Distortion-Free Piston Ring Installation : Pistons with Stacked Crowns

[BronxFigs]

Just recently, I read an article describing, and showing a method for making model engine pistons with crowns consisting of two or more pieces, stacked, in the crown area for easy, distortion-free ring installation. I cannot find the article, nor the photos. I don't even know where to begin to look. A search led me to no results.
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The conventional-looking piston(s) had a modified and removable crown section which was made up of stacked, discs (piston material), some having a smaller diameter, some having full-bore diameter (plus clearances). Conventional, cast-iron rings with correct clearances, end-gap etc. were made, and became part of the stacked assembly, so the piston crown, once installed, captured the ring(s). The piston rings were never distorted for installation, but simply slipped onto the outside of the smaller diameter discs, and capped with a larger disc, or the final, crown section.


CCCCCCCCCCCCCCCCCC
CCCCCCCCCCCCCCCCCC
RR DDDDDDDDDDDDD RR
DDDDDDDDDDDDDDDDDD
RR DDDDDDDDDDDDD RR
PPPPPPPPPPPPPPPPPPPP
PPPPPPPPPPPPPPPPPPPP
PPPPPPPPPPPPPPPPPPPP
PPPPPPPPPPPPPPPPPPPP
PPPPPPPPPPPPPPPPPPPP
PPPPPPPPPPPPPPPPPPPP
PPPPPPPPPPPPPPPPPPPP

C= crown section
R= rings
D= stacked disk
P= Piston skirt


Anyone use this method? I would like to re-read the article....where do I find this information? I hope the above diagram reproduces as I have originally drawn it. It should all be the same diameter...and represent a stacked piston with captured rings.


Frank

 

[Lakc]

Frank,
Your ascii art worked perfectly

Distorted for installation is probably the wrong way to look at it. Iron rings are spread open to maintain wall pressure to make them work, the fact that allows installation is just a bonus. Proper piston ring design will keep installation "distortion" well below its strength so it has no effect.

With conventional construction, iron ring in iron/steel cylinder, and with an aluminum piston, the piston is aluminum for weight reasons. If you use an iron piston you have no need for rings. A multi piece piston would be a challenge to keep the weight below cast iron, and to keep from flying apart. While the idea would certainly work, aside from a little math, there is no real problem with conventional rings that this would fix.

 

[kvom]

The pistons for my A3 locomotive (and many others as well) are made this way. Should be pics in my build thread.

 

[BronxFigs]

Lakc:

Thank you for the response.

I wish I could find the original article, but according to your great answer, it may be just an unconventional solution to a non-existing problem. Why go through the bother of stacking when conventional rings can be installed, and, without problems. The original explanation within the article was certainly plausible, that's why it stuck in my mind, but strangely, I can't remember how the whole piston was fastened together, but it involved screws.

******************************************************************************************************************************************************


Let's assume that one uses this construction with an aluminum piston with cast-iron rings running in a steel cylinder...how do you prevent the crown screws from loosening over the many heating/cooling cycles? Would a certain formula Loctite work?

 

[Lakc]

Let's assume that one uses this construction with an aluminum piston with cast-iron rings running in a steel cylinder...how do you prevent the crown screws from loosening over the many heating/cooling cycles? Would a certain formula Loctite work?

Loctite has a solution to almost every problem, but you may not like the price.
In a home shop environment, I would only use some type of mechanical lock. Safety wire would be tempting, but more then likely some type of keyed washer with foldable tabs.

 

[kvom]

Here's the post with pics of the parts: http://www.homemodelenginemachinist.com/f31/kozo-a3-1-5-scale-10775/index13.html#post142377

The head and rings are cast iron, and the piston rod and screws are stainless.

Post #118 on the previous page has a drawing.

 

[enfieldbullet]

simplicity usually pays off.

imagine the engine rev'ing at 4000 rpm and that screw decides to go loose and play pinball with the piston and the valve head.

it is indeed an interesting design, but not very practical IMO because there's no good way to fasten the parts.

 

[BronxFigs]

kvom, bullets, et al:

The links and photos were great. I can't recall just what type piston the original article was discussing, but the fasteners coming loose is not appealing at all.

I just thought it was an interesting construction technique, but there is little reason, if any, to use this method, I have learned from the comments.

Thanks for all the thoughts.



Frank

 

[enfieldbullet]

don't get me wrong, as a horology geek i love things that are complicated without need.

discussing every mechanism is valid.

but i would be more confortable with brazing on this case.

the threaded fasteners would probably never go loose during service on a model engine. on a full scale they might. the key problem here is that it would be very difficult to check for that(remove head) during normal service. thus it would most likely only be noted when it came completely loose and smacked the valves.

on a slow rev engine i don't see a problem and would make it easier for some kinds of rings, PTFE comes to mind.(such as a steam engine)

 

[kvom]

The Kozo design is for a steam engine operating at 300 degrees and relatively low RPM. The piston needs to be thin as it's double acting. Red loctite thread locker is reliable at these temperatures (often used for ring gear bolts in auto differentials at higher temps and RPMs).

An IC engine obviously has different needs. OP didn't specify the type of engine he's building.