Author Topic: A new attempt at making piston rings  (Read 10428 times)

Offline Laurentic

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Re: A new attempt at making piston rings
« Reply #135 on: May 28, 2021, 10:54:50 PM »
Good question!!

Offline Brian Rupnow

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Re: A new attempt at making piston rings
« Reply #136 on: May 29, 2021, 01:51:58 AM »
Chris---Best answer---I don't know. I am still dabbling in something that I don't fully understand. This ring making business is one of those things where I know all (or most of) the theory, but am failing badly with the practical application of it. People with less technical background and less practical experience than me are making rings successfully, but somehow it eludes me. I'm sure that there are all kinds of model engine builders who are making  their pistons .002" smaller than their cylinder bore and making perfectly acceptable rings to hold compression, but I'm not there yet. I don't want to start any more engine builds until the fall, and I had hoped that if I spent a bit of time on ring making this summer I would have it mastered by fall. Right at the moment, I'm not nearly so sure of that. I know that an aluminum piston must be .002" smaller than the cylinder bore, because aluminum has a higher expansion factor when heated than iron has, and will "seize" in the bore when the engine heats up if it doesn't have that clearance. I think that the people who have had success with ring making do leave .002" clearance on their cast iron pistons as well, but I'm not 100% sure of that.

Offline Brian Rupnow

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Re: A new attempt at making piston rings
« Reply #137 on: May 29, 2021, 05:44:40 PM »
The news is positive. With new cast iron cylinder, piston, and rings, the engine has high (to coin a new word) Suckability!!! It fired, it ran for three short blasts, but I'm having head gasket issues. The aluminum head has a ring of material that fits down into the top of the cylinder about 0.060". Or, let me rephrase that--It is supposed to extend down into the cylinder about 0.060" minus the 0.030" thickness of the head gasket. A close examination of the cylinder head shows that that ring of material is "hanging up" on the inside of the bore and not letting the head bolts tighten the cylinder down evenly all the way around. I'm going to set the cylinder head up on the lathe and trim a tiny bit off that diameter that is giving me problems.

Offline Brian Rupnow

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Re: A new attempt at making piston rings
« Reply #138 on: May 30, 2021, 09:48:09 PM »
I'm having a no work Sunday---I need it. But the old brain cells keep clicking away. Every thing I read about rings recommends that the ring thickness should be  1/30 to 1/25 of the cylinder bore (and the depth of the ring can match the width so your ring actually has a square cross section). On a 1" bore, that gives a ring width of 0.033" to 0.040". The width of the groove in the piston should be 0.001" greater than the width of the ring itself. So---If I made my rings 0.038" wide, then the groove in the piston should be 0.039"---And 1 millimeter is .0394". Okay, cool!!! Now, where do I buy a 1 mm wide grooving tool? I have googled this and still have no clear answer. I can turn a lathe tool for general purpose turning from HSS, but I don't trust myself to be able to make a 1 mm wide grooving tool. I don't want to spend a zillion bucks on this either. My lathe has a 12" swing, and it has a quick change toolpost on it. I prefer 3/8" square tooling, but in a pinch I can mount 1/2" square tooling.  I'm not averse to using inserted carbide if the price for the carbide and the appropriate holder don't break the bank. Since the piston groove will be in cast iron or aluminum, I could even work with  1mm wide HSS and a 3/8" square shank.---Thanks.---Brian

Offline simplyloco

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Re: A new attempt at making piston rings
« Reply #139 on: May 31, 2021, 08:30:15 AM »
I'm having a no work Sunday---I need it.
SNIP
Since the piston groove will be in cast iron or aluminum, I could even work with  1mm wide HSS and a 3/8" square shank.---Thanks.---Brian

Hi Brian. Carbide grooving tools get expensive in small sizes. I have found that a Dremel rotary tool fitted with resin discs cuts HSS relatively easily, removing much of the drudgery associated with grinding fine tooling. You need just a small stub tool, and rake and clearance angles aren't that critical for this job so it wouldn't take much effort to get a result. Have a go!
John

Offline jonesie

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Re: A new attempt at making piston rings
« Reply #140 on: June 01, 2021, 01:33:09 PM »
why not use a 1/32 parting tool and just step over to get the right width,or just grind down a 1/16 one undersize and do in 2 cuts ??

Offline Brian Rupnow

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Re: A new attempt at making piston rings
« Reply #141 on: June 01, 2021, 08:40:42 PM »
Time has come to cry FAIL!!!! I can not get the engine to fire, even with new piston, purchased new cast iron rings and new cast iron cylinder. Valve timing has been reset, ignition timing has been reset, carburetor has been adjusted to every possible configuration, and the engine doesn't fire. Compression feels cheesy when flywheel is turned by hand. If I squirt a bit of oil down the sparkplug hole, the engine has great compression and starts right up and runs like a trooper until the oil on top of the cylinder has burned away. Then it quits. This is purely the result of too low or no compression due to leaky piston rings and "fit" of piston in cylinder. I have monkeyed with this engine (which ran just great with a Viton o-ring) until I am sick of it. I have received a lot of interesting opinions on ring making, piston making, and state of the world in general from forum members and yes, I do read and pay heed to all of it, thank you. Later in the summer, when I have had time to digest all the new information and I'm not so burned out, I will use my new heat treat oven and toolpost grinder and make a new set of rings 0.038" wide and possibly another new piston with grooves to accept the new rings, I may try again. For now though, I'm whipped. Thank you to all of you who have followed this thread, and for the information given.----Brian

Offline propforward

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Re: A new attempt at making piston rings
« Reply #142 on: June 01, 2021, 08:43:04 PM »
I hope you will come back to it in a few months, because I think you will get it. Good idea to take a break for a bit though.
Stuart

Offline Dave Otto

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Re: A new attempt at making piston rings
« Reply #143 on: June 01, 2021, 11:10:28 PM »
Brian

You have spent a considerable amount of time working on the rings and cylinder bore, but have you taken a critical look at the condition of your ring grooves?
I have been thinking about this, and have not see anyone else mention it. The grooves of coarse need to be of proper size but the grooves also need to have a good finish, and the walls perpendicular to the axis of the piston. The bottom surface of the groove is as much of a seal as the interface between the ring and cylinder wall.
Have a look at the attached drawing and you can see what I'm talking about.

If your grooving tool is not dead sharp and square to the world your ring lands may be tapered and not sealing properly. This is a common problem in the old one lung antique engines where they have worn this way over the years. Guys will hone the cylinder and put new rings in the old grooves and then still have low compression. The common procedure to repair this is to re-cut the grooves to square them up and and add a ring spacer above the ring. Or to cut it large enough to stack two thinner rings in one grove.

Hope that maybe this helps.
Dave

Offline Brian Rupnow

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Re: A new attempt at making piston rings
« Reply #144 on: June 02, 2021, 12:06:19 AM »
Dave--the ring grooves were new, on a new piston, and they were as square and true as I could make them. There may have been a bit of chatter in the bottom of the groove, but the sides were both very clean and well machined.----Brian

Offline Dave Otto

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Re: A new attempt at making piston rings
« Reply #145 on: June 02, 2021, 01:00:58 AM »
Ok sounds like you have it dialed in.

Offline Brian Rupnow

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Re: A new attempt at making piston rings
« Reply #146 on: June 02, 2021, 01:44:23 AM »
This is a good time to ask a theoretical question. If I had a Viton ring on a piston, and shoved the piston half was down a length of cylinder, there would be no leakage at all past the Viton ring. A Viton ring seals perfectly around it's full circumference. If I stood the cylinder on end and filled the top half with Varsol or any other fluid, none of it would leak past the Viton ring---ever. That is a given. I have done that.---Now, if I take a piston with two cast iron rings on it, each ring having a 0.004" gap between the ends, and the gaps set at 180 degrees to each other, it will leak. If placed half way down a cylinder and stood on end, and if I fill the top half of the cylinder up with Varsol or some similar fluid and left overnight, some of the Varsol would leak past the rings and be puddled in the bottom half of the cylinder. Iron rings are not a perfect seal. The only reason that they work in an engine is that during a typical engine cycle, there isn't enough time for the compression to leak down past the rings. Some does--but there isn't enough time for the compression to degrade below a point where ignition is no longer possible. I ask this, because I would like to make a testing rig which would let me know if a piston and rings were "good enough" to make an engine run.-----------Why would I do that? ---Because on a real engine there is always a lot more involved in changing the rings and piston than just the cylinder bolts. All I can think of is a leak-down pressure test. Problem is, the volume of a 1" bore x 1" stroke engine is so small, it would require far better and more sensitive equipment to do a leak down pressure test than anything I currently have.---And even a leak down pressure test is only comparative. I would have to perform the test on a "good" piston and ring set to form a baseline from which I could measure the effectiveness of my own piston and rings. The ONLY way I know how to test the effectiveness of a ring and piston set, it to put it in an engine and see if the engine runs.---Any comments???

Offline Art K

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Re: A new attempt at making piston rings
« Reply #147 on: June 02, 2021, 03:49:22 AM »
Brian Et Al,
I am trying to remember something I saw about piston rings. I can't find a reference but. The gist of it goes like this, If you look at Dave's diagram the force of compression as the piston moves up towards TDC would be sort of like the arrows on top and behind the compression ring helping to force the rings out. It also seems that the ring is also tilted out at the top, also so due to compression forces. This force pushing out the ring is higher as the compression ratio goes up. Don't know that this is helpful but I thought I'd throw it out there.
Art
"The beautiful thing about learning is that no one can take it away from you" B.B. King

Offline Roger B

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Re: A new attempt at making piston rings
« Reply #148 on: June 02, 2021, 05:05:21 PM »
I still think the problem is the cylinder bore. If I look at what you do compared to what I do that is the key difference:

Piston rings: made with far less care and attention than you give.

Ring groves: cut with a cheap 1.5mm parting tool.

Bores: Lapped with an Acrolap and 40 micron diamond paste. Piston and rings made to fit using a plug gauge.

This has always given me good compression and instant starts (not instant good running, that takes more effort).

This engine is nominal 1" bore 2" stroke. All I did to prepare for this video was to remove the generator drive belt.



Best regards

Roger

Offline Dave Otto

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Re: A new attempt at making piston rings
« Reply #149 on: June 02, 2021, 05:48:19 PM »
I kind of had to cringe when Brian described the lapping process of his latest cylinder, it all sounded pretty brutal to me. :shrug:
I agree Roger there still is no guarantee that his bore is truly round, and most of us do understand that roundness can't be measured with a two point contact measuring tool.

Dave