Author Topic: Silicon piston rings help.  (Read 3310 times)

Offline Gas_mantle

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Silicon piston rings help.
« on: June 13, 2016, 06:51:45 PM »
Hi,

I'm wanting to replace the silicon piston rings on a small 1" bore engine I've just about finished building and hope someone can help.

When I installed the existing 'O' rings they were an afterthought to already machined cylinders and pistons - I wasn't having much success with graphite yarn as rings so I just put in the closest fit of rings I had onto the existing pistons, in this case I used 15/16 OD in 25mm cylinders and stretched them to fit over the pistons - they work ok but having now stripped the engine down for painting they seem very worn and I like to have another go at it replacing the pistons if necessary to accomodate new rings better.

It's not really practical to alter the cylinders and at 25mm they are between sizes for the rings I've ordered (I've ordered 15/16 and 1").

I'm not really sure how things should fit once assembled - If I use 1" rings in a 25mm bore will that be too tight ? The ID is 3/4" so do I cut the piston grooves to exactly that bearing in mind 1" may be a bit tight in the bore ? How close a fit should the pistons be in the bore ?

I appreciate I'm doing this job a bit late in the build and it will be a bit of a compromise but I'll have to work with the situation I'm in.

The engine is only intended as an ornament and is unlikey operate under any load and once run in will probably only run for short periods once in a while.

Any help would be great, thanks.
Peter.

Offline Jasonb

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Re: Silicon piston rings help.
« Reply #1 on: June 13, 2016, 07:17:26 PM »
As you have a metric cylinder bore why not get metric O rings.

2.4mm section 20.6mm ID ring will fit the bill

Make the grooves 2.6mm wide and the bottom of teh groove 19.6mm dia.

As the OD of the ring is over the nominal bore it will squash down to a nice fit and teh wider groove will accomodate the oval shape it will take on.

Vitron is fine, does not need to be silicon

http://www.ebay.co.uk/itm/METRIC-VITON-75-HIGH-TEMP-O-RINGS-2-4MM-CROSS-SECTION-SIZES-16-3MM-ID-69-6MM-ID-/371503929586?var=&hash=item567f5c18f2:m:mLV1qpGwPEaiHdLOk7CJ4oA
« Last Edit: June 14, 2016, 01:24:35 PM by Jasonb »

Offline Gas_mantle

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Re: Silicon piston rings help.
« Reply #2 on: June 13, 2016, 07:30:40 PM »
Thanks Jason,

If I'm honest I never thought of metric rings - the supplier I have used in the past has only imperial so I without thinking just ordered 2 of each size slightly above and below. I'll order some metric as you suggest.

At the moment the width of the groove is a tight fit on the ring so it can't rotate, am I right in thinking the rings should be able to 'roll' a bit - maybe that explains the wear ? 

At the moment the rings both have a flat on the outer edge - they still give a good seal for now but my worry is they will soon wear.

Offline Vixen

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Re: Silicon piston rings help.
« Reply #3 on: June 13, 2016, 08:42:55 PM »

2.4mm section 20.6mm ID ring will fit the bill

Make the grooves 2.6mm wide and the bottom of teh groove 19.6mm dia.


Do what Jason suggests and make the groove wider than the o-ring section, the compressed o-ring needs room to expand into. The width of the groove should never be tight.

Mike
It is the journey that matters, not the destination

Offline derekwarner

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Re: Silicon piston rings help.
« Reply #4 on: June 13, 2016, 11:48:37 PM »
Silicon elastomer [MVQ] o-rings will be difficult to source in the sizes you are after, however Viton [FPM] will be readily available
A standard 2-117 0-ring is 20.29 ID x 2.62 section

Viton is suitable for steam to ~~ 200 degrees C and has far greater dimensional stability at elevated temperatures. This is why most elastomer manufacturers list Silicone as not suitable for dynamic applications, and marginal only as a static seal

The fluid compatibility table below confirms these recommendations................ Derek

 
« Last Edit: June 14, 2016, 05:05:50 AM by derekwarner_decoy »
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Offline Brian Rupnow

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Re: Silicon piston rings help.
« Reply #5 on: June 14, 2016, 01:37:31 AM »
For what it's worth---I use Viton rings on all of my internal combustion engines. (11 and counting). I only use one ring on the piston. A 1/16" cross section ring is actually 0.070". I make the ring grooves .093" wide (I have a 3/32" parting off blade) and I cut the groove .057" to .060" deep. They last very well, are very inexpensive, and seal absolutely air-tight.---Brian

Offline Jasonb

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Re: Silicon piston rings help.
« Reply #6 on: June 14, 2016, 07:44:22 AM »
Derek, how do you find the imperial 117 ring performs in a 25mm bore. As its usual to run model O ring piston seals at less compression than commercial practice I would have thought the larger OD would make things tight.

Similar with Brian's groove sizes as these are for imperial spec rings in imperial holes

Peter, make sure you have a lapped bore so the ring does not wear on any ridges left by the tool

Offline derekwarner

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Re: Silicon piston rings help.
« Reply #7 on: June 14, 2016, 10:12:40 AM »
Jason.......... I have no personal experience in using a 2-117 o-ring in a 25.0 diameter bore, however simply looked at what was readily available as a standard sized element [the 2-117 has a theoretical OD of 25.53 diameter]

Viton elastomer of ~~70 Duro is relatively soft, and will compress without any loss of mechanical or physical properties at our model steam temperatures of ~~ 150 degrees C, providing diametrical clearance by way of cavity width/ID is provided

We must also remember that all of the cavity dimension tables be they British, American or Australian Standards are based upon either 200 or 250 Bar fluid design criteria and so these tables are created for these humble o-rings being subjected to these pressures

We naturally are talking model applications of say 3 to 5 Bar

A Japanese [JIS Series] o-ring # PO21 is 21 ID x 2.4 section, however I am not aware of a 2.4mm section x 20.6mm ID o-ring element

Derek
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Offline Gas_mantle

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Re: Silicon piston rings help.
« Reply #8 on: June 14, 2016, 10:50:12 AM »
Thanks guys,

What I have now done is ordered some 25mm Nitrile rings as well as the 15/16 and 1" once they all arrive I'll have a try at fitting the 25mm ones first.

Part of the problem I have is as this is an after thought I'm now stuck with 2 cylinder / valve chests permanently fixed together and it's difficult to do much to the cylinders. With the limited tooling and experience I have the bores haven't been lapped.

To get a smooth bore when I made the cylinders all I did was while they were still in the lathe was use some very fine emery paper to smooth out the finish left by the boring bar. I appreciate that is a bit amateurish but it seemed the best I could do with the tools available. To be fair the bores seem very smooth to the touch and the engine has had a few hours running with the existing silicon rings.

Do you think it's worthwhile buying one of these hones ?

http://www.ebay.co.uk/itm/SEALEY-VS029-Engine-Cylinder-Hone-Honing-Kit-4-in-1-/161979844801?hash=item25b6c048c1:g:S3kAAOSwPc9Ww0jo

Once this engine is finished I'm intending to make 2 other different engines so a hone of some sort seems to be the way forward.

Thanks
Peter.

Offline Vixen

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Re: Silicon piston rings help.
« Reply #9 on: June 14, 2016, 11:00:43 AM »
The materials used to make 0-rings are elastomers.  When the material is compressed in one direction, it expands in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect (ref. Poisson ratio). The material is not actually compressed as it still retains it's original volume. That is why it is essential to make the groove wider than the section of the o-ring to accommodate the sideways expansion.
Mike
It is the journey that matters, not the destination

Offline Jasonb

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Re: Silicon piston rings help.
« Reply #10 on: June 14, 2016, 11:08:39 AM »
Derek, the e-bay link I posted has the 20.6 x 2.4.  I have happily used these in a model.

There are sets of tables that show groove widths and depths to give the  reduced amount of compression for model use as if you use the regular standards the friction becomes too high. Reeves and ME Handbook are two that spring to mind but both are based on imperial bores

Peter, as you have already used Emery that should be OK unless it was very coarse. The Hone you link to will be OK for iron rings but a lapped finish is finer and less abrasive to the rings, Also your aluminium cylinders will soon clog the stone.

Offline derekwarner

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Re: Silicon piston rings help.
« Reply #11 on: June 14, 2016, 11:57:08 AM »
Mike....

Whilst we have digital surface measuring devices down to Um [microns] of units termed as Ra & Rt  we also have an HB pencil to rub in or across  & see the carbon deposit, or to gauge by our thumb nail to feel a level of surface [roughness]  :ThumbsUp: we feel is suitably smooth ......[which is not a technically quantifiable unit]  :headscratch:

The latter has been universally used for some 100 years to understand the surface of cylinder bores

Jason....

I acknowledge the WEB reference to 20.6 x 2.4 o-rings, however these were nominated as Nitrile which is suitable to application's to <100 degrees C

Derek
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Offline Jasonb

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Re: Silicon piston rings help.
« Reply #12 on: June 14, 2016, 01:24:11 PM »
Opps wrong ebay link, these are the vitron ones from the same company, I have corrected my earlier post

The Nitrile will be fine for air running

http://www.ebay.co.uk/itm/METRIC-VITON-75-HIGH-TEMP-O-RINGS-2-4MM-CROSS-SECTION-SIZES-16-3MM-ID-69-6MM-ID-/371503929586?var=&hash=item567f5c18f2:m:mLV1qpGwPEaiHdLOk7CJ4oA

Offline Gas_mantle

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Re: Silicon piston rings help.
« Reply #13 on: June 14, 2016, 01:38:21 PM »
No probs Jason, I can see what sort of difference the Nitrile ones make and consider Viton at a later date if need be.

For the time being the engine will only be running on air but at some point in the future I'd like to try running things on steam.

Offline crueby

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Re: Silicon piston rings help.
« Reply #14 on: June 14, 2016, 03:52:29 PM »
I don't know about the Amazon. com in UK, here in US they have the viton ones in bags of 25,  50 or 100 for just a few dollars, depending on size.