[cf-drg]

Have this Evo 26 gasser NIB and was just fitting it to mount in a aircraft.. But noticed this gap when the piston is at TDC.. Have 2 others and never noticed, and can't check (mufflers mount and in cowls).. Just wondering if there's a flaw in this piston? Don't want to tear it down if I don't have to..

[michel gravelle]

Hi

If the piston is at TDC , you can go ahead and open it up , THATS NOT NORMAL , any way you only have 4 screws to remove . Nice pic BTW .

Michel

[kip51035]

It is called "Sub Piston Induction" and was invented a long time ago. On some engines it made no difference and on others it made them a fussy runner. As long as the engine does not make any weird noises when you turn it over go ahead and run it.

Kip

[michel gravelle]

Hi

Really ........................... I find this very interesting , I,ve seen it also , in snowmobiles , it,s when water gets into the gas ................ something to learn every day But I have NEVER seen it in an RC engine . Still it wouldn,t hurt to open the engine and have a look , .................. would it ?

Michel

[Arceenut]

Remember the COX .049s? A lot of them used the "sub piston induction" system.

[cf-drg]

Your right Arceenut, I remember the cox 049's piston going above the port.. Mine seems funny though that it's only in the one corner.. Checked my Mvvs 35 gasser and no opening..

[cf-drg]

O'man.... I pulled the back off.. "when I doubt pull the back plate off".. Well the piston skirt is machined at the front, for clearance of the crank counter weight. That's why it's only at the front corner of the port.. My other 2 must have it to, or their skirts have been smashing the crank..

[RAMJET]

it may have nothing to do with the actual piston machining,if there is too little base gasket it can also cause a problem like this,or if the block has been decked to low or exhaust port cast wrong.like mentioned above it may be normal for this type of motor but I would look more into it as it is not comon and no motor I have is like that ,cox had no exhaust so their was no back pressure and the motor could gasp a bit of fresh air below the piston @TDC but with your engine it would be breathing exhaust gasses witch is a inert gas and could hinder combustion .sort of like a EGR valve on a car, it introduces inert gas (exhaust)to the incomming charge to lower combustion temp but even with this system it only does it at higher RPM as it will stall the engine at lower RPM if stuck open.possible it's normal but it would be news to me ,post your findings as I love to learn new things .my 2 cents Roger

just a thought ,where is the piston at BDC the top edge should in most 2 cycle engines be level with the bottom of the exhaust port .if it is higher my guess is it needs a thicker base gasket .

WOW just looked at the enlarged picture that is a pretty big gap, thought you were talking about a small slit under the piston skirt .that gap would take quite some time to close as it's a vertical cut .now I just have to know if it's normal LOL.if you loosen the cylinder can it turn a bit ,does the barrel look off square like the mounting bolts were machined wrong .

[cf-drg]

Rammy, its a Evo/MVVS 26 cc gasser, so there is no base gasket and you can't turn the cylinder..(only a head gasket and back plate).. It has to be normal, because the only way to eliminate the gap would be to have a longer connecting rod.. Both the front left and right side of the piston are machined down for the counter weight clearance.. And the piston at BDC is flush with the bottom of the exhaust port..

[Arceenut]

How much clearance is there between the piston and the counterweight? Is it possible the piston was cut away a little more than necessary?/machining error?.