Piston Ring questions

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vcutajar

vcutajar

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Good morning

I need again to tap your expertise.

I have just finished honing the cylinder liner for my Kiwi MK II and would like to ask some questions about piston rings. I have searched the internet and this forum but could not find exactly what I need.

The piston rings that came with the kit are slightly large. If I compress them they will not enter the liner. So I guess I have to file one side of the gap so that they will fit inside the liner. I understand that when the ring is in the liner there should be a gap (I think they call it end gap). How much should be the gap? I have found that the gap on the first ring should be 0.004" times the bore diameter and 0.005" times the bore diameter for the second ring. These are for full size engines. What gap would you use for a model engine with a bore diameter of 1"?

How do you insert the piston with the rings in the bore. For full size engines a ring compressor is used but I could not find any which handle a 1" piston.

Also am I correct when I say that the piston with rings and conrod should be inserted from the top of the cylinder?

Thanks in advance

Vince
 
HI
When I built my Nemett 15cc engine I turned a shamfer on the bottom edge of the liner and inserted the piston and conrod from the bottom. I eased the rings in with a small screwdriver, working round the edge.

The designer specified 0.125mm for the ring gap.
The bore was 1".

Roy
 
Hi Vince;

.004'' is a good target for all ring gaps, I usually try for a little less and a careful break in.

When measuring the end gap use the piston to push the ring into the bore to ensure it is
square to the cyl.

If the gap must be filed, go easy, a couple strokes at at time as the gap opens quickly.
Keep the ends as square as possible.

Depending on engine design the piston usually installs from the head end.

A small chamfer in the cylinder is a good idea.

I use thin brass shimstock as a ring compressor, about one and a half wraps tied with
brass wire to squeeze the rings. Make sure the rings are seated in the grooves when commpressing
and plenty of oil on the rings and in the cylinder. Gently push or tap the assembly into the bore.

Peter


 
Hi Vince,
In answer to some of your questions. As to the ring gap, for a cylinder of 1.00 bore I would set the end gap at no more than .003. To insert the piston/rod into the bore there are a couple of ways to do this. The first way is to make your own ring compressor. Just take a piece of shim stock (.010) and wrap it around the piston. With a piece of tape pull it tightly to compress the rings and then tap the piston into the bore. The second way is to turn up a bushing to match your bore diameter. It doesn't need to be very long, just about as long as the piston. Put a nice chamfer on one end to ease the piston into it. Set it on the cylinder with the piston slightly protruding to get it started in the engine bore. Now holding it tightly against the head face just tap the piston into the bore.
As a note it will help to put a small chamfer at the top of the bore. Just so you don't have a sharp corner.
On some engines the width of the conrod won't go through the bore so the assembly will have to be inserted from the bottom but that depends on the configuration of the engine, open or closed crankcase.
gbritnell
 
Hi Vince. Just a precaution, when filing the gap clamp as much of the ring as possible with just the bit to be filed exposed. Bob (Maryak) shows a good setup in his "Hit me - miss me" article. Good luck
Herbie
 
George;

I like your idea of a sized bushing to install rings, I think that would work well on very small bore sizes.
The inside of the bush could even be tapered to the bore size to smoothly compress the rings.

Peter
 
What Peter has described is what is commonly used in production and race engine building - a tapered ring compressor. Generally a dedicated ring compressor is made for the bore size being dealt with (production remember). However; lately they have developed a slotted compressor that at least allows you to deal with a limited range of bore sizes with one compressor and would probably be better for model engine builders given that each piston/bore will be SLIGHTLY different. Similar to this;

http://www.benchmarkarizona.com/Com...sor__4_250_quot__to_4_310_quot_-8-37-196.html

I don't know what taper angle they use but for us I don't think it will be critical.

Cheers Garry
 
here's another design in plastic that looks interesting (although I'd be concerned about sharp rings shaving off some plastic and jamming between the ring/piston/bore);

http://forums.triplezoom.com/showth...d.-how-do-you-use-Piston-ring-compressor-tool

Don't be thrown off by the angle cut on the bottom of the tool because it looks like the engine is a VW V6 which has a cylinder block face machined at an angle - not 90 degrees to the bore, so a tool cut 90 degrees would not work.

A flexible tool like this in brass or aluminum might work well on a nominal bore size where each cylinder is a few thou different in bore diameter. Plenty of ways to skin a cat!!!

Cheers Garry
 
Just use a flat auto hose clamp, they work perfectly as a ring compressor, and come in all different sizes.
Install it on the piston, and tighten just enough for a sliding fit, and just enough to compress the ring(s).

Pat J
 
Quick and dirty

split piece of copper pipe and self locking pliers.
 
Thanks guys for the replies.

I have the chamfer at both ends of the cylinder liner so I guess I can insert the piston from either end.

Now I have a good target to aim for regarding the end gap (I could not find any info in Westbury's article or I missed it). And also you gave me a couple of ideas to experiment on regarding the piston ring compressor.

Again thanks. Much appreciated.

Vince
 
I'm reading this thread with interest as I'll need to be able to do this in the near future. It occurs to me a variation on what has been suggested could be some thin sheet wrapped round the piston and pulled together with a couple of 'tywrap" cable ties would do the trick.

Pete

 
Or else, I was thinking of making an aluminium bush, split heightwise and compressing it with a jubilee clip. Still need to test it out.

Vince
 
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