Make piston rings easily


There's quite a bit of information on the web about making piston rings, and much of it over complicates what is basically a simple process.

This article deals with making new or replacement piston rings for model engines, and obsolete, worn or antique engines, where parts are either unobtainable or economically non viable.

Or maybe you just want to try your hand at making some rings for your old lawnmower, line trimmer, or garden blower that no longer goes.  

Got a metal lathe?  No problem then.

Here's how I do it.

Material

Piston rings for engines with relatively low to medium piston speeds are generally made from cast iron.

The rather brittle nature of cast iron is the reason why rings sometimes break under stress.

If piston rings are made of steel this will not occur - eg many modern engines now use exotic materials .

As speeds rise rings become thinner and have to work a lot harder, so manufacturers move to more durable materials which are outside of the scope of this article.

However, if the original rings are cast iron then you can make replacements.

You can easily test this by trying to break the ring.  If it snaps it's cast iron.  If it bends, it's steel  - simple as that.  Anyone that has ever fitted piston rings will know how easily cast iron can break.

Suitable cast iron can be sourced from:

(1) new round bar - type 2P,

(2) scrap items such as  "V" belt pulleys, brake caliper pistons, used cylinder liners, vehicle axle hubs etc.,

(3) any thing else made of quality grey cast iron -  NOT gym weights as these are mainly furnace dregs/slag.

Measurement

The external diameter of the freshly machined, unbroken, piston ring must be the same as the diameter of the engine cylinder bore - NOT the external diameter of the piston. 

The cylinder bore should be uniform in diameter throughout it's full length.

If the cylinder bore is worn and can't be re-bored to a uniform diameter, the external piston ring diameter should be the same as the smallest diameter of the bore (usually crank shaft end) - to avoid possible ring breakage due to an insufficient expansion gap.

You can make the rings slightly oversize to allow for a worn bore, but you MUST set the ring gap in the least worn section of the bore to prevent breakage from insufficient clearance.

On a worn cylinder bore, it is also usual for a lip to be worn into the cylinder bore from heat/ring compression, at the top of the piston stroke (cylinder head end).

If the cylinder is not re-bored, this should be honed back/removed to prevent possible breakage of the replacement top compression ring.

Use the old rings to determine the internal diameter of the rings, and the thickness.

The ring cross section (horizontal width) should be slightly less than the depth of the ring lands in the piston.

The width of the ring lands is pretty obvious and measurement/fit should exclude any vertical movement and sticking/binding.

If you want to make a perfect piston ring, the following section may interest you.

Scientific paper on piston ring characteristics

Click here for a detailed look at piston rings and their design features.

Machining the ring

This is self explanatory in that you are simply machining a cylinder of the correct internal and external piston ring dimensions, from which segments are sliced off to the required thickness.

Polish the external surface of the ring cylinder with fine wet and dry emery paper prior to cutting.

Most people will use a parting off tool to slice off the rings.  Keep the cutting blade overhang as short as possible to reduce wander, chatter, grooving and corrugation marks - these are your greatest enemy.

If you look closely at the "bad ring" image at the top of the page you can see corrugation marks in it.  This is a throw away from using a parting off blade.

A thicker blade is likely to be more rigid.  Why some web sites say to use a thin blade is beyond me.

Any irregularities in the ring faces will have to be ground/polished out using fine emery paper (and kerosene as a flushing agent) on a very flat surface - eg. glass pane.

You could do the preliminary clean up dry on a very slow belt sander or linisher with a fine grit belt.

The finishing process will throw out the thickness dimension, and can make the ring undersize and useless.

So make them one at a time and adjust the thickness when cutting to compensate, until you get the process just right.

A better way

The best option is to use a tool post grinder with a thin carborundum cutting disc (1 mm) to slice off the segments as below.

tool post grinder cutting piston rings

This gives a near perfect finish to the ring faces and is 100% dead accurate every time.  No rejects.

Importantly, the low pressure cutting action of a friction disc does NOTimpart hidden stress fractures into a ring segment, that can easily occur with a parting off blade.  So the piston ring will be stronger and more reliable.

You can see the dial indicator and carriage stop used to accurately set each cut above.

Below are some freshly sliced compression rings destined for a Kawasaki generator - worn originals on block.

home made piston rings

It's not necessay to have a mirror finish on the ring faces.

Many commercial rings have a matt finish.  Some even have visible light emery cross hatching (finishing marks) when examined closely.

The faces/surfaces must however be uniform.

Regardless of whether you use a parting off blade or a cutting disc, you will have to remove a small burr from the inside edge of the ring where the segment has broken away at the end of the cut.

No tool post grinder? 

Here's a cheap alternative to using a full sized tool post grinder.




This is especially handy for cutting very small and thin piston rings, and for grooving and cutting relief slots in oil scraper rings.

No chatter marks, no grooving, etc, just like it's big brother, but it can do even more.

It can cut rings down to the thickness of a piece of paper with the low stress little friction discs it uses.  Try that with a parting off blade.

Click here for a full review of the air die grinder I used in the video.

Suitable ring types

Two stroke and four stroke compression rings are easy to make.   You can also incorprate a step in the bottom ring face to help reduce oil consumption.

Relief type oil scrapers are much more difficult to make, particularly small ones.  You can copy the originals but it will be necessary to drill or grind oil passage ways through the ring edge - tricky. 

Once again a tool post or air die grinder greatly assists as it is able to grind a double edged oil scraper ring with a central groove and oil relief slots - using a suitably thin friction disc.

Breaking the ring

Lightly clamp the ring in a vice at the vertical centre point as seen below.

Apply (front to back) finger pressure close to the edge of the jaws.

break a pistop ring

The ring will snap easily at the jaw edge, with a fairly clean break.

Lightly grind the faces of the break parallel on both planes - removing the least amount of material possible.

Tensioning the ring is dealt with on the next page.


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