pistons?

[Aydelott]

my print say to fit the piston to the bore but how much play should i leave for the rings im making them from nylon

 

[lensman57]

Hi,

That is an interesting question as when I made the piston and the rings for the Webster engine following instructions, the result was not good. There also appears to be some disagreement amongst the more knowledgable and experienced members as to the size of the piston and as a result the dimensions of the rings required. In the end I opted for a viton O ring, as there appears to be a more unfied approach to the size of the piston and the dimensions of the groove ring. Perhaps you could follow the viton route and used the published data from the ring manufacturer to build the piston.
If you search for Brian Rupnow's build log of the Webster engine on this site and navigate to page 14 you should be able to see a photo of the chart with different sizes of O rings and the required piston and ring groove sizes. Maybe this would be of help to you.

Good luck,

A.G

 

[Brian Rupnow]

My knowledge of piston to cylinder fit is as follows. First make the cylinder and bore or ream to finished size. If you have a hone, use it to get a very smooth inner surface on the bore. I use a brake cylinder hone which is 3 spring loaded arms with a stone about 1" long on each spring loaded arm. They are inexpensive to buy. Make the piston .002" less in diameter than the bore of the cylinder, and good luck with that!!! Bigger won't go in, smaller falls through and you can start another piston. Attach the con rod to the piston, coat the piston with 600 grit compound and work the piston back and forth thru the bore with a twisting motion. Some very brave souls mount the cylinder on their lathe and turn it at a low speed while doing this. (I've grown rather attached to my two arms over the past 66 years, and I want to keep them.) I strongly advise and promote Viton o-rings, as I have used them with great success. As Lensman says, look up my Webster thread and you can use the same spec I did for ring groove. The groove should be .055" to .057" deep and 0.095" wide, for a 1/16" nominal cross section o-ring. The outer diameter of the o-ring should be whatever the bore of your cylinder is. The smooth finish on the inside of the bore is critical so that you don't tear up the o-ring on a rough surface. The end of your cylinder opposite the cylinder head should have a nice angled lead into it (about 30 degrees) and you should "break" the sharp edge with some emery paper while the cyl is turned at about 1000 rpm in the lath---And for Gods sake, be carefull of your fingers!!! The reason for this angled lead in and the breaking of any sharp edge is because the piston with the Viton o-ring on it is NOT going to want to go into that bore. Coat the entire piston with the slimiest, gooiest oil you have access to (STP works good hear). You will need a small press or the old hammer and block of wood to get that sucker into the cylinder----And if there are any sharp edges, you will shave peices of the o-ring, rendering it useless. It will also be stiffer than hell after installation, so again, slop lots of oil into the cylinder, hook the con rod up to the crankshaft, hook up your variable speed drill and turn the engine over at about 1/3 your drill speed for about 1/2 an hour to loosen things up. If you are going to put a ring groove in your cylinder, do it before you lap the piston into the bore, and be aware that when you cut the ring groove this is going to create a "bulge" of material on each side of the groove, so you will want to take some fine emery held against the side of a flat bar or file and dress the o.d. of the cylinder very lightly as it turns in the lathe to get rid of these bulges of material but not take any more of the outer diameter of the piston.---Brian

 

[ART]

Hello,
I might have missed something, but are you asking the question for a steam engine or gas engine. I'm not sure nylon rings would work very well in a gas engine. I have used them in steam engines which I run only on compressed air and they work fine with many hours of running time.
Art

 

[Aydelott]

its a steam engine Rudy K/s model marine engine the pistons are 1.25 i doubled the size of the prints

 

[Brian Rupnow]

My information given was for internal combustion engines. However, all the info holds true with the exception of the Viton o-ring. You can buy ready made cast iron rings, and there are numerous articles on building your own cast iron rings. I don't know of anyone who has succesfully used a nylon or plastic ring in a steam engine. Many of the nylons are hygroscopic which means they soak up the moisture in the steam and swell to the point where they seize the piston in the bore. However, I am not the worlds foremost authority on steam engines, I'm just repeating what i have read on this forum about plastic and nylon rings.

 

[Tin Falcon]

I run all my engines on compressed air. no rings except my PMR engine that has teflon rings.
if you plan on running on compressed air delrin should be fine . steam I am not sure would have to do research.
tin

 

[lensman57]

its a steam engine Rudy K/s model marine engine the pistons are 1.25 i doubled the size of the prints

Hi,

You may be able to get away from using rings all together , if you are using air or none super heated steam. Cast iron rings and steam, I am not sure about, but the other stuff will work. You may have to experiment in the end to see which one suits best. Nylon, Teflon, Ptfe, Viton and even some felt are all fine so long as the operating temp does not exceed the design Temp. Dimensions are up to you.

Good luck,

A.G