Interesting 2 stroke info from Max RPM's

Thought this might be a good read. Taught me some interesting stuff I never new, particular about cyl exhaust port bridges. I got my previously pooched YZ144 cylinder back today, and they did an awesome job !

Care and feeding of 2 stroke cylinders.

Most of our customers have been around 2 strokes for many years and are pretty savvy. We do get a few customers that don’t understand the great responsibility that maintaining and operating a race bike entails. They will seize their bike “for no logical reason.” They’ll say “I was just putting in 2nd gear when….” Or “I’ve been riding for 30 years and this has never happened before….” In almost all cases, they don’t understand the care required to keep a race bike alive. Often, we can help over the phone or the Internet. In virtually all cases we can tell what happened by inspecting the piston, cylinder and spark plug. To prevent having to send us a broken engine, please read this entire pamphlet. It is a refresher course that we feel will greatly reduce the pain and increase the fun involved in maintaining a race bike.

If you’re planning to just bolt your cylinder on and gas it, remember, this is a race bike.

There is no expressed or implied warranty as to the life or longevity of your engine.

If you seize it or break it for any reason, we will charge you the normal rate to repair it, you’ll have to pay shipping all over again, and you’ll be scheduled in line with all other jobs. We care about our customers, but we care more about the ones that are careful.

FUEL and OIL

Our kits are normally set up with a compression ratio that allows them to run best with a mixture of pump gas and race gas that equals 100 octane. This is what most modern MX bikes should use in stock condition. This allows for tune-ability and a safety cushion. If you’re headed to a sand race on the beach or another severe use situation run straight 108 octane for increased reliability. If you’re doing a super cross or tight woods race at high altitude, run closer to 92 octane for quicker throttle response and a lower fuel bill. If you hear detonation, you need higher octane and richer jetting. Detonation can cause a catastrophic failure in less than one lap.

Over the past 25 years we’ve found the best piston, ring and bearing life with Yamalube-R. We suggest Yamalube mixed at 32:1. We have no confidence in wild 50:1 or 80:1 claims by some oil manufacturers. Oil is cheap. Pistons and cranks are expensive.

ASSEMBLY

Assemble the top end with 2-stroke oil on the bore and wrist pin. Push the circlips in with a small dull screwdriver and your fingers. Using a needle-nose pliars will often minutely bend the clip and it will fall out. Push the pin against each clip and look closely at them.

Don’t forget to put a clean rag under the piston to catch any dirt or a fallen circlip.

Blow the pipe out with compressed air to prevent dust, rust or carbon in the pipe from being sucked back into the engine. You can use a garden hose if you allow the pipe to dry before putting it on.

BREAK IN

When starting a cold engine NEVER rev it up any higher than a fast idle. This is very hard on a new piston. Also, do not blast down the street in front of your house. The piston will expand from combustion, but the cylinder will stay small because the radiators are getting a fresh blast of high-speed cool air. A motocross bike is jetted to blast from turn to turn, extended running in one throttle position can lead to seizure. This doesn’t only happen at “Wide Open Throttle”. Steady ½ throttle use will seize a Yamaha Blaster quad in no time. This is because the Blaster is too lean at ½ throttle and many riders keep it there when putting around.

Warm the bike up with a fan blowing into the radiators 3 or 4 times before riding. When the radiator is as hot as the water you wash dishes in, stop the engine and let it cool off. Repeat 3 or 4 times.

Remove the pipe and inspect the piston face with a flashlight every few rides. This will reveal a mild seizure before it does further damage. If the piston is slightly scuffed remove it and sand it lightly with 600-grit emery cloth. The marks should be crosshatched at 45 degrees just like the hone marks in the cylinder. The crosshatch helps to retain oil.

RING WEAR

If the rings look sharp or worn flat on the face they should be replaced. On center-bridged exhaust ports (used on most 80’s and 125’s and some bigger bikes like the 02-04 CR250) the ring will tend to wear flat were it runs on the bridge if any dirt is present. This is because the small contact area of the bridge is supporting the entire piston. To understand this, try sliding down a 1” wide banister on your bottom! Pistons in an 80 or 125 will tend to wear much more rapidly than in a 250 or 500. This is because of the higher rpm the smaller engines turn. Between the 4 to 10 hour mark an 80 or 125 will show a noticeable power loss due to piston and ring wear. Most manuals will suggest replacing a 125’s piston at 7 to 10 hours. Vet riders that putt around at low rpm can sometimes go 20 or more hours before power drops off sharply. Dirt can wear out a piston and ring in 10 minutes. Literally.

If your rings are wearing out before the piston, you’re getting dirt from the track or grit from a dirty solvent machine, etc. into the engine. Nikasil plating will not wear a ring unless dirt is present. In a perfectly clean, well-lubricated engine, the piston will wear out, or more accurately, collapse, causing excessive clearance, before the ring will. This is because the piston is subject to “slapping” against the cylinder. High RPM use greatly reduces piston life. If the piston and ring wear out in fewer than 4 hours it’s usually caused by: 1) dirt in engine 2) lots of high rpm running 3) not enough lubrication. Running a piston and ring that are worn out not only hurts power but can cause the sharp ring to snag on the exhaust port and create a mess. If you’re willing to spend the money on high performance engine work, suspension, a pipe etc. Don’t wimp out on routine maintenance! We’ve seen many 125’s gain 5 horsepower on the dyno with just a new piston and ring. That’s more than the power difference between a factory works bike and one on the showroom floor!

CHECKING RING WEAR

Piston rings are much more consistent than they were 20 years ago; it’s still a good idea to check the end gap with a feeler gauge. Minimum end-gap is .012”. The ring can be filed by clamping a fine flat file in a vice and drawing the ring ends over it.

When the gap on a used ring gets past about .025” the ring is getting old. We get cylinders in for replating that have as much as .080” end-gap on occasion! The face of a ring is rounded from top to bottom. If the ring face is flat, it is worn out from dirt.

If you have a caliper, measure your ring thickness from the ID to the OD (front to back). If it is thinner in front, you’ve gotten dirt in the engine. Now push the ring into the bore with the bare piston as you would to check the end-gap. Push it a few mm above the exhaust port and pull out the piston. Can you see daylight between the ring and the bore? Any daylight at all will cause a massive loss of horsepower. Now push it up a little higher and check again. If you see daylight with a brand new ring, the cylinder has been “warped” from overheating. In most cases, it will need to be replated.

PISTON WEAR

If your piston is dull gray with small vertical scratches running top to bottom, you’re getting dirt in the engine. Under our microscope the dirt particles look like meteors that dig a trench and come to rest. KX80’s, KX100’s, KX250’s and late model RM250’s have intake bridges. This allows the intake port to be larger, but creates a narrow/high pressure area for the skirt of the piston to ride against. As with an exhaust bridge, any dirt caught between the bridge and the piston will cause rapid wear. If you see a grayish groove in the piston, it was caused by dirt. If the piston seizes on the intake side, it will look “gouged” by the intake bridges. The bridges didn’t seize the piston; the piston grew too much and pushed against the bridges. Often, this will crack the bridges.

Many riders replace just the ring and run an old piston. In most cases, this is nuts! Remember, the only time a ring will wear out before the piston is if dirt enters the engine. Also, the ring must seal not only against the bore, but also against the bottom of the ring land that is machined into the piston. Can you picture this?

CHECKING PISTON WEAR

A piston grows more than a cylinder, so there must be clearance between the two that allows for piston expansion.

If you bought a big bore kit from us or had us plate a cylinder and we had the piston here, we’ve honed the cylinder to fit the piston with the correct amount of clearance. If you bought a piston after we finish honed the cylinder, you should check the clearance to be 100% sure that the piston measures the same as previous ones that we’ve checked. . Pistons vary much less in size than they did 20 years ago, but there is always a chance you could get a small one or a big one that causes the bore to be too “loose” or too “tight”.

The only accurate way to measure piston clearance is with a bore gauge. A feeler gauge or caliper is about as accurate as a wooden ruler.

It is always a good idea to take your cylinder to a knowledgeable dealer and have him measure the piston and bore with a micrometer and a bore gauge. If he whips out a caliper or a feeler gauge……………RUN!

BRIDGE RELIEF

On models with a single exhaust bridge, we relieve (buff down) the bridge. On some models, we also relieve the intake bridges. If you don’t see a hone crosshatch on the bridges, don’t panic, it’s intentional.

Kawasaki relieves the intake bridges on the KX100 starting in 2002. Nearly all manufacturers relieve exhaust bridges.

We relieve the bridges, not because the bridge “grows” into the bore, as many people think, but because the piston exerts the most pressure on this small area. By relieving the bridges, the piston rests more on the more sturdy circumference of the bore. If your piston grows larger than the bore and seizes, most of the damage will be to the thin bridges of the cylinder and the area of the piston that pushes against them. This often cracks the bridge or bridges. A worn out piston “slapping” in the bore can also crack a bridge.

AIR FILTER

Inspect and or clean your air filter after every ride. Use only squeeze in air filter oil from a motorcycle shop. The air boot between the air box and the carb should be removed and siliconed on any new bike. While you’re at it, pull the pipe off and inspect the face of the piston with a flashlight. Doing this on Monday can prevent a mad scramble on Friday night!

Edited by Polar_Bus

Thanks, I just replaced my rings only for the first time ever.

I thought it might be a bad idea but heard of so many others doing it.

Time to order a piston kit.

Why silicone the air-boot ?

Why silicone the air-boot ?

Airbox-to-airboot junction, just to make sure it's sealed. The way it's written in the article it almost sounds like they want airboot-to-carb siliconed!

I will agree and disagree with the bridge relief section. Yes, pressure on the small area is a big deal, but there is also expansion happening as well. The bridge itself has very little cooling compared to pretty much any other portion of the cylinder walls, about all it gets is the intake charge that exits the exhaust and is then stuffed back in by wave tuning, plus any conduction to the piston skirt when it's covering the exhaust port. Just by physics a hotter portion of aluminum will expand more, and it's growing in all directions, including inwards towards the piston. This may only be a few thousandths of a millimeter, but now we go back to that small area large pressure thing, and galling/scuffing occurs.

Lol 1/2 throttle will make the bike run lean. Not if you jet it to how you ride. I've had well over 150 hours on my banshee with used blaster pistons. Like every article, ill take in what i read and use my own judgement.

Edited by RagunCajun

Lol 1/2 throttle will make the bike run lean. Not if you jet it to how you ride. I've had well over 150 hours on my banshee with used blaster pistons. Like every article, ill take in what i read and use my own judgement.

Ha, I skimmed over that part. Also claims Nikasil will not wear a ring without dirt present. If that's the case then a top end should last a lifetime with proper clearances and good filtration.

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