Cylinder bore surface help

[Brian Rupnow]

this is a message copied and pasted from the website of a man who makes and sells model engine rings for a living:
Piston rings will rotate in the cylinder when the ring side clearance
to the ring land is excessive. A new ring in a new groove has 1 1/2
thousandths of clearance built into the ring. Once the piston
grooves wear the ring is not held properly and will rotate. In a
horizontal engine the force of gravity will work the gaps to the top.
With proper clearances this will not happen. If you see all the
gaps lined up you had better check for ring groove wear because
trouble is just over the horizon. The greatest old wives tale about
piston rings is that there is great importance to the gap whether in
size or orientation. The gap is only about 5 percent of the
equation that includes the ring side clearance. When there is
compression leakage past a ring 95% goes past the sides of the
rings and never enters the gap. If you bother to check the end gap
why don't you check the most important thing - the fit of the ring to
the groove? Most people don't. Then when they don't get any
compression they call me. You can live with as much as 5
thousandths of side clearance but 6 or more means you will not be
happy with your results. Machine work will be needed. Pins in the grooves will force a ring to seat in one spot but there again when the ring grooves wear the seal will fail. Gap size should be a minimum of 3 thou per inch of bore for easy break ins. Engines that will work hard and hot immediately after overhaul need 4 thou minimum end gap per inch of bore. As much as 8 thou per inch of bore is acceptable.


Honing
The other thing people overdo is hone. They want to see a
pretty 350 chevy cross hatch pattern but that is not necessary in
our old engines. You are grinding away precious cylinder wall
material. The extra piston to cylinder wall clearance will allow the
piston to cock around in the cylinder and will break the seal of the
rings. If you can't resist the extra honing get a piece of emory
paper and do it by hand. When your arm is tired you know that
you are done. Leave the power honing to the professionals who
have modern marvels like oversized pistons in their bag of tricks.

 

[WOB]

Flex-hone is a very good method of finishing cylinder bores if the boring process is precise so that bore is uniform dia. and straight.

WOB

 

[bluejets]

cylinder bore was smooth as glass with a mirror finish.

Problem here of course is, it is not desirable to have the bore smooth as glass with a mirror finish otherwise the rings will never bed in.
The "glassing" property is well known with diesel engines run on no load for extended periods.
In this instance they burn oil, smoke like a chimney and overall run like a half dead chook.

 

[Cogsy]

Glazed bores (mirror finish) in full size applications are a problem. Bores need to have some surface roughness to retain lube and create a running seal. As modelers, we are all aware that the smaller we make things the tighter the tolerances need to be. The point is, our surface finish needs to be finer than that seen in full size because otherwise we will lose our fine tolerances once break-in/run-in is completed. In addition to this, it's my opinion that with our machining practices, we are not able to machine a bore finish so fine that it will not hold lube or allow for break-in to occur, unless we go out of our way to polish a bore.

So if I achieve a nice, uniform finish on my bores, with a known diameter, there's no way I want to rough them up with a coarse abrasive just so they have a cross hatch pattern, especially as I'm now not sure what size the bore will end up at once the roughness has been worn away by the rings.

 

[lantain1982]

Having the need to generate cylinder bore sizing and the target of a consistent surface texture, inparticular on multi cylinder engines, I bore to the smoothest finish ex tools that is practical with emphasis on size control and geometry within the cylinders. If you are
making you own rings then a minor variation from specified size is not a great worry as long as all bores are the same size.. I leave 0.001" to 0.0015" maximum under the specified size and using a "brake cylinder hone" [purchased from a auto parts store] I give each bore a few longitudinal strokes with the cylinder rotating in the lathe. The hone is kept oily. The honing being used to correct size variations to all cylinders thereby giving all cylinders of equal size. The resultant "cross hatch" has negligible depth. End result is a constant surface finish, a degree of crosshatch and a device to give good size control. Having uniform surface finish and bore size, pistons and rings are then made to suit.

 

[Joump5]

I finished bore with 400 wet/dry over hone,
and then put in crosshatch as best I could.
Looks much better this time.I’ll check for
taper (with snap gauge) one last time, and
if it’s not tapered I’m going to call it good,
and move on to the next phase, which I
think will be casting the piston and rod.