Contra Piston Material

[Clive Brown 1Participant @clivebrown1]

I’m making a “BollAero 18″ 2-stroke diesel, and find my stock of cast-iron bar a bit low. Is mild steel, or even brass a useable alternative for the contra piston or does the higher expansion rate make it a no-no? The bore is 1/2”.

TIA

[BazParticipant @baz89810]

Not sure about using brass but a steel contra piston should be ok.

[bernard towersParticipant @bernardtowers37738]

I personally use cast iron as it is different to the liner so less possibility of galling and also seems to be the material of choice in the drawings I own

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Mild steel will be fine. I have used it on the last 6 diesels I have made without problems. When I assemble the engine,I coat the top bore and the CP with copper rich grease. I try to avoid machining C/I if I can.

Keith.

[Ramon WilsonParticipant @ramonwilson3]

I used brass as the moving part of a composite contra piston in recently converting an OS Max FP25 glo engine to diesel without issues. However, I too prefer to use CI for the CP but steel will be fine – it would be preferable to harden it if possible to prevent galling potential but grease, as Keith suggests, will go some way to help.

Tug

[bernard towersParticipant @bernardtowers37738]

Happy New Year Tug

[Clive Brown 1Participant @clivebrown1]

Thanks for the replies, all helpful.

Happy New Year and a nice warm workshop to everyone.

[SillyOldDufferModerator @sillyoldduffer]

Umm, I’m small-c conservative when it comes to substituting materials.  I prefer to trust the designer, unless he’s made a lot of mistakes.

Cast-iron is a classic good choice for pistons.   It’s self-lubricating, has good thermal properties, is strong enough, doesn’t gall, absorbs vibration, and resists corrosion.    In comparison, mild-steel isn’t self-lubricating, has inferior thermal properties, galls, rings like a bell, and rusts easily.  On the plus side it’s more than strong enough, and maybe the piston could be slimmed down to reduce weight, though that might not be smart in a diesel!   Apart from strength everything  about mild-steel is inferior to cast-iron for this application.

Not so inferior that the engine won’t work at all – it will!  The question is, for how long?  Keith reassures by saying he’s built 6 engines, and we’re safe to assume they all work.   But Keith leaves out the all-important information, which is for how many hours his engines run before failing.   Firing up successfully for 30 minutes says nothing about reliability, for that we want to know the Mean Time Between Failures, which requires the engine to be run in anger for much longer.   How many run-hours do you get Keith?

When run in anger for a long time, I suggest the time to fail of an engine fitted with a mild-steel piston will be much worse than the same engine fitted with a cast-iron piston.

Reliability may not matter, especially in models.  When an engine is built for fun, and only fired up a few times to prove it works before going on display, no-one knows or cares that the piston is made of a sub-optimal choice of material.  However, where operational reliability matters, perhaps to frequently power an expensive model aircraft, then the most suitable material should be used.   When reliability matters, spend precious Shekels on some cast-iron, filthy stuff though it is.

Brass I’d be wary of! About double the thermal expansion of Cast-Iron and Mild-steel, increasing the risk of binding.  The engine will still work, but might seize after getting hot, or wear the Brass so the piston soon slops out and loses compression.  Again, long-term reliability is the issue, and I guess Brass will be inferior to mild-steel.

Piston rings help.

Dave

 

 

 

[Paul KempParticipant @paulkemp46892]

Clue is in the title Dave “contra” piston.  It’s not really a piston as per your comments, more a false cylinder head flame face that can move to increase or decrease compression, normally adjusted with a screw so it moves a very small distance, very slowly, once in a blue moon in terms of the power piston movement.

Paul.

[John HaineParticipant @johnhaine32865]

Moves very slowly down, but very smartly up!

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Paul, you beat me to it ! Dave can be excused this time. He has probably had too many mince pies.!

Keith.

[Ramon WilsonParticipant @ramonwilson3]

No offence Dave but I think your reply shows a lack of understanding to the original question and to the task in hand.

For the home made engine there is nothing better than the combination of a leaded steel liner with a cast iron piston if access to heat treatment and grinding is not available. The wear combiination, though not up to hardened surfaces will wear considerably well from the piston liner point of view. As far as the contra piston is concerned the fit is more important than the wear – to tight is difficult to set too loose and it’s going up and down the bore in sympathy with the piston – the difference in all diameters is a matter of tenths if not microns. All reasonably easily achieved with home made kit BTW.
I’ve been messing with diesels since ’58 and made and restored quite a few. There is nothing wrong with using brass as a contra piston but usually if so it is set within a steel or cast iron housing the CP considerably smaller than the engine bore. The biggest issue with using brass is getting it to the fine limit required becausee it is so soft in comparison to steel or CI.

Anyone setting out to make a home built engine needs good advice – I believe I and others have done that based on personal experience with them – there is no ‘black art’ in making these little marvels in the home workshop other than care and attention to detail of fits.

Best – Tug

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