What sort of finish is required on pistons and cylinders ?

I have spent the day turning some wooden mandrels on the lathe. It is the first time I have machined wood...much easier than metal . These will be used to polish the inside of cylinders once they have been bored out to the correct size. What grade of wet and dry should I be using for various engines ?

I have read that steam engine cylinders should not be too smooth as the surface needs to hold steam oil. So what is ''not too smooth'' ...1500 grit ?

Flame eaters and Stirling engines are different and do require a very smooth finish. The finest wet and dry I can get in Cairns is 2000 grit. Do I need to go to metal polish and some sort of polishing mop to get it to a mirror finish ?

I bought one of these to hone the cylinder bores of my 5" Pansy loco:-

http://www.ebay.co.uk/itm/252030027513?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Great for removing machining marks and not too aggressive with material removal.

Alan.

IF you want to polish the bore, and not remove much metal then use something like Brasso, or even toothpaste. I tend to use toothpaste with a drop of oil, direct on the wood.

You need to aim for a smooth bore to your cylinders not necessarily polished, they are not the same thing. You can have a highly polished bore that is like the 'rocky road to Dublin' ie you have polished the machining marks in instead of polishing them out. In a perfect world to remove machining marks you work at right angles to them, in practice this is difficult which is why you end up with the figure of eight pattern in the bore. The better finish from the boring tool the less work you have to do removing machine marks. The idea is to remove all the 'umps and ollers' not make them shiny.

Very much depends on what the materials are and what sort of piston rings if any are being used.

For example a cast Iron ringed piston in a cast iron bore will just need a honed finish. If that same piston were fitted with an O ring then you would want to be looking at lapping the cylinder bore.

That Entablature engine I posted a video of last week has a honed bore and no rings to teh piston, machined finish on teh piston and runs a treat.

The Robrenz channel on YouTube just put out a video on lapping cylinders or bores , although he is doing much larger bores than we do the basic principle is the same .

Probably worth a look for those who have never seen or done any lapping .

I just keep and eye on what seems to be general practice in case I ever do feel like making a steam engine.

For honing given that the bores are a suitable size car brake cylinder hones have been popular. Larger sizes are also available for honing car and other engine bores. This sort of suggests cast iron cylinders but all brake cylinders I am aware of have been made of aluminium for rather a large number of years. Most cars have discs all round now anyway. These will need using with a honing oil or will just clog. WD40 might be an alternative.

Various styles of hones have been shown on here in threads. Also the Model Engine Maker site

None of this answers your question. Maybe because wooden hones are usually used with some grade of honing paste and are expandable to some degree. Going on that I would have thought several grades would be a better option as 1500 will clog very quickly and sounds way to fine to me especially initially. If a very highly polished finish is needed metal polish would probably be better. Lapping and honing pastes don't tend to clog so much so are a better option than papers.

Coarse and fine valve lapping paste is still around. I understand people have used that. Diamond pastes need less and last longer. Various grades could be made up from the powders telescope mirror makers use. Toothpastes use a very fine grit.

If you use emery cloth you are effectively using a multipoint cutting tool which is why grades like 180 to 400 can produce a decent finish on steel. This is true of pastes as well. The "cutting teeth" are very close together so pressure and the rate of movement has an effect on the finish. Grades like 180 also wear. The multipoint aspect is why cylindrical and other forms of grinder give such a high quality finish with what are petty coarse grit wheels.

Some one like Jason may use flap wheels that are a bit bigger than the bore and probably a bit worn out as well other wise it might be difficult to get them in.

John

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I usually get a decent enough finish for steam engines with the boring tool but I am just enquiring about the ''final finish'' . Stirling engines and flame eaters, in particular, need a very smooth bore. I usually use 1500 grit but I wonder if it should be even smoother.

Don't go anywhere near car valve grinding paste, way too coarse.

I have only used a wooden lap once many years ago on a bronze cylinder that was going to have silicon rings, used metal polish if I remember rightly.

For a honed finish I now use brake cylinder hones which adjust to sizes we are likely to need, lubricate with paraffin.

Lapping I have used home made aluminium and copper laps loaded with grades of silicon carbide powder 320, 400, 600 and 1000. These have been for small IC engines with no ring so the piston was then lapped into the bore with 1000g ( all grits mixed with oil to make a paste)

Not used a flap wheel.

I just think I have seen a comment about using flap wheels Jason and thought it might be you,

Main things as I see it is Brian needs paste not paper or cloth.

One other alternative might be the 3M film. It's also available with an adhesive backing. It might be possible to make something similar to those rather expensive Swiss hones using that as the abrasive. Thinking in terms of a small size morse reamer to provide the taper for adjustment. I have been playing around with a version of the film for polishing HSS tooling. Looks promising but needs plenty of liquid to prevent it clogging as the grit size is so small. Water works well in this case. Also a bit of that under it stops it moving around. As the material is white it seems the grit is under 1um. Probably 0.5um

John

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