Hi
I wrote here earlier on a mission to solve some engine noises my bike has.
The noise was when i was ideling and when i had low RPM - heavy load does
not change the noise.
Ive had alot of ideas what could be wrong. but now i have splitted my engine
so i would hope that i could get some new ideas maybe!

Anyways when i splitted the engine then i saw the piston rings didn'tt
seem damaged.
The top of the piston seemed blac and like i suppose it should.
But the black color was also on the SIDE of the piston in two places. could
it mean that the piston rings are not good enough??

sincerly
Andras

jimmy joe wrote:

> Anyways when i splitted the engine then i saw the piston rings
didn'tt
> seem damaged. The top of the piston seemed blac and like i suppose
> it should. But the black color was also on the SIDE of the piston in
> two places. could it mean that the piston rings are not good enough??

If you are using a petroleum based oil that doesn't run too clean,it
will leave hard black carbon in the combustion chamber and on top of
the piston. And that carburized oil can make the piston rings stick in
their grooves.

Piston ring designers have cleverly designed piston rings so the sides
are perfectly parallel and the piston rings actually rock a little bit
in their grooves to keep them from sticking. Those are called "keystone
rings". There are other ring designs too...

A certain amount of fuel/oil blowby is to be expected, that's when the
hot combustion gases get by the piston rings and burn oil onto the
sides of the piston. The burned on oil is often dark brown or black...

Is the inside of the cylinder scratched and scored? It's normal to see
the cross-hatched patterns of the hone in the cylinder when the
cylinder walls were ground to a precise clearance dimension. But
scratches and scoring go straight up and down the cylinder bore...

Is the piston all scuffed and scratched looking on the side that faces
the exhaust port? The piston skirt might be a little collapsed. Try to
get it measured...

If I was you, I would find the specifications for the piston diameter
at whatever point on the piston skirt it's supposed to be measured, and
measure the cylinder bore diameter and subtract the first measurement
from the second measurement to see if the piston *clearance* is still
within specification...

You'd probably want to take the piston and cylinder to a machine shop
to have an experienced machinist measure it for you, he would have all
the tools. Sometimes you can find an old retired machinist who likes to
help people for free. If you can make friends with somebody like that,
you're lucky...

If you go to a motorcycle shop not knowing what you really need, the
mechanic may want to sell you a piston you don't need or re-bore the
cylinder when it doesn't need re-boring...

If you have a trade school nearby that teaches machine shop courses,
maybe you can find an instructor who will measure the cylinder and
piston for you?

In English units, the piston clearance might be only about 0.002 to
0.003 inches, that's about 0.06 to 0.07 millimeters, I think. Piston
clearance is fairly critical, if there's too little clearance the
engine can seize when it's really hot or if there's too much clearance
because the skirt has collapsed the piston will be rattly in the bore,
making "slapping" noises and you might have a lot of blowby past the
rings, so fresh mixture can't get into the engine, because there's
compression gasses pushing the fresh air backwards out of the
carburetor...

You can check to see if the piston skirt is collapsed by sticking the
piston without the rings up into the cylinder from underneath so that
the skirt is all the way into the bottom of the barrel and use thin
feeler gauges to see if the clearance is really too big. That won't
tell you anything about wear in the upper cylinder though. The piston
rings slide from about 6 mm from the top of the clinder to about 6 mm
below the exhaust port. That's where piston ring wear will occur in a
cylinder. You need
somebody with a bore gauge and a large micrometer to measure the wear
and taper of the cylinder...

If you find you can still use your piston, you can very carefully
scrape any carburized oil out of the ring grooves. While you have the
engine apart, you might as well replace the piston rings...

Hey Krusty.

Thanks for your previous answers, i value them

"krusty kritter" <(E-Mail Removed)> wrote in message >


> Is the inside of the cylinder scratched and scored? It's normal to see.
---
There is one scratch inside the cylinder its not big, it goes vertical, but
its not in the high pressure combustion area!

> Is the piston all scuffed and scratched looking on the side that faces
> the exhaust port? The piston skirt might be a little collapsed. Try to
> get it measured...

I study mechanical engineering in DTU so im pretty lucky we have a machine
shop for hobbyists that i happpen to be a member of. I will try to messure
the clearances there. We should have the tools (micrometers) for it too. I
need to do some things there too. The bolts between the exhaust pipe and
cylinder head broke when i tried to loosen the nuts. The bolt is casted into
the cylinder head! (it seems a little odd to me that someone would do that)
.. I will make a hole and cut a new threat.

> In English units, the piston clearance might be only about 0.002 to
> 0.003 inches, that's about 0.06 to 0.07 millimeters,

I will use those dimensions if i can't get them from a dealer or anywhere
else.


> You can check to see if the piston skirt is collapsed by sticking the
> piston without the rings up into the cylinder from underneath so that
> the skirt is all the way into the bottom of the barrel and use thin
> feeler gauges to see if the clearance is really too big. That won't
> tell you anything about wear in the upper cylinder though. The piston
> rings slide from about 6 mm from the top of the clinder to about 6 mm
> below the exhaust port. That's where piston ring wear will occur in a
> cylinder. You need
> somebody with a bore gauge and a large micrometer to measure the wear
> and taper of the cylinder...

What did you mean there?
Did you mean that i should messure the diameter of the cylinder in many
places (from TDC to BDC) to get a better picture?


> If you find you can still use your piston, you can very carefully
> scrape any carburized oil out of the ring grooves. While you have the
> engine apart, you might as well replace the piston rings...

Good idea!


Would you recomend me to file a little bit from the hole where the exhaust
port is?. I noticed that the edges were very sharp compared to the intake
port.

jimmy joe wrote:

> I study mechanical engineering in DTU so im pretty lucky we have a
machine
> shop for hobbyists that i happpen to be a member of. I will try to
messure
> the clearances there. We should have the tools (micrometers) for it
too.

Good. The cylinder bore should be measured front and back at the top of
the stroke and the bottom of the stroke and it should be measure side
to side at top and bottom to see how much the cylinder is tapered. If
it's not tapered too badly, you might be able to clean up the score
mark by just boring the cylinder 0.25 mm oversize. The next oversize
would be 0.5 mm...

> The bolts between the exhaust pipe and cylinder head broke when i
tried
> to loosen the nuts. The bolt is casted into the cylinder head! (it
seems
> a little odd to me that someone would do that). I will make a hole
and
> cut a new threat.

It's a stud, threaded on both ends. The aluminum and steel have
corroded together. Is there any of the stud sticking up so you can grip
it with vise grips?

I would first recommend soaking the broken off stud area in penetrating
oil and hitting the stud with a hammer and center punch a few time to
start a vibration that loosens the stud a little and starts penetrating
oil flowing down into the hole. After a day or two, I would use the
punch at the endge of the stud to see if it starts turning, if there's
no broken off part of the stud sticking out...

Some people may suggest using a stud extractor called an EZ-Out or
something similar. You drill a pilot hole in the stud and screw the
EZ-Out into the pilot hole. It has reverse threads with a very large
pitch. The threads look like spirals. When you turn the EZ-Out, it
removes the broken stud, if you're lucky. Sometimes it breaks the
EZ-Out and you're worse off than you were. I don't recommend EZ-Outs
for that reason...

Other posters have said that drilling a pilot hole with a reversible
electric drill will sometimes cause enough vibration to make the broken
stud loosen up enough so you can unthread it with your fingers...

The last resort is to take your cylinder to a shop that specializes in
stud removal. They may have an electric discharge machine that erodes
the broken stud out with an electic arc discharge.

If the stud hole is absolutely ruined and you have to go to an
oversize, you can also use what is called a "helicoil". Kits come with
the steel coils and you drill an oversize hole and thread the helicoil
into the hole with a special too supplied in the kit.

> > In English units, the piston clearance might be only about 0.002 to

>
> What did you mean there?
> Did you mean that i should messure the diameter of the cylinder in
many
> places (from TDC to BDC) to get a better picture?

Yes, that's the way taper and out-of-round conditions are measured, by
checking multiple points...

> Would you recomend me to file a little bit from the hole where the
exhaust
> port is?. I noticed that the edges were very sharp compared to the
intake
> port.

The edge of the exhaust port has to be chamfered at a shallow angle
with a small file or an oil stone after the cylinder is rebored. Maybe
that was the source of the clashing noise you heard? Maybe somebody
already bored the cylinder out and didn't chamfer the port to make the
rings slide past the sharp edge easily?

I only have one 2-stroke manual left. It's for my old Honda CR-250M. It
says to chamfer the edge of the exhaust port at a shallow angle to the
axis of the cylinder to a depth of 0.07 to 0.10 millimeters...

Good luck fixing your Yamaha...