PISTON AND CYLINDER SIZE. (O-Rings)

[EVAN WYNES]

Hi all....I recently finished my Hoglet and after a little fiddling' got it running. But after a few minutes it stopped and I could not get it to start. To make a long story short while taking it apart I noticed that the cylinders pulled off the pistons with very little resistance. I replaced the O-rings, which are Viton rings, and after putting it together she fired right off, but after about 2 minutes it stopped and again...couldn't get it running. Took it apart and noticed the same thing when pulling off the cylinders. The cylinder bore, by spec., is supposed to be 1.000-1.001 and the piston diameter is supposed to be .996. My cylinders are 1.002 and piston #1 is .998 and #2 is .999. The groove depth is supposed to be .068. The grooves measure .067 deep. I realize that I am over by .001 on the cylinders, over by .002 and .003 on the pistons, and the o-ring groove is short .001.
It appears from examine the rings that have worn down causing a loss of compression. So, my question(s) is/are...are my measurements off that much to cause the rings to wear way too soon? Is there a standard clearance between the size of the cylinder bore and piston diameter and ring groove depth. Any advice you all could give would be greatly appreciated.
Thanks

 

[Andrew Stroud]

I would suggest you check the surface finish on the cylinder walls and compare with the O ring supplier's design advice - in a reciprocating application a rough surface will eat the rings.

 

[Vietti]

Agree with Andrew. My Hoglet ran for a few hours at first and then the compression fell way off. Put in new rings and all was OK from then on. I think the O rings need a really nice smooth surface to run with. Previously I tried them in engines that had been running cast iron rings and the cylinder bores were really smooth, almost polished, and have never had to replace the O rings.

You might consider making the grooves a little deeper, I think the O rings should be allowed to float in and out a bit and expand when the compression hits them. Might even widen the grooves a little.

 

[Andrew Stroud]

Regarding your groove dimensions again I would suggest you check with the supplier's design guide, it will give the groove dimensioning needed with tolerances for your application. I use the James Walker guide, always works for me.

 

[actinaria]

I am in the process of building a Hoglett. What do you use for fuel? Do you add oil to it?

 

[Jasonb]

You really want a finely lapped surface to the bore, even a brake cylinder hone leaves things too rough.

I would also avoid using most makers suggested groove sizes as they compress the rings too much for our use. Aim for a groove 5% shallower than the actual rings diameter. So if using nominal 3/32" rings which have an actual cross section of 0.103 make the groove depth 0.098" less any piston clearance.

 

[danallen]

I am in the process of building a Hoglett. What do you use for fuel? Do you add oil to it?

I run mine on 50:1 premix.

 

[Lloyd-ss]

The surface finish (cylinder bore) should have a very smooth finish, almost polished if possible. Have you looked at the failed o-rings under magnification to see if the outer surface is worn or damaged? Given your stated actual dimensions of your engine, the groove depth including the piston clearance is .069. If you are using a size 020 o-ring (???), the actual dia of the o-ring is .070 which only gives .001 compression on the o-ring.

I realize this isn't a standard application, but as a starting point, for a 1/16 nominal (.070 dia) o-ring, for a dynamic application, should have a gland depth of .052-.060 and a width of .093-.098.

For a 3/32 nominal (size 117) (.103 actual dia) the actual gland should be .085-.093 deep and .140-.145 wide.

O-rings are usually pretty forgiving and you can play all sorts of tricks including changing material (more wear resistant), going to higher duro o-ring of the same material, using back-up rings, and using something other than a standard round cross section. Check McMaster Carr. They have lots of odd metric sizes that can fit a groove that is a bit off.

But first, look at the failed o-ring and see what happened. There are lots of options on fixing the situation.

 

[Tidza]

I run my hoglet on methanol, which is more compatible with the silicon hoses. It doesn't mix with conventional two stroke oil but is OK with Castrol R which is castor oil based. 50 to 1 is plenty strong enough.

 

[EVAN WYNES]

Thanks to all that replied with your suggestions, I greatly appreciate them. I will definitely take a closer look at the rings and the finish of the cylinder bore and give it another go.

 

[Stefan-K]

Hi
In my judgement all IC engines should ever have Metal Piston rings. Just to avoid the said Problems. In the end you will have to constantly change the Viton Rings due to wear. No matter how polished your Cylinder surface will be. This does not happen with precisely made metal rings. The additional expence pays off everytime. There is a reason why all Engines have Metal Piston rings.

I´m using a 1:40 Gas/Oil mixture for all my engines. As long as you dont have a completely closed Oil circuit with Oilpump and all what is needed you should allways use a mixture. As the Hoglet has an open Crankcase and no oil circuit at all, you must use a mixture. Due to the piston size of the Hoglet there are ready made Piston rings available. So you dont have to make them on your own.

Stefan

 

[Jasonb]

The other option is to change the aluminium pistons for iron and then you don't really have to bother to fit any rings. 1000s of glow and diesel aero engines running with no rings.

Just finished this engine and tried it yesterday, although grooved for an O ring I did not bother fitting it but was running within 10mins of first trying it.

 

[a41capt]

Hi
In my judgement all IC engines should ever have Metal Piston rings. Just to avoid the said Problems. In the end you will have to constantly change the Viton Rings due to wear. No matter how polished your Cylinder surface will be. This does not happen with precisely made metal rings. The additional expence pays off everytime. There is a reason why all Engines have Metal Piston rings.

I´m using a 1:40 Gas/Oil mixture for all my engines. As long as you dont have a completely closed Oil circuit with Oilpump and all what is needed you should allways use a mixture. As the Hoglet has an open Crankcase and no oil circuit at all, you must use a mixture. Due to the piston size of the Hoglet there are ready made Piston rings available. So you dont have to make them on your own.

Stefan

Agreed! The knowledge gained from making your own cast iron rings is extraordinary. My early examples left a lot to be desired, but with practice added to gained knowledge, they’ve become an integral and looked forward to part of the experience.

I highly recommend giving it a try!
John W