Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The reports of early users were of early ring failure, and what appeared to be heat of friction induced damage to the rings, and if I remember correctly, cylinder wall scoring from piston scuffing.
The prevailing notion is a single ring set up is far more durable.

With my M90 running so nominally about 30 hits/min with long coasting time & not exhibiting any frictional binding, tightness or squeaks, I'm reluctant to disturb it's current o-rings fitting...With such a close (prob < 0.001") piston to cyl bore clearance, an unfortunate "stack up" of production fitting tolerances could produce the occasional "tight" engine & with it's extremely hard steel cylinder liner & piston material used, a normal running wear self break-in won't happen. When/if O-ring replacement is needed, I'll sure try the single O-ring upgrade.

No O-ring(s) check until ready to add some piston oiling & cyl sleeve finish/polish mods.

When I had the piston out to cut 100/1000 off the top, I also added a tiny chamfer (so small it nearly unnoticeable)to the top and bottom of the piston. I also used some #1200 paper and Maas or Simichrome and gave the entire piston a good rub by hand against a folded towel on the table top. This actually took a little of the coating off the bottom of the skirt. The bore had a few very minor scrapes so I bent a coat hanger wire to hold a folded strip of #1200 paper about 6 " long dripping with WD-40 and gave the bore a quick "honing" followed by a cloth strip with Maas or Simichrome that was large enough to fit the bore fairly tightly. I spun this with an electric drill. (the sandpaper strip spun by a Dremel tool on the slowest setting). This took care of the very minor scoring that was on the cylinder wall. Perhaps I went a bit overboard but it sure did work well. Only one O-ring went back on the piston.

The result was a very smooth running engine that fired 24 times per minute and ran for a very long time (nearly 5 hours) on a single fill-up. I also add a couple drops of oil to the skirt and lower bore before firing and once or twice during the run.

Gil

Hi Tom, I honestly don't know enough to form an opinion of your special dimpled liner idea. However I do believe you know more than enough to make a fantastic running machine out of it and I encourage you to do this if you can find the time. Please bring us along for the ride if you go for it.

I know yours already runs and starts great so "fixing" it isn't a major priority and I understand this well. 

For the piston, I started out cutting the top off perfectly flat by about .100" to .105". On more recent engines I have taken up to .125" and carved a .075" depth bowl into a couple of them. I would like your opinion here.... Is there a point where taking too much of the top off would change the piston geometry enough to affect wear patterns. If so, I could reduce the amount taken off the top and make up for it by cutting a much deeper bowl into it. That said, My engines will run mostly all day for 2-3 days straight at shows.
Gil

I think there is a catch twenty two with regard to bore finishes. I am mindful of my dad’s experience back in the sixties,  manually machining bearings for gas turbines. They were large diameter sleeves cut to +.0003/-0.0”, with mirror finishes.  They lived or died on the idea that the smoother you got a bore, the less surface oil film required.  By contrast, assembly line, pre-CNC car engines were iron bores of far sloppier tolerances, which used textured crosshatching to retain essential lube. I also remember a ‘59 Ford 289 that was huffing oil past the rings at under eighty thousand miles when dad got rid of it.  By contrast, stainless sleeved engines are a thousand times smoother, and require less oil because an oily fingerprint would be enough to protect them. A friend ran an 08 Toyota 145000 highway miles on the original factory oil.  It was exactly half a quart low when changed.  We beat the daylights out of that car, often hundreds of miles at a rip at 90-100 mph.  It never complained.

Nope. She changed the AIR filter religiously, and followed a rule of thumb I’d mentioned casually about oil long ago. “Oil changes are a rip off. Three thousand miles is ridiculous if the engine isn’t loose to begin with. Watch the color of the oil. If it looks dirty, it is.”  She took that literally, and when I was driving one day, as we refueled during a blistering run from SC to MO, she showed me the dipstick. “Looks like it might be time”. The oil was a dark caramel color, no longer Pilsner tan on the stick, and it was down almost half a quart. That was at 140,000 miles. I changed it the following week at 145000.”  That little Toyota engine was bulletproof. We regularly ran it at near wide open for hours, crisscrossing the states as we travelled. The struts, brakes, window regulators, and two sets of hub caps kept falling apart, but that motor was amazing.