CNC machines are not a replacement for grinding/lapping machines. See at these photo of pistons photographed under microscope
Also they are centerless grunded or lapped. I searched after the tool marks by CNC lathe and there are the tool marks who are not complete removed by centerless grinding machine in piston from unused OS 21 SE-M and there are lapping marks in piston from Thunder Tiger 07 GP.

See at the photo, the pic tell more. Also not barrel as we allways thinked before. Sure not all engines has allways been barrel grunded piston in ABC/ABN engines as 2 difference engine manufacture are making the cylindrical grounded pistons are showed there i took measure of piston. The last pic of crankcase are showing the engine are unused.

It is important to remove tool marks first before chrome/nickel are applied. It is to preventing top of "mountains" after lathe tool are grunded/lapped off and there will be visible brass/aluminium spots. After the chrome/nickel are applied, then lap again to make true bore (there are not same thickness overall in chrome/nickel layer) and smooth surface + a lot of oil pockets.

Jens,


It seems you have done your homework very well.

The TT GP .07 engine is ABN, and I am not sure if it is lapped, since honing, or sanding by hand, i.e. not by a machine, can also produce lines across the normal movement of the pistons.


MVVS ABC engines, before they went fully to CNC, used to match pistons and sleeves and to do the final sizing of the sleeve, with nothing more than a finger and some fine emery/carborundum paper/cloth.

Some of these very tight, older sets are now sought after...

This TT 07 engine, however' is too small to use a finger...


As to barrel grinding the piston; some manufacturers replace this with a final process that removes a 'phase', a few microns from the top 1.5-2 mm of the piston, so its contact area with the sleeve is below the crown.

I have seen this on the OS FX.


The MVVS .49 is clearly barrel-ground. The shiny contact area is 1.5-2 mm below the piston's crown and there is no 'phase' machined into the piston's top.
This attests to the barrel-profile, although I have not a micrometer to show you the actual measurements.

Thanks..

At the photo all will see lappingmarks are too coarse, it is better to see with own eye in actual size when you are holding the sleeve in your hand.. The sleeve are handlapped.

The lapping marks can have varied surface depent on which grinding paste are used such as coarse, medium, fine and superfine.. Dry lapping the surface will remove all lapping marks and make surface smooth as mirror without marks..

Surface smooth as mirror without marks are not recommented since there are lack of oil pockets to smear oil at the piston.

The barrel-grunded piston are depent on how are the engine running and how well are the piston/sleeve lubricated by oil. Before the engine are ready to set in production, the engine will go through painful tests and the constructor will learn out about the engine need barrel shaped piston or not..

The barrel shaped piston has two advantages:
Firstly a wedge is formed to press the oil between the piston skirt and the cylinder to improve lubrication.
Secondly it allows the piston to "find its way" if the cylinder is deformed from thermal reasons.

OK, Jens, you are the expert and you have the parts in your hands and can see them.


But this is rarely so with modern engines, as you did see in the OS.21 (and many other).


What is the difference in appearance between a surface that was 'hand lapped' with lapping paste, using the corresponding part and a 'hand sanded/honed' surface?

Machine-/hand lapping:
It take less material then there are less risk to weakening the chrome/nickel layer by mechanical stresses.
you can feel there are cylindrical, bellmouth or tapered and make true bore while you are lapping since there are not springness in lapping tool. You can adjust more or less tapered shape in sleeve depent on which sleeve are made for Mehanite/steel- or ABC/ABN engine.

Hand honed: Can make true bore, there are the risk to distorting the sleeve since the hone stone can tilt in wrong way and make unwanted shape such as bellmouth or tapered (i experienced when i honed the sleeve in Sunnen honing machine, i got bellmouth shaped sleeve. Maked 2-3 new sleeves without luck. Learned out the machine was worned or bad maintenance.) The stone need maintenance with special restoring tool before honing can start. Difficult to feel there are cylindrical, bellmouth or tapered while you are honing since there are springness and tilting stones in honing tool.

Grinding machine can you see in this link : http://www.modelenginenews.org/pb/index.html

Both honing and lap tool can you adjust diameter each time the bore diameter are increased + stone/lap tool are worned out.

Training in grinding technique make master either one are using grinding-, honing and lapping machine.

To make lapping/honing short and economical as possible will be done if surface are smooth and true as possibe to examlpe the bore are reamed then tool marks by lathe tool are removed before honing/lapping can start. The boring tool can spring and make chattering marks in sleeve if incorrect use or the tool are not sharp enough. It is important to save so much time to use grinding tools effective and less waste at loss of grinding material. The trained machine operator will hone/lap in some minutes for each sleeve.

The other are the roughness as we can see in these photos in early post there both pistons from Thunder Tiger 07 GP and OS 21 SE-M has difference surface, in smaller model engines are rougness more important to get short running-in and less loss of material of piston/sleeve. The bigger engines can tolerate more roughness without greater loss of compression.

Surface roughness is defined by the minute variations in height of the surface of a given material or workpiece. The individual variances of the peaks and valleys are averaged (Ra reading), or quantified by the largest difference from peak-to-valley (Rz).