacturers use different methods for specifying the grid size. Some use the term "mesh" and some "grit" and each has a different definition of them. However all is not lost as they all eventually refer back to the grit size in micron. For all my engines I finish with 10 micron compound. It is sometimes shown as 8 to 12 micron in the specifications and many manufacturers refer to it as 1500 mesh. There is no need to go to any finer compound for a cylinder in my opinion. The only other compound I ever use is 30 micron if I need to remove quite a bit of material, but I haven't needed to use it for a long time.

I typically bore my smaller cylinders 0.0015 under size and just and just laugh at the size with the 10 µm compound. Using a barrel lap for through holes you can feel if you have a tight spot or taper or anyplace and just dwell there a little bit more by your lapping to get a parallel bore. On compression ignition engines it is customary to lap a little taper into the cylinder so the cylinder is tighter at the head end and this is when we do that.

There are several different ways of charging the lap with compound and if you should go with the Acro type lap and diamond compound I will be glad to tell you how I do it and some of the alternatives.

Here are links to the Acro website showing both the complete lap and the replacement barrels. Just remember that you want a through hole lap and NOT a blind hole lap.
http://www.acrolaps.com/index_009.htm
http://www.acrolaps.com/index_010.htm
You can find diamond compounds on Amazon and eBay quite inexpensively. Light loaded compounds are quite suitable. The loading refers to how much diamond percentage there is in the compound. Industrial suppliers also have the compounds available but they are normally much more expensive. Here is a link to an assortment of compounds on Amazon. You would only use one or possibly two of the coarser compounds for your cylinder but you get the idea.
http://www.amazon.com/dp/B015HLK31W/?tag=skimlinks_replacement-20

Gail in NM

 

[Brian Rupnow]

Gail--I was really hoping to hear from you on this subject, because I knew you had made a lot of engines by this method. Many of the web-sites I have looked at infer that lapping is only done after grinding, but I don't have any grinding capability here. Some people have posted that what I want to do is impossible, because making a piston such a close fit that no rings were required would cause the engine to "seize up".----Some have suggested that the fit for an ignition fired gasoline/oil burning engine is not as critical as that of a compression/ignition engine. Please give me your comments on these points.---Brian

 

[GailInNM]

Brian,
I always follow your posts and try to contribute if there's something I have personal experience with. Not able to do much machining because of the I situation these days but I do try to do some anyway.

To the people that say it's impossible to achieve fits like this, they were never around model airplane engines in the 40s 50s or 60s. Most of the low-cost midsized model engines, that is up to 0.60 CI placement were of a lapped piston/cylinder construction. This was both spark ignition and glow plug engines. I think the only real reason the larger ones used rings was because of the heavy cast-iron piston caused too much vibration in the speed range they wanted to run at. Typically a 0.30 to 0.50 class glow engine ran at from 12 to 16,000 RPM. At the speed your thinking of vibration will be no problem.

Yes, compression ignition engines do require much tighter tolerances on the cylinder fits. Typically model compression ignition engines run from 15 to 18:1 compression ratios and any leakage will not give enough compression for ignition. Most of the ones I have built have had tapered cylinders so there is virtually no clearance at top dead center. It feels like the piston is jamming just before it reaches the top dead center when you build one, but it loosens up a little bit after a few runs. The most critical part of the fit is the contra piston, which is used to adjust the compression ratio, and is located in the cylinder above the piston and is adjustable. All my compression ignition engines were three eights inch bore and I had a rule of thumb for fitting them. it was that if I could push the contra piston into position with my thumb without it feeling uncomfortable the piston was to loose.

I do have internal grinding capabilities here, but I have found that it takes me longer to set up the tool post grinder than the extra time it takes to just lap it well bored cylinder. I have not ground a cylinder bore for over 10 years and should probably just sell the tool post grinder. I do use a good grade, Micro 100, solid carbide boring bar in the lathe and consider it a very worthwhile investment. It gives a far superior finish to the bore.

Some people have suggested leaded steel for the cylinder. While this works very well, I prefer to use cast-iron. If I were doing a high-speed engine I would probably use the steel. I did so on my Lobo Pup Twin. This is because the steel expands slightly more than the cast-iron piston with temperature and the crown of a piston will run hotter than the cylinder upper end. For your engine I would use cast-iron for the cylinder and piston as it is a lot less critical of lubrication. And for low-speed operation your temperatures will not be as high as a engine built for high-performance. But, mostly I would use cast-iron just because I like to work with cast-iron.

Once the cylinder is bored to your satisfaction and cleaning it, turn the piston to about 0.001 or so oversize so it won't quite fit in the bore. After doing the internal work and the wrist pin hole, cut off and fit on a simple internal expanding mandrel. I then finish the piston to size using EZE Lap and laps. I will attach a link to them on Amazon although other places are cheaper. I have a set of four like are shown in the link but I never use the coarse grit one. After a bit of research the grit sizes of the set turn out to be coarse-250, medium-400, fine-600 and super fine-1200. I start with the medium and work the piston to wear is just thinking about entering the cylinder. Then I switched to the medium and bring the piston diameter down to where I can push the piston through the cylinder with a little bit of effort. I finish with the superfine until the piston was sliding easily from one end of the cylinder to the other. With the superfine I'm only taking off about 0.0002 inch. It is easy to feel if you're getting any taper on the piston and adjust it with the superfine.

The purests will argue that doing it this way will make the piston out of roundand technically they are probably right. But from a practical standpoint how much out around am I going to get since I am only going to be taking off about 1/1000 of an inch on the diameter anyway. The advantage of using the EZE laps is that you have no mess to clean up on the piston while you're fitting it to the cylinder. A simple wipe with a paper towel is all that you need. I do have an external hone that I built up but I never use it anymore except sometimes for polishing a long shaft that needs to be accurately finished to size the entire length. Otherwise I just use the EZE laps.

Here is the link to the laps. Like I said they are available elsewhere for less money. They are handy to have around the shop anyway for touching up lathe tools, both HSS and carbide.

http://www.amazon.com/dp/B000UVTDZC/?tag=skimlinks_replacement-20

Gail in NM

 

[Tim1974]

im very new to all this but a very slight taper on your bore so at tdc very cloce fit should do the job ?I no a little off topic but trying to help

 

[Charles Lamont]

I forgot to mention. Any time you are using abrasives round the lathe, move the saddle well out of the way and cover the bed with newspaper to catch any drips. You definitely do not want diamond paste in the slides.

Actually I use silicon carbide paste for iron & steel, and also have some fine garnet paste from Brownells (who were happy to ship to the UK) for softer materials.

For a little easing generally, liquid Brasso works very well.

 

[canadianhorsepower]

Brian,
if you can get a copy of
Model Engineers' Workshop #237 January 2016
there is a very good article how to build a
sacrificial Table for your rotary table,

If you can't let me know

cheers
__________________

 

[holmes_ca]

When I built my Sealion 4 cylinders (cast Iron liners) I finished to size and honed with an Auto Flex Hone, http://www.brushresearch.com/flex-hone.php?gclid=CJeGkKaM_8oCFQgxaQod01ILJg

placed the block in a coffee tin container fill with oil submerged the block and with the Flex Hone in a elec drill gave each bore a few ups and down, came out beautiful in just a few minutes, then made the pistons (with cast iron rings), my bores were 7/8

Edmund........Alberta

 

[Brian Rupnow]

Blogwitch--what is the name of the type of lap shown in your post? Is it lap or a hone? I an trying to do an internet search to find one like it and not having much luck. Maybe if I new the exact correct name I would have better success trying to find one to buy.---Brian

 

[GailInNM]

Here is a link to a home built one similar to Blogwitch's.
http://thebloughs.net/external-hone-inspiration-and-design/#more-328

If you have back issues of Model Engine builder Magazine, Issue #25 has an article on external hones with construction drawings. The one I built is included.

Mine is functional, but not near as nice looking as John's.
Like I said, I don't use it much anymore. Just for long stuff.

Gail in NM

 

[Brian Rupnow]

Thanks Gail---I will probably skip making one of those and go right from a turned finish on the piston to using a piece of brass plate about 1" thick with a 24 mm nominal hole drilled thru it, a sawcut from center to the outside, and one pinch-bolt to tighten it up and use that as my adjustable lap, with the piston spinning in the lathe or on the mill if the lathe is holding the liner and I am trying for the "best fit".

 

[GailInNM]

It was just for informational purposes. I would not make one again for as little as I use it. I have fund that other methods are easier/faster and yield results that are just as good for my purposes.
Gail in NM

 

[petertha]

Brian,
- turn the piston to about 0.001 or so oversize
- finish the piston to size using EZE Lap and laps
- work the piston to wear is just thinking about entering the cylinder.
- where I can push the piston through the cylinder with a little bit of effort
- superfine until the piston was sliding easily from one end of the cylinder to the other

This has always fascinated me. So if both the liner & piston are cast iron (same material = same hardness = same lapping stock removal rate) and assume the liner starts out as a taper-less parallel bore and the piston is worked down to the point it just enters the skirt bottom & now progressively lapping upwards to the crown for the resultant fit... To me that means the piston is always seeing lapping over its entire surface, but the liner sees higher % lapping strokes near the bottom than near the top.

So... what is the dimensional 'end result'? A rather parallel (cylindrical) piston OD over its length and an ever-so-slightly tapered bore from this final conditioning? Or maybe a slightly tapered piston as well as a tapered bore? I cant visualized a perfectly cylindrical piston and bore regardless of how good the fit is, or it would just slide with the same resistance from bottom to top. Friction the whole way & no compression 'pinch' as 2S RC guys like to say.

Curious if you ever mapped the geometry of one of your engines to gain insight to this. They must be pretty teeny tolerances. On commercial 2S RC engines I've measured, the cylinder taper is evident like a sore thumb. But the piston crown diameter vs the liner dia near TDC... now that's a hard dimension to distinguish to 0.000x, at least with the tools I have in my shop. Lapping is a feel thing, isn't it?

 

[GailInNM]

Petertha,
I think you are misunderstanding the process that I use. Probably my fault for not explaining it well enough. The following is what I do for my low-speed gas engines that have no taper in the cylinder bore. I modify this process slightly to add taper to the bore for higher speed glow and compression ignition engines but will not cover that now.

The cylinder is finished first with the more straight and parallel throughout. I do size them to be dimensionally accurate as it makes life easier later on, but it is not necessary to do so as the piston will normally just be fitted to the cylinder. Besides a tenth reading digital inside micrometer, I have a set of Deltronic plug gauges for the boards that I normally do. These are a set of 21 gauges in 0.0001 inch increments from minus 0.0012 to plus 0.0012 inch from the nominal. They are accurate to 40 micro inches and have about a five or 10 micro inch finish on them. When checking a cylinder for size I select the gauge that fits the bore all the way through with light finger pressure. It is easy to feel any errors in the bore this way. Selecting a plug gauge that is 0.0001 smaller it will free fall through the bore under its own weight. If the bore is capped with a finger the gauge will not fall through for several minutes. These tests are done with the bore and gauge both dry with no oil on them.

After the cylinder is finished in this way it never sees abrasive again. It is left dry and the piston is lapped with the EZE laps using the medium to superfine laps as I described above. The EZE laps are used dry and are like a fine file in their operation. The piston is wiped down before test fitting each time in the cylinder and occasionally I wipe down the lap, both which is a paper towel. For the most part I don't do any test fits in the cylinder until I am very close to being to dimension. Since I accurately know the bore of my cylinder I just measure the piston with a tenth reading micrometer until I hit my known board dimension. Then I take off a little more to get the fit I want. For my 3/8 inch bore hit and miss engines that I have been building for the last few years the Pistons have been measuring about 0.0001 inch under my known more dimension. The piston will slide through the bore with just a hint of finger pressure.

All this testing is done dry and then the piston and cylinder are oiled to prevent corrosion until I am ready to use them.

Gail in NM

 

[GailInNM]

By the way. Some people think that lapping a piston/cylinder set is a very time consuming process. I have spent more time telling about it in this thread than it takes me to lap a set.

After the machining is done, a set tor my Tiny H&M engines it takes me less than an hour to lap including clean up and most often a cup of coffee. Of course my first lapped set many years ago took a lot longer.
Gail in NM

 

[Brian Rupnow]

This quest is taking me in new and different directions than I have ever gone before. The consensus seems to be that what I want to do regarding the piston/cylinder fit with no rings is indeed possible, but I am going to have to learn lapping skills that I currently do not have. So---I have today ordered a 15/16" internal barrel lap (which will easily expand to 0.945" or 24 mm) and a tube of 8 micron to 12 micron "light" diamond lapping compound. I looked at external manual hones, but as they cost upwards of $400 I will probably be making my own external lap, (pending some information from Ramon on MEM) from a piece of brass with a hole bored thru it and a pinch bolt. Wiser heads than mine are suggesting that I use leaded steel for the liner, and as I have a piece of 12L14 left over from my last engine I will probably use it for the liner with cast iron for the piston. My course of "step by step" actions will be to first make the cast iron piston, turning the outer diameter of the piston to about .001" to .002" oversize from my desired 24 mm (0.945"). I will then set the piston aside. I next machine the outside diameter of the piece of material for the liner to ensure it's roundness over the full length, including an inch or more to be held in the chuck jaws. Next I will bore the liner and chucking stub to about .0015" to .0001 undersize and then turn the outer diameter to finished size, turn the "lip" at the top of the liner to finished size, and still leave the liner attached to the "chucking stub" held in the lathe. Then over to the rotary table to cut the ports and sparkplug hole in the sides with the mill. Then back over to the lathe to lap the inside bore to exact size, using a lap mounted in the lathe and the cylinder liner in my hand (I try for a tighter fit at the top of the liner than at the bottom--this is sort of a "by feel" thing.). Now, assuming the cylinder liner is exactly finished to "On size", I put the chucking stub end of it back into the lathe. No farther machining operations will be carried out on the liner, except to part it off from the chucking stub after the piston is fitted. I then use the external lapping tool on the piston by hand, and bring the piston down to a point where it will enter the liner. If I don't lap the piston into the liner, then how do I know that the liner has been lapped enough. Am I required to make up a lapped plug gauge to a very closely held and lapped tolerance to check the liner with, to know when I have lapped it enough?