Hi Chris - mind if I jump in here again?
I have built several diesels now all bar one at 5cc and all have cast iron pistons. The first a 'Nova' used En1a leaded steel (12L14 equiv) for the one piece cylinder fins and liner. This was primarilly chosen because of machining the very thin fins required. Despite my initial doubts over whether this would stand up to running for any length of time the piston seal on this is as good as when first made and it runs well.
However, not quite sure that this was actually a good combination I sought advice on the ME forum the upshot of which the next two engines - 5cc 'ED Racers' - used a cast iron liner in one and a high tensile steel in the other. The 'cast iron in cast iron' soon lost its initial seal as it bedded in but still works ok. The high tensile steel version, though providing a lovely finish and seal proved tough to cut the porting.
Personally speaking I would not go down this High Tensile route again....
Just after these were made I found I had an article squirreled away up the loft. This was by a 'George Aldrich' who was recognised as an extremely knowlegeable person in the model aircraft world when it came to piston/liner fits particularly on competitive motors. Indeed in later life he earnt a living by re-working model engines.
The article extolled the virtues of using the 'CIS' system - ie Cast in Steel. The steel recommended was leaded freecutting 12L14
The next engines I made then - the Eta's - used this for all three cylinders - very easy to machine and lap, no distortion and very good verging on excellent piston/liner seals. They run extremely well.
The last engines made - the Super Tigres (described on here somewhere) also use this combination. Since making those I have become aware that several engine manufacturers also used this combination on commercial engines - perusal of the pages on MEN will confirm this and it has also been used in some home built racing engines where out right performance is obviously the goal but for me personally it was/is the article by 'GMA' that swings it for me.
Cast iron in leaded steel is a good 'bearing' combination and it appears that this is also true for this application. Given the engines we make are unlikely to ever be required to work long and hard my initial doubt over the longevity of piston seal has proved to be an uneccessary one.
The only thing I would say is that as the bore gets smaller the quality of fit of piston to liner needs to be as near perfect as you can get it. It maybe that on very small bores a much tougher (harder?) surface is required as any wear will be quickly noticeable but I would certainly not hesitate to use it (12L14) in the first instance.
I've described how I approach fitting these parts on the Tigre thread if that's any help - not my ideas originally - yes, you guessed it - those of GMA
Hope this goes some way to allaying any doubt you may have.
Regards - Ramon
