Author Topic: Piston/Cylinder lapped fit--no rings  (Read 3265 times)

Offline Brian Rupnow

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Piston/Cylinder lapped fit--no rings
« on: February 16, 2016, 08:55:48 PM »
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

Offline Ramon

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Re: Piston/Cylinder lapped fit--no rings
« Reply #1 on: February 16, 2016, 09:27:15 PM »
Hello Brian, quite some time back I spent some pleasurable time putting this together for the benefit of members. As it's faded down the list you probably are unaware of it. As before I think this will be worth a look
http://www.modelenginemaker.com/index.php/topic,1908.0.html

You should have no problem lapping a 1" diameter piston/liner set up for a spark ignition engine following the info on there. The compression ignition requires a much better tolerance but the principle can be eased a little. I would strongly advise using a leaded steel liner with a cast iron piston however as this makes for a very good combination of materials. That's not to say cast on cast is not ideal - just that in my opinion it's a better set up. Besides steel is much cheaper  ;)

I'm surprised no one has mentioned this before but the way piston rings are prevented from catching in the exhaust and transfer slots in the liners is to segment the slots with bars though this wouldn't allow the use of O rings of course  ;)


regards - Ramon
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(a very apt phrase - thanks to a well meaning MEM friend)

Offline Brian Rupnow

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Re: Piston/Cylinder lapped fit--no rings
« Reply #2 on: February 17, 2016, 01:42:21 AM »
My Gosh Ramon!!! What an incredible thread you have on lapping!!! That was just marvelous. Thank you so much for the link.---Brian

Offline Jasonb

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Re: Piston/Cylinder lapped fit--no rings
« Reply #3 on: February 17, 2016, 07:36:33 AM »
Follow Ramons thread and you should not go far wrong, Its worked for the ringless engines I have made. I think I also suggested a steel liner the other day.

J

kettrinboy

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Re: Piston/Cylinder lapped fit--no rings
« Reply #4 on: February 17, 2016, 08:08:15 AM »
Hi Brian , I always spend more time on getting a very finely finished and parallel cylinder during the boring process and keep time spent honing and lapping to a minimum as too much of this can risk bellmouthing the bore or even getting it a bit out of round, and the same with pistons really good turned finish and then polishing with 600-1200 grit emery paper, i think an external hone is only really necessary if you need a specific grade of finish with a crosshatched pattern.
regards Geoff
« Last Edit: February 17, 2016, 08:15:07 AM by kettrinboy »

Offline Brian Rupnow

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Re: Piston/Cylinder lapped fit--no rings
« Reply #5 on: February 17, 2016, 04:33:55 PM »
Ramon--was the external hone you showed made from brass? I just checked out the price of an external scissor type hone as shown in your posts, ands it was over $400.00----Brian

Offline Ramon

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Re: Piston/Cylinder lapped fit--no rings
« Reply #6 on: February 17, 2016, 06:03:18 PM »
Hello Brian - yes just  a simple brass lap as shown is more than sufficient.

I have two lovely sets of Delapena external hones bought off Ebay for a song but having tried them quickly came to the conclusion that a brass lap was infinitely easier to control over a piston length as the hones tend to curve the ends of the piston. This is not to condemn the scissor type as they have other uses such as on crankshafts but I still prefer to use a brass lap on a piston. 

Looking ahead I had also bought a bench mounted Delapena honing machine for bores but so far it has not been used. Though I say it myself (forgive the self praise  :-[) I have had total success on all the diesel engines made so far with just the laps shown. To be fair the diesel does need a much better piston/liner fit than glow or spark ignition but were I to make the latter (it is on the cards at some stage) I would lap first for the fit then very lightly hone the liner only with a sprung brake hone to break the surface.

All this has done is shown to me that desirable as the hones were at the time, for the average home built engine home made laps are more than adequate to give the accuracy and finish required

Hope that helps - Ramon
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline CHP

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Re: Piston/Cylinder lapped fit--no rings
« Reply #7 on: February 17, 2016, 06:09:37 PM »
12x36 lathe,Seig7x10 lathe, Taig lathe
9x29 Mill, Emco 55 CNC mill.......

Offline Brian Rupnow

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Offline Brian Rupnow

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Re: Piston/Cylinder lapped fit--no rings
« Reply #9 on: February 17, 2016, 07:20:08 PM »
Follow Ramons thread and you should not go far wrong, Its worked for the ringless engines I have made. I think I also suggested a steel liner the other day.

J

Yes Jason--I have some 12L14 here , left over from another job. I will make the liner from it.

Offline Brian Rupnow

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Re: Piston/Cylinder lapped fit--no rings
« Reply #10 on: February 18, 2016, 09:40:13 PM »
Thank you to everyone who responded to this post and answered questions I have asked. I have been taken to task on other forums for starting two very similar threads, but my intent was to keep the two issues well separated ,because I figured this thread would probably take on a life of it's own, while the engine build will diverge into different directions. i will not post anymore in this thread.---Brian