What's up with my cylinder?

[HondaRidr]

I seized my fooler so I took the head off to check out the cylinder. I noticed the piston didn't go down below the ports ( the piston is about 2mm above the bottom of the ports). I bought the cylinder from manbearpig and the only thing I know is that it has some portwork. Does any body know if that is normal?

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[El Camexican]

I don't think it's abnormal, but it may not be the optimal configuration. I think I've seen photos where guys ground down the tops or their piston crowns just at the ports to give the port the maximum window to flow through, but I would think that the charge created by the piston coming down, or the exhaust pulse coming back into the cylinder diminishes as the piston nears the bottom of the stroke, so it likely doesn't do much, but it would alter the squish band and that might have a more adverse effect to the way the engine runs. If it was mine I would either leave it alone, or "slightly" round the edges of the piston crown where they match with the ports.

What's more important for you to check is your squish band and then ask some guys that work on these things what the norm is just in case there was some machining done that might necessitate that you lift the barrel up with a thicker gasket. Particularly if you bought the cylinder separately and bolted it to parts obtained elsewhere. If that's the case you have a few things to check in order to ensure it runs as good as it can and doesn't lock up on you again.

[FPMXer]

The piston should dip below the ports. I'd say you roached the big end rod bearing and the cylinder is for surely scored and needs to be cut to the next size. I had an 82' fooler along time ago and kept blowing the exhaust skirt off due to a bad rod bearing. Anyway the piston crown should dip below the ports to suck the full fuel/air mixture into the cylinder.

[atc300r]

The top of the top ring will be flush at the bottom of the transfer port. So the crown will be just above the bottom of the transfer port.From the looks of your cylinder it looks like a 4 corner sieze. you may want to check cylinder to piston clearance. it should be .002-.004.

[RIDE-RED 250r]

Yeah, most times when exhaust and transfer timing/duration is altered the work is done in the ceiling of those ports in order to raise them. You don't necessarily need the piston crown to dip below the exhaust and transfer floors, what is more important is timing, duration and blowdown, all of which are measured at the ceiling of the exhaust and transfer ports. I'd be curious to know the numbers on that cylinder.

[HondaRidr]

I would have to check the clearance but I think it must have been a little off on the jetting. I have the rubbing of the stock and ported cylinders. I will have to try and find it to compare

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[RIDE-RED 250r]

A degree wheel is the best method to measure port timing and to start going about altering it for a different recipe. Port area and flow dynamics are a whole other thing.

[John Tice]

Red Rider is spot on; every good mechanic should have a degree wheel in the tool box. Check out every 2 stroke cylinder you have & record the dope in a notebook. Over time you’ll learn from your notes. I’ve recorded the port heights of around 150 cylinders & loaded them on a spread sheet with interesting results. We measure down from the top of the cylinder instead of using a degree wheel. Either system works well as long as you know the blow down dimension.
I’ve never seen a cylinder where the piston completely exposes the transfer port at BDC. Since the transfer ports register in degrees of port opening at BDC, the port height isn’t as important. When I machine a new sleeve the top of the transfer port is usually up angled around 10deg or so netting a mid range power boost. I’ve never had a customer complaint about the power band of the new sleeve installation.


John Tice
503-593-2908 Alternate 541-595-0310
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971
http://forums.everything2stroke.com/...d-the-Shop.com

[Scootertrash]

How many miles on the piston/cylinder?

[HondaRidr]

I don't know about the miles. I had the top end on there for a year

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[yaegerb]

The piston sitting slightly above the port is not abnormal. What concerns me more is that you have zero piston wash. Either you have very few hours on the motor or you need to take a look at your fuel delivery. Perform a squish test, then, pull the cylinder and check your ring gap and look on the underside of the piston for discoloration.

[HondaRidr]

What do you mean piston wash? I have never heard of it before

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[barnett468]

I would have to check the clearance but I think it must have been a little off on the jetting.

Post a photo of the porcelain end of your spark plug from a slight angle.

Measure the piston clearance.

What is the number on the spark plug?

What oil were you using?

What ratio was it mixed at?

Approximately how many hours did you ride it?




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[yaegerb]

What do you mean piston wash? I have never heard of it before

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http://s20.photobucket.com/user/TheC...nwash.gif.html

[barnett468]

What do you mean piston wash? I have never heard of it before

The following will hopefully explain it sufficiently . If it does not, feel free to ask any questions you have . Instead of spending 20 minutes typing an explanation for you, I simply copied the comments in quotations below from another site to save time.

Basically, "piston wash" is a partial, and occasional indicator of jetting on a 2 stroke engine . This means that it can sometimes provide a rough idea of whether an engine is jetted to lean or too rich or just about right . The spark plug color is also another part that can provide an idea of how an engine is jetted in both 2 and 4 stroke engines . The spark plug should always be used as one primary indicator of jetting because it's color changes quickly with jetting . A new plug should always be used when jetting to get a decent and accurate read of the color.

Below are some of the problems with using piston wash to determine the jetting that are not mentioned in the post in quotations . This is not to suggest that the shape/pattern of the build up vs the cleaner areas on the top of a piston have no value, it mainly means that it is not the best or most practical or most accurate thing to use . That being said, the spark plug color alone is also not the only thing that should be used to determine jetting either, however, it is far more accurate and far easier.


1. Unless one has a bore scope, which is a fairly expensive tool, the cylinder head must be removed to see the top of the piston.

2. In addition to jetting, the buildup on top of the piston is affected by the type of fuel and oil being used and the amount of oil being used . Different brands of gasoline have different detergent levels of detergents than others, and in general, gas that is referred to as "Tier 1" has the most detergents and therefore more cleaning abilities than gas with less detergents if all else is the same . Also, alcohol will clean better than gasoline, and since E85 gasoline/ethanol is 85% alcohol, it will clean better than E10 gasoline/ethanol or gasoline with no alcohol at all if all else is the same.

3. In general, the longer an engine is run, the more color/deposits it will accumulate . For example, it may take as many as 50 hours for a piston to accumulate enough deposits that it can give a half way reasonable indication of the jetting . At the very least, it will take several hours, and if an engine is jetted too lean, it will seize long before there are sufficient deposits on it to use it as an indicator of jetting.


"The first thing you need to realize is that the top of the piston is not
a uniform temperature. The area in front of the exhaust port that is
exposed to all of the hot gases exiting the cylinder is naturally the
hottest. The area in front of the transfer ports that is cooled by the
fresh air and fuel entering the cylinder is obviously going to be the
coolest. And, the center of the piston will be somewhere in between.

The second important thing is that there is some small range of
temperatures where the piston is hot enough to char the oil that comes in
contact with it, but cool enough not to melt the piston.

Putting those together, you have "piston wash".

When the engine is jetted very rich, most of the piston (except near the
exhaust port) is too cool to char the oil; and you will have large areas
spreading out from the transfer ports that appear to have been "washed"
clean of carbon by the air and fuel entering the cylinder... that is "piston
wash".

If you jet down leaner, the temperature of the whole piston will
increase... and more areas will be hot enough to char the oil. The "washed"
areas will be large circular areas, located just in front of the transfer
ports. At this point, the center of the piston is quite hot; and is
actually charring the oil on the underside of the dome, right in the center.

Jetting down further will cause the whole piston to get still hotter.
Now, nearly the entire piston is hot enough to char the oil... and the
"wash" will be just a small area about the size of a half of a dime, right
in front of each transfer port. The bottom side of the piston will be
charring the oil over a large area by now also. At this point, the area
near the exhaust port is getting almost hot enough to melt. This would be
considered (at least by me) to be jetted correctly... but near the "edge".

If you go leaner yet, the whole piston will be hot enough to char the
oil on top of it and there will be no "washed" areas left. The entire
piston will be covered with carbon, and the aluminum at the edge of the
piston, right in front of the exhaust port will actually be slowly melting
away and smearing onto the rings... more than a few seconds of this and it's
time for new pistons.

That, for whatever it's worth, is my own twisted look at piston wash.

Some things to keep in mind include the fact that the size of the
"washed" area depends somewhat on the upward angle of the transfer port. If
the port is angled flat across the top of the piston, there will be more
cooling and more wash... in spite of the fact that the area in front of the
exhaust port might be just as hot as it would be on a motor with upward
angled transfers that shows much less wash. The upward angled transfers
don't cool as much of the piston top.

Also, the size of the "washed" area has to be somewhat proportional to
the size of the piston. A "half a dime" sized wash area on a 600 triple
(very small piston) means the piston is a lot cooler than a "half a dime"
sized wash area on an 800 twin (with coffee cans for pistons)! What you're
really interested in is how much of the piston isn't clean, because that
tells you how much of the piston is hot enough to char the oil... and you
know the exhaust area is hotter yet.

All this really means is that you can't just say, "Every motor should
have a half-dime sized wash area." You have to correlate the size of the
wash area to other indicators and engine specs... and use it as just one of
your tools for jetting."



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