Piston size?

[Mark Gould 1Participant @markgould1]

Gents,

We are about to start machining the piston for our Stuart no. 1. I have a few questions regarding this part.

1. How much clearance should we allow the piston to have relative to the cylinder?

2. I have seen a few youtube videos where silicone piston rings are used instead of the Stuart supplied rings. Which should we use?

3. The original drawings (imperial) show a 2 ringed piston and my build book (metric, also from Stuart) show a strange looking part that doesn't even seem to have a piston ring groove in it.

Any help is greatly appreciated, many thanks,

Mark

[Mark Gould 1Participant @markgould1]

[JasonBModerator @jasonb]

Are you going to run on steam or air?

The odd looking part is possibly one of two "half pistons" when joined together small diameter to small diameter that creates a groove.

[Mark Gould 1Participant @markgould1]

Jason,

Thanks, that explains it then. I couldn't figure out why it would be made from 2 halves though while the original imperial drawings call for a single piece, 2 ringed piston.

We will be running in on air, at least to start. We have no way of creating steam as of yet and have no plans to make a boiler.

Mark

[JasonBModerator @jasonb]

I seldom measure my piston fits but generally I give the cylinder a hone first and then fit the piston to the cylinder so it just slides freely but if one end of the cylinder is blocked will take about 20secs on your size stroke to slowly drop to the bottom of the cylinder. Probably a thou smaller.

The best method is to turn the piston but leave say 10thou oversize, then mount on teheston down to fit the cylinder. This way will ensure the OD of the piston is concentric to the piston rod and hopefully avoid tight spots when the engine is assembled. Here the cylinder cover is fitted and I have my thumb over the port, no ring fitted yet.

 

Edited By JasonB on 31/12/2019 16:40:46

[Mark Gould 1Participant @markgould1]

Jason, thanks again. Can you say anything about the choice between silicone poston rings and the ones supplied by Stuart?

Mark

[JasonBModerator @jasonb]

I've certainly gone over to rubber rings on both steam and IC engines but tend to use Viton rather than silicone as they take the heat and are more oil tolerant.

The main thing to bear in mind are that you want at least a honed bore and possibly lapped otherwise the ring will wear. Also the groove sizing needs to be right, most published sizes are for commercial use where much higher pressures are involved and this will create unwanted friction so the ring just needs to be squashed a little so that it seals but allows free movement which is the hey to that nice slow tick over on a couple of psi. I'll have a look at the No1 drawings later and suggest a ring and groove size.

[Ron LadenParticipant @ronladen17547]

Posted by JasonB on 31/12/2019 16:39:01:

I seldom measure my piston fits but generally I give the cylinder a hone first and then fit the piston to the cylinder so it just slides freely but if one end of the cylinder is blocked will take about 20secs on your size stroke to slowly drop to the bottom of the cylinder. Probably a thou smaller.

The best method is to turn the piston but leave say 10thou oversize, then mount on teheston down to fit the cylinder. This way will ensure the OD of the piston is concentric to the piston rod and hopefully avoid tight spots when the engine is assembled. Here the cylinder cover is fitted and I have my thumb over the port, no ring fitted yet.

Edited By JasonB on 31/12/2019 16:40:46

Wow, that's impressive Jason what a great fit the piston is, excellent.

[Mark Gould 1Participant @markgould1]

It is indeed. I tried to hone the bore with a cheap and nasty honing set I bought of eBay. I can’t say I was overly impressed by it, but my own newbie level of honing tai chi might have played a part too.

[Mark Gould 1Participant @markgould1]

Posted by JasonB on 01/01/2020 07:12:45:

. I'll have a look at the No1 drawings later and suggest a ring and groove size.

Jason, I don’t mean to prod you but have you had time to have a look yet?

Many thanks,

Mark

[JasonBModerator @jasonb]

Sorry about that

So with a 2" bore I would suggest a single O ring of 1/8" nominal section which should be easy to find if you search for "BS224 0-ring" and would also suggest Viton as the material.

As there are no gaps in the rings like you get with CI rings there is no need to use two which will reduce friction and make it easier to get a nice slow tick over on just a few Psi pressure. Viton is more resistant to oils and solvents so little chance of it swelling and making the piston tight, don't use steam oil unless you are actually running on steam as at room temperatures even the "light" version is too thick and sticky.

Although this size ring has a nominal 1/8" section it actually measures 0.139" so to get a seal we need to squash it a bit but that will make it take on an oval shape so the groove needs to be wider to allow for this. The proven groove size for model use is one that is 0.132" deep and 0.160" wide, this is less than you will find on most manufacturers websites but that is because they are quoting for much higher pressures.

As for the piston, you could stay with the two part design and just cut the groove into one half in the form of a rebate which will be easier to cut and use the now plain other half to form the second side of the groove. Or as it will be easy to stretch the ring over the piston a single piece one 3/4" thick would do but I would make that out of aluminium to keep the weight down. Infact I would even do the two pat option from aluminium or if sticking with cast iron increase the hollowed out parts where the two inner faces meet. Reason to keep weight down is to get a smoother running engine as speed goes up as the No1 has no balance weights on the crankshaft

J

[HopperParticipant @hopper]

Another alternative is to fill the piston ring groove with rolled-up teflon thread tape. Less friction than an o-ring and works ok on compressed air.

[Mark Gould 1Participant @markgould1]

Wow what a wealth of information. Just what we need to proceed. Many thanks for taking the trouble to explain all that Jason, it is much appreciated. We are taking steps in the dark here and this website is providing all sorts of relevant guidance.

Most excellent.

Mark

[Mark Gould 1Participant @markgould1]

Thanks Hopper, I will try the Viton o-ring first though. Our second Stuart is already waiting to be built so I’ll archive your advice.

[Mark Gould 1Participant @markgould1]

Sorry to bother you again gents, but would this be the o-ring to use? The thing I wasn’t sure about is the OD which is 44mm. Seems tight for a 2 inch bore?

https://simplybearings.co.uk/shop/p512731/BS224-3.53mm-Section-44.09mm-Bore-VITON-Rubber-O-Rings/product_info.html

[JasonBModerator @jasonb]

Have a closer look at the sizes towards the bottom of the description in the link you posted

[Mark Gould 1Participant @markgould1]

Bugger I knew I had replied too fast

Thanks Jason

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