mylamo

My Feedback: (1)

I have a slightly used Modela .12 that runs great
but the contra fits so loosely that when it is cool
it will practically fall down the bore. Funny thing
is that it seals well and has been like this since new.
After the engine is warm it seems to be snug. It
has always hand started easily and so far presents
no problem. I was thinking,(always dangerous) that
this may at some time cause an interesting failure.
Does anyone have an idea on what should be done?
Ralph

gkamysz

My Feedback: (19)

Do you know what material the contrapiston is made of? It seems that the coefficient of expansion of the contrapiston is greater than the liner causing this problem. The cure would be a tighter fit or a different material.

I have one of those russian 1.5cc (ABC sleeve) with the brass contra in a steel head. It locks up tight a few seconds after it starts not leaving enough time to make adjustments.

mylamo

My Feedback: (1)

Yeah Greg ,ABC diesels are hard to set because you have to
be real close when they are started. Once adjusted you pretty
much have to live with it. I wouldn't be a good team racer. No
these Modelas and MVVS are iron and steel. Actually the engine
is a pleasure to adjust,it only concerns me because I have
visions of the contra following the piston at some point and the
embarassing noise that probably would ensue.[:-]
Ralph

gkamysz

My Feedback: (19)

I was wondering if there was a mistake in manufacturing or somebody made a replacement part. You could try growing the contrapiston by the methods described in the forum.

JMP_blackfoot

Hello,
I have three of those, none seems to have the problem (only two bench run so far).
Out of curiosity, what prop size/rpm are you getting?
I haven't seen a test of this engine, so I'm a bit hesitant as to what size model to fit it to.

mylamo

My Feedback: (1)

I have found that an 8x3 was not enough prop
so went with a MAS 8x4 and get a solid 11.500
rpm with a 3600 idle.That seems to fly 26 oz.
planes pretty good. My latest weighs 25.5 oz.
with 245 sq. inches wing area.
Ralph

JMP_blackfoot

Thank you for these figures. They give me a better idea. I guess they were with the silencer?
About the contra-piston, I don't think it is at all likely to fall during motor run, because there is always positive pressure in the cylinder. The only way would be while turning the motor over by hand.

Motorboy

Sleeve of steel and contrapiston of cast iron. Cast iron expanding less than steel, therefore you can move the contrapiston while the engine are running. The fit in contrapiston are a bit bigger than sleeve and it shall be press fit. You can not press the contrapiston with finger. With drill press and rod to press the contrapiston in sleeve, it feel light to press with drill press.

Jens Eirik

mylamo

My Feedback: (1)

Those figures are with muffler on DDD fuel.
The contra is very easy to move when cold
but firms up nicely when warmsd up and the
screw doesn't try to back out so I'll leave it
be for now.
Ralph