IVE BEEN TESTING 4 DIFFERENT ENGINES WITH OIL RATIOS FROM 24-1 TO 50-1 AND ALL THE ENGINES ALL GET A HIGHER RPM WHEN THEY ARE FIRST STARTED COLD. I LOOSE 250-300RPM ON ALL 4. I BEEN THINKING THAT MAYBE THE ROUND PISTON IS THE TROUBLE. I KNOW ON SB CHEVY WE USE ELIPICAL PISTONS FOR THIS REASON. IVE USE DIFFERENT MUFFLERS FROM STOCK TO NO MUFFLER AND GET THE SAME RESULTS. ENGINES ARE TORO 2BB, TORO 1BBL, ECHO 23.6 AND A FACTORY G23. ANT THOUGHTS GUYS
FARMER TED

I've had the same experience with Homelite 25 & 30, Mac 32, Ryobi. I remember TKG talking about the slow down some years ago. He didn't offer an explanation. Just that it happened.

Enjoy,

Jim

You know Guys...I notice the same RPM drop also.......maybe that is the answer....got to get the right cam grind on the piston ? Now that will take some thought...how much and where? Capt,n

I thought all pistons were cam ground. Years ago the skirt needed to be slightly larger to prevent breaking the piston when the engine was first started and cold.

Bill

I think bore,s over a certain size, pistons are cam ground.....but not very small dia bore/pistons.....just my tought??? Capt,n[:-]

I get around this by hand lapping the piston to the bore...of course this is for larger 2-strokes (>125 cc). This is subsequent to oversizing the bore and using a clearance of 0.03 mm (for a 50 to 58 mm bore). The final clearance comes to between 0.05 and 0.06 mm. With this setting there is light rattling of the piston when the engine is cold but this disappears once it warms up.

In my opinion the taper of the piston (i.e., its cylindricity) is more important than its cam. You can get the same effect as a cam-ground piston for a fraction of the cost by hand-lapping. Usually a cylindricity of 0.01 to 0.02 is desirable.

The purpose of cam grinding is to maintain close to the same clearance when hot and cold. With proper piston design, you would have between .001 and .002 thou. clearance when cold, for instance. When heated up, the piston expands along the length of the wrist pin and the clearance perpendicular to the pin stays the same. I think part of the reason the motor has more power at start up is the viscosity of the oil is higher making for a better piston and ring seal.

The answer is pretty simple:: HEAT. When the engine first starts the crankcase is cool and more air/gas gets in, as the engine warms up, the air/gas is heated as it enter the crankcase and expands, not as much gets in. Good old PVT = C.

Steve

Contrary to popular lore, thinner is better when it comes to oil doing its job. It does best at about 250 degrees. The old saying that says something that "can't get out of its own way" applies to thick oil, when it gets in a tight spot it does what they call shear down and leaves bare spots. Thin oil removes heat better and thick oil contributes to heat by being harder to move around. For example YS recommends lower viscosity oils for the fuel injected ones so more oil will flow past the piston to the lower end.

Please don't let this wonder off into automobile or motorcycle land as these kinds of discussions sometimes do.

A piston with a cylindricity of .01 to .02 would soon break the skirt off from rattling around in the bore...Probably meant .001 to .002...Anything over .005 to .006 is worn out....

Maybe he ment measurement in milameters or metric

.01 mm is only about .0004, hardly worth the trouble, but I haven't tried it, so maybe [8D]
The manual for the G26 shows stock clearance to be .03-.06mm and worn out at .15mm..OR
.0011 to .0023 and worn out at .0059

Glad to find a current thread relating to bore diameters and clearances.

RCIGN1
Could you elaborate on or direct me to a reading source for a guideline know whether a bore or rings are worn out.
I have no idea where or how that measurement in quote 1 would be made. I suspect that it requires more than a set of dial caliper. They would not reach into the bore to measure where the ring(s) would contact.

Yesterday picked up a Echo PB 4600 (S/N 040410) It ran low RPM with lots of smoke before tearing it down. Cylinder is marked but not scored to where anything can be felt.
Only engine specs from Echo are Bore 40.0mm (1.574 in) Stroke 35.0mm (1.378 in) 44cc (2.69 cu in) No wear limits avail at Echo that I know how to access.

As long as compression is in spec I wouldn't worry about bore limits if the thing doesn't rattle like a diesel.


Not to disagree but the conversation is about two stroke gasoline engines , YS nitro four stroke recommendations are not relevant to the lubrication needs of a two stroke gasoline engine.

Yes it is, and the exact same principle applies. You need the oil to go and stay where it might not want to.

According to the Zenoah manual the bore is not worn if you can still see the chrome, there are no measureable wear limits..The piston and rings are what wear in a chrome bore two cycle engine, usually about 6 thousandths less than new.. The wear on a piston is measured at the bottom of the skirt at right angles to the wrist pin...The rings are worn when they don't hold compression..If you can see black carbon deposits on the piston skirt the rings are either worn or have not seated...

You need to measure the piston and bore separately; the former using an outside micrometer and the latter using a bore gauge (preferable) or an inside micrometer. Then you have to subtract and divide by 2. For the bore, a total of 6 readings are to be taken, 3 of them at three points along the height of the bore (area of ring travel only), and the other three perpendicular to the first 3 at the same heights. Then average out the readings. Piston is to be measured at maximum diameter (usually perpendicular to the wrist pin and at the bottom).

The thumb rule I use for clearance is 0.03 - 0.04 mm per 25 mm bore. For ring end gap the rule is 0.1 mm per 25 mm bore.

Why the " divide by two" ?

My guess would be to have the results be the clearance on each side of the piston?

Piston to bore clearance is always measured as a total reading, the difference between the bore diameter and the piston diameter, Then apply whatever limits you use...Zenoah's limits are .03-.06 smaller than the bore for a new piston, and if the piston is .15 smaller than the bore it's time to replace...I suspect Echo and others use the about the same limits...
No one would ever replace a cylinder when the chrome is still good, regardless of whatever miniscule wear could be measured...by the time any wear is detected the chrome is gone..
Cast iron cylinders are another story, they wear out at different places in the bore...

That's what had me confused. Normal procedure is to stick a feeler gauge on one side.

I've seen the rpm drop off after warmed up in every two stroke application I've ever run . It can be attributed to incoming mixture heating , to seals leaking , bore expansion and so on.
250 rpm isn't much really. I wouldn't worry about it too much.

I dont know what I was thinking. You dont need to divide by 2. Sorry for the error.

For comparison: of factory specs and pipefather's thumb rule based on clearance per 1 inch (25.4mm) the list below shows the factory specs converted to a measurement per 25.4mm or 1 inch of bore as pipefather thumb rule uses.

Example Zenoah G26 34 mm bore
34/25.4* .0224mm = .03mm minimum clearance
34/25.4*.000882" = .0012" minimum clearance
34/25.4*.03mm = .04mm (pipefather thumb rule)
34/25.4*.04mm =.055mm (pipefather thumb rule)
34/25.4* .0448mm = .06mm manual maximum
34/25.4*.001763" = .00236" manual maximum
and the numbers to guide on whether a new piston is required
34/25.4*.112mm = .15mm worn out
34/25.4*.004408" = .0059" worn out

Could anyone with factory specifications for engines larger than the 26cc post some additional specs. I'm curious how the thumb rule compares. The worn out clearances are important as most conversions are from used engines, (often well used by the time they make it to my local second hand shops!)

I DONT WANT THIS TO GO TO AUTOMOTIVE EITHER! I DONT THINK THE PISTON KNOWS IF ITS 2CYC OR 4CYC WHEN IT COMES TO HEAT EXPANSION. I HAVE 30+ YRS OF SB CHEVY RACING ENGINE EXPERIANCE AND WOULD LIKE TO RELATE THE PISTON SHAPE AND SIZE TO OUR DISCUSSION.
SKIRT LENGTH IS 65% OF THE DIA. SO A 1.0 DIA. PISTON SHOULD ONLY BE .650 LONG
NO SKIRT BELOW THE WRISTPIN
PISTONHEAD DIA IS .005 UNDER BORE DIA
SKIRT DIA IS .0018 UNDER BORE DIA
RING END GAP .004 2ND RING & .005 TOP RING
IF YOU CAN PICTURE THAT PISTON IN YOUR MIND ITS ONLY 1/3 THE MASS WE HAVE IN THESE 25CC ENGINES AND WE RUN THESE 350CID 4CYC AT 9000RPM FOR HOURS IN A RACE. CAN ONE OF YOU 2CYC EXPERTS EXPLAIN THESE 2CYC PISTON ARE SO MASSIVE? SOMEONE TLAKED ABOUT LAPPING THE PISTON. DO YOU LAP IT IN THE BORE WITH AN ABBRASSIVE COMPOUND AND JUST MAKE IT SMALLER LIKE PUTTING IT IN A LATHE AND TURNING ITS DIA DOWN? WE CANT DO ANYTHING ABOUT THE HEATED INCOMING AIR REDUCING THE VOLUMETRIC EFFICIENCY BUT PISTON TO BORE INTERFERANCE CAN BE HELPED. HAS ANYONE REMOVED OR REDUCED THE QUARANT BELOW THE PIN OR TURNED DOWN THE HEAD OF THE PISTON? I AHTE TO DO ANY EXPERIMENTING BECAUSE YOU CANT GET A REPLACEMENT PISTON.
LONGWINDED ENOUGH
FARMER TED