New (to me) internet myth
 

A customer told me today he heard that the A stamped on top of a Sachs piston means top of the line, a D on top means industrial grade...What a CROCK...Maybe we should start a thread with all these internet MYTHS listed for newbies..We ALL know the old timers don't perpetuate these things, don't we ? :eek:;)

RE: New (to me) internet myth
 

I heard from these old guys that your plane will stall on a down wind turn.

RE: New (to me) internet myth
 

This piston has a "AB" ?

http://img.photobucket.com/albums/v1...g?t=1328638530

RE: New (to me) internet myth
 

Between aircraft grade and blower grade?:D

RE: New (to me) internet myth
 

I have no clue about the Sachs pistons but many (not necessarily 2 stroke) pistons have numbers or letters stamped on them to denote size / bore fit / tolerance, etc. As far as the Sachs piston, if it were true, I would think that industrial grade would be top of the line!

RE: New (to me) internet myth
 

AB means that it can be used in both A and B size cylinders...The differences are in the tenths of thousandths, only shows the attention to detail the chainsaw manufacturers go to....A D piston will fit in an A cylinder, but won't run for long....An A piston in a D cylinder is borderline loose....

RE: New (to me) internet myth
 

OK RC, while talking about engine bores .... do quality cylinders have any bore taper? I have to say I've never measured this on any 2 stroke gasser I ever worked on though I do check ring end gap from top to bottom of stroke. I know some full scale aircraft cylinders have a bit of taper to allow for the top of the cylinder running at a higher temperature than the bottom .... theory being that when up to temperature the bore would be straight. Just curious.

RE: New (to me) internet myth
 

Off to go make some popcorn and grab a soda. Be right back as this is going to be interesting.:D

Industrial grade is just fine by me.

Bliksem

RE: New (to me) internet myth
 

And what is that comment supposed to mean? At least this isn't another stupid oil or ethanol in gas thread. I would like to see more real technical questions and comments on this forum. But no, we'll probably have more of the same old stuff that has been covered dozens of times before.

RE: New (to me) internet myth
 

Yes, in a true high performance engine such as a full sized Aero enigne, the bores do in fact have a taper when at room temperature. This is to allow it to be "round and parallel" when at operating temperature.

Generally on our RC sized Gas engines there will be no engineered taper. The cylinders are too small and the power outputs are too low to require it. Even on the best quality engines, the amount of taper would be very small even if it was intentionally added.

So the bores should not only be parallel but they should also be round. Very few (by numbers sold) actually are

This is because the majority of the engines that are bought nowadays have a very basic manufacturing technique used. Teh cylinders are molded on a mandrel, chemically cleaned, flame plated with Chrome then the final surface finish is achieved with a basic hone unit.

That technique results in bores that are neither parallel nor round. They are however cheap to produce that way and they will allow the engine to run. What they do not allow is good ring sealing so power is lost.

Roundness is difficult to measure due to the open port designs commonly used (another source of power loss).

RE: New (to me) internet myth
 

Thanks, Ssteve, beat me to it, my computer is screwed up again tonight.;)
Hign quality ABC glow engines are tapered at the top, a new one can't be easily rotated...

RE: New (to me) internet myth
 

Thanks, I didn't think our small engines would exhibit much if any taper but thought I would ask.

RE: New (to me) internet myth
 

Yes, regarding taper in the ABC glo engines. I watched years of development and tried every trick I could find during many years of pylon racing. From the early no taper ST designs to some of the highly tapered F-1 ST customs such as Terry Prather put out, etc. For a local Q500 class that required reasonably stock engines, I once crudely honed as much taper into an existing almost straight walled ST cylinder as I could removing all of the chrome from the bottom 1/3 of the bore. Left the pinch area untouched as piston fit was excellent there. Also relieved the bottom 2/3 or so of the piston skirt. Worked great and won many trophies with that engine. I still have it in my collection and it would run as good today as it did back then. Modern ARF buyers and overnight wonders wouldn't understand trying these sorts of things though. But then, you did what you had to do to be competitive. Sorry, not gasser related but I thought it interesting.

RE: New (to me) internet myth
 

What I fail to see explained here, Ralph scratched the surface of it, is the system behind the A,B,C.... markings.
To continue from where Aussiesteve left off:
Once in production, after all those many operations, the operator is satisfied with roundness and taper he stops working on the bore. The next station is a hydraulic measuring device that defines the true mean dimension of the bore at height specified in specs. Dependine on that dimension, the cylinder is marked.
Same with piston dimensions. rather than working to the EXACT dimension, which is nearly impossible regarding tool wear and operator intervention, the piston when done is measured. According to measurement it is "classed" to match the class of the cylinder sizes.
There you have it in a nutshell. Minimal production costs, and accuracy that competes with multiple cost production systems.
BTW,
MVVS, my favorite manufacurer, does not use this system. Any Cylinder will fit ANY piston in their gas engines (costly, costly). They have to learn a bit still.

RE: New (to me) internet myth
 

RE: New (to me) internet myth
 

Some numbers from an older Sachs set of pistons, dimensions are in MM
A= XX.919-XX.923
AB= XX.923-XX.933
B= XX.933-XX.937
So for being "just" an dumb ole chainsaw they were petty fussy when they built them.
TKG

RE: New (to me) internet myth
 

You cannot buy a cylinder without a matching piston but you can buy a piston by itself

RE: New (to me) internet myth
 

I didn't have a need to mention the final measurement / acceptance because I was referring to how the majority (by numbers sold) of RC engines are made (In china mostly). - there they set up for an approximate size, then use pretty much everythign that comes off the line.

A REAL cylinder is made slightly differently.

Casting
Rough Machining
Finish Machining (with a different cutting tool to what was used in the Rough Machining)
Inspection for acceptance / rejection
Plating / Coating (Either Electro-plating of multiple metallic layers with a Chrome alloy as the final run - or with a Silicon Alloying coating such as Nikasil)
Inspection for acceptance / rejection
Final Grinding to size
Finish Honing with fixed hone tooling
Final measurement
Acceptance or rejection
Grading.

Pe - the beauty of such manufacturers as MVVS is that the product is consistently great from one engine to the next - it took me a while to really "get" that but once it is experienced, it sure is a nice thing.

RE: New (to me) internet myth
 

:D:D:D

RE: New (to me) internet myth
 

As does Zenoah and DA....:D

RE: New (to me) internet myth
 

I hear newbies insist that a glow engine won't run without nitro methane in the fuel. Tell'em that international FAI competitions use a spec fuel with 0-nitro and they won't believe you.

RE: New (to me) internet myth
 

dont use that piston,,,,they were made for weed wackers only ! the dred ad piston,,,,,,,,,,

RE: New (to me) internet myth
 

The piston in my post is from a current production Husqvarna 346XP 50cc chainsaw. They are one of the hottest stock 50cc saws made and I happen to own one.

RE: New (to me) internet myth
 

You gonna do a conversion on it?
What prop will it spin, how many RPM?
How much will it cost?
Which oil?

:D

RE: New (to me) internet myth
 

I like chainsaws and most of them will remain to be chainsaws. I have a lot of them.