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Discussion Starter #1
96 Scrambler 400. Started it cold and didn't wait. Piston skirt blew up. I have engine out.

What other maintenance should I do?

The jug looks fine so I am honing and replacing piston and rings. I am not boring unless someone can give me a good reason why I should. Seems fine to me.
I am replacing oil line and gas lines and old breather lines
Cleaning carb.
Cleaning gas tank.
Draining and refilling counterbalance.
Removing recoil and gear behind to clean up stator

Any other recommended maintenance or tips?
 

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The jug looks fine so I am honing and replacing piston and rings. I am not boring unless someone can give me a good reason why I should. Seems fine to me.
The piston broke due to excessive clearance - the piston was rocking in the bore and banging against the cylinder wall - the constant 'slapping' of the skirt induces a vibration in the piston that over a long period of time causes the skirt to crack. At some point the piston breaks. It is common and can only be prevented by routine maintenance - routine maintenance means the engine is rebuilt every couple of years or X number of miles - on my CRF250 race bike it's every 30 hours. If the piston broke, it's an indication of several factors: 1) the piston to cylinder clearance was excessive - 2) The air filter was not serviced or replaced regularly or there was a leak of unfiltered air between the filter and the carburetor - 3) the engine was subjected to long periods of idling - 4) the engine was subjected to long periods of high speed operation - 5) the oil mixed with the fuel was insufficient, low quality, not approved for use in an injector system, the fuel used was alcohol blended (most oil will not mix with alcohol - if the oil will mix with water, it will mix with alcohol) - 6) the engine reached the end of it's service limits and was simply due for rebuilding.

Have the cylinder measured or just have it bored and a new over sized piston fit - at it's age it is probably already been bored at least once anyway - inspect the top of the piston for an over size mark. 0.5 mm = .020 inch, 1.0 mm = .040 inch, 1.5 mm = .060 inch etc. Typically 400 cc two strokes are bored in increments of .020 with an nominal clearance of .0025 and a service limit of .006 inch.

Ring end gap should be a minimum of .007 and a maximum of .015 inch.
 

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Discussion Starter #3
The piston broke due to excessive clearance - the piston was rocking in the bore and banging against the cylinder wall - the constant 'slapping' of the skirt induces a vibration in the piston that over a long period of time causes the skirt to crack. At some point the piston breaks. It is common and can only be prevented by routine maintenance - routine maintenance means the engine is rebuilt every couple of years or X number of miles - on my CRF250 race bike it's every 30 hours. If the piston broke, it's an indication of several factors: 1) the piston to cylinder clearance was excessive - 2) The air filter was not serviced or replaced regularly or there was a leak of unfiltered air between the filter and the carburetor - 3) the engine was subjected to long periods of idling - 4) the engine was subjected to long periods of high speed operation - 5) the oil mixed with the fuel was insufficient, low quality, not approved for use in an injector system, the fuel used was alcohol blended (most oil will not mix with alcohol - if the oil will mix with water, it will mix with alcohol) - 6) the engine reached the end of it's service limits and was simply due for rebuilding.

Have the cylinder measured or just have it bored and a new over sized piston fit - at it's age it is probably already been bored at least once anyway - inspect the top of the piston for an over size mark. 0.5 mm = .020 inch, 1.0 mm = .040 inch, 1.5 mm = .060 inch etc. Typically 400 cc two strokes are bored in increments of .020 with an nominal clearance of .0025 and a service limit of .006 inch.

Ring end gap should be a minimum of .007 and a maximum of .015 inch.
It was 5 degrees on the day it broke and it had been run about 30 seconds before I started moving and a minute before I gassed it. Wouldn't the cold create that increased clearance causing piston breakage. I was being careless.

Of course it could have been a number of factors but I was thinking; same jug new piston and wait til warm next time and I should be good. Assuming my initial failure was due to cold.

But $80 and a couple trips to the machine shop is probably better than replacing the piston again and the possibility of more damage next time it fails.

I'm cheap and indecisive. Thanks for your response.
 

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A 100 inch long piece of steel will change it's length by .0036" with a 100 degree increase or decrease in temperature - if the cylinder has an ID of 5 inches and the temperature changed from 70 degrees to -30, the diameter of the cylinder would shrink appx. .0002 (2 ten thousandths of an inch)

Now the piston is aluminum it expands and contracts more and faster than steel. In an engine there is a phenomenon called cold seizure. It happens when the aluminum piston expands faster than the steel cylinder, decreasing clearance and causing the piston to press against the cylinder wall. Happens more commonly in two strokes vs 4 strokes, but will happen in either.

Now a 100 inch long piece of aluminum will expand or contract .25 with a 100 degree change in temperature. If the piston had a 5 inch circumference and .0025" of clearance and the temperature changed from 70 to -30, the clearance would increase to appx. .0030 - the service limit of the piston to cyl. clearance is .006, so for the cold to have been the cause of breakage, the temperature would have had to change appx. 1200 degrees.

No, the running hard cold was not the cause - it's highly more likely to experience 'cold seizure' when the piston is heated more quickly by high engine RPM before the cylinder with it's liquid cooling has time to reach normal operating temperature.

Find out what size piston you have, measure the bore and calculate the clearance - I think you will surprised.
 

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Complete engine rebuild is about all you can do beyond a top end rebuild - you might service the water pump, but it can be serviced in the frame if it need be - other than that, inspect the recoil rope and replace if frayed anywhere along it's length - change the oil in the counter balance and maybe install a new starter Bendix - they fail at some point in time, but they usually start acting up before going completely out providing an opportunity to install a new one before quitting completely. Put some grease on the Bendix bushings if they are in good condition, or just install new ones as a matter of preventative maint. When the new piston is installed, install a new piston pin bearing if the piston kit does not come with one. If you get a Namura TE kit, it will include gaskets and a bearing.
 

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Discussion Starter #8
Thanks for the info and for not being a d***. I almost expect any response on a forum to be condescending based on experiences on other forums. So nice to get real feedback without the attitude. THANKS AGAIN!
 

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Oh, there's attitude here! Just ask a really dumb question like if the windshield washer bottle on your Polaris Sportsman will work on other models. You will get attitude from me and others.
 

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Latebird gives great advice! Here is what I have learned I just did two of these engines,(Will be reassembling today/this weekend from crank up) The machine shop thought there must be a defect, I told them "No, just a couple of folks who didn't maintain their equipment and both are 20 year old units. Make sure your cases are not cracked as the piston skirt pieces tend to lodge between the crank and case - and i would really consider a complete rebuild with new crank bearings and check the radial and side to side of the con rod/crank bearing. The "slotted nut" on the Stator cover is Left Hand thread I have done a total of 4 of these in the past 6 months and every one had a water pump bearing out. The shaft on the counterbalance gear where the waterpump sits sometimes needs repair as well because the waterpump bearing seizes and spins on the shaft. When disassembling make sure to see if there is a spacer on the PTO end of the crankshaft. Some have them some dont. a complete rebuild will cost in the neighborhood of $500 I get all my parts from Shannon at MFG Supply in Medford Wisconsin They have all the parts in stock and great people to deal with. They have lots of stuff that is NOT on their website i learned. Nice thing since I am old you can call Shannon and talk to a live person. Old kodgers like me hate emailing back and forth for days! Keep us posted let us know how it goes. I will try to post some reassembly pics of my 400 reassembly if I remember. (Stave and FDP are always on my ass to post pics) 1579870398538.png 1579870473071.png

136819


136820
 

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Just an FYI - side to side clearance on the big end of the rod is not a concern - only up and down movement - the side to side movement may be as little as .014 or as much as .050 (spec. is .016 - .036) and still be fine - up and down movement is .000 - .001 is operational, but is too much - with .0005 up and down movement it's life expectancy is about a year depending on frequency of use and engine speed during use - high RPM operation will decrease the length of time before it goes out completely.

Most important is the amount and type of oil mixed with the fuel and the quality of fuel used. Oil will not mix with alcohol, so alcohol blended fuels should not be used. The oil should be synthetic and if the oil injection is utilized the oil must be approved for injection use. Any oil (including injector oil) can be used for premix. The fuel to oil ratio should be no less than 24:1 for engine longevity.

Back in the 50' or 60's, the boat motor manufacturers did a series of dyno tests on various engines using a variety of fuels, oils and mix ratios. One thing proved to be a constant; regardless of the fuel or the oil combination, every time the amount of oil mixed with the fuel was decreased, the engine lost power. The tests were conducted using ratios as high as (perhaps - as I do not remember the exact amount and can no longer find the article) 10:1 and as low as 100:1 - now changes to the carburation were required because oil displaces fuel and synthetic oil does not burn during the combustion cycle, so as less oil was mixed with the fuel, the jetting had to be adjusted to decrease the amount of fuel being mixed with the air. This was done to maintain the desired 14 - 16:1 air to fuel ratio as the oil to fuel ratio changed.

So, when you get the engine done, set the oil injector as specified in the service manual or premix at 24:1 to 32:1 and use non-alcohol blended fuel.
 

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Hey Latebird,
Does setting the oil injector as specified in the service manual result in a ratio close to 32 to 1? Or does the premix ratio vary depending on engine RPM?

Roughly what is the gas/oil ratio for my oil injected 1995 Xplorer?

On my 1987 Trail Boss I do not use the oil injection system and have been premixing at 40 to 1 with semi-synthetic 2 cycle oil. Should I be mixing at 32 to 1?

Thanks!
 

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The oil injection varies the ratio depending on the throttle setting, but it's calibrated to set the ratio at about 24:1, but it may be about 32:1 at idle and 20:1 at full throttle. This is because at idle the oil tends to drop out of the fuel and pool on the crankcase floor, then when the throttle is opened and engine speed increases, the turbulence caused by the rapidly rotating crankshaft picks the oil up off the bottom of the crankcase and throws it up into the cylinder. Therefore when the engine is operated at full throttle after a cold start or after a period of low speed operation it smokes like crazy. It will do this whether injected or premixed. Premixing sets the fuel to oil ratio the same for all RPM's. Race engines operated primarily at full throttle benefit from premixing - utility engines and engines operated at constant speeds benefit from oil injection.

Recommended oil ratio in a engine with a steel or cast iron lined cylinder (both the 250 and the 400) using a premium synthetic oil is about 24 to 32 to 1 - if using a mineral oil 20 to 24 to 1 - using Castor bean oil 28 to 40 to 1. It takes a bit of experimentation using the fuel you normally use mixed with the oil you choose to use to settle on the mixture that provides the best results (performance and economy).

As to the 87 Trail Boss - is the injection system removed or just not used? If it is run dry, the pump will seize and break the drive shaft from the crank to the oil pump possibly necessitating engine repair. Also, a dry injection system will bleed air into the engine causing lean running and possible engine damage. Never run an oil injection system dry and never premix and inject oil at the same time. The injection pump and drive needs to be removed and the attachment point blocked off and sealed. Removing the oil pump will increase crankcase capacity improving low end power at the expense of high end power. To get maximum high end power in a two stroke engine, the crankcase capacity needs to be decreased to the point of being equal to the top end displacement.

I personally would not run 40:1 unless I operated at full throttle constantly and that would be an engine with a plated cylinder using non-ethanol 91 octane fuel and a premium synthetic or vegetable oil mixed with the fuel. (Vegetable oil is pure Castor bean oil) It is a superior lubricant at low ratios, but will support the growth of mold, so it is best used on engines run frequently and at high temperatures - another downside of castor oil is it produces a lot of carbon, so the exhaust needs to be decarbonized routinely, like maybe once a year or during routine ring replacement or annual engine rebuilding. Greatest advantage of castor oil is once applied to metal, it requires machining to remove it. An engine might be run for hours with no oil mixed with the fuel before engine damage might occur after having been run on castor oil.

The more fuel you can run with out fouling spark plugs, the more power the engine will produce and the longer it will last. You have to remember that oil displaces fuel - running less oil means the engine is using more fuel. Some guys thinking 'more fuel means more power' cut back on the oil to get more fuel into the engine, then they can't understand why they foul spark plugs, so they cut back on the oil more - they repeat this until they do engine damage. In reality, when they cut back in the oil, it was the increase in fuel usage and 'cold' burn that fouled the plug. If they had increased the oil mixed with the fuel leaning out the fuel mixed with the air, the burn would be hotter and the plug less likely to foul.

It's a science and the answers are as individual as the engine, it's use and the operator using it. There's things the operator can do to extend the engines life and reduce fouling spark plugs. Like not using any more enrichened fuel/air mixture during cold starts than is necessary to keep the engine running until warmed up and warming up by operating immediately upon starting at a low load and moderate speed rather than idling with the enrichener on.

Damn, that a lot to digest, but it's as simple as I can make it - the full story fills volumes.
 

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The oil injection varies the ratio depending on the throttle setting, but it's calibrated to set the ratio at about 24:1, but it may be about 32:1 at idle and 20:1 at full throttle. This is because at idle the oil tends to drop out of the fuel and pool on the crankcase floor, then when...
Don't apologize for providing a long post with incredibly useful information. I thank you and when others read it they will appreciate it too.

Back to my situation. CRAP!!!, the only thing I've done right was use non-ethanol premium gas. It is all I use on all my small engines. Everything else I've done the wrong.

Regarding the 1987 Trail boss I did leave the oil pump in the engine and removed the oil tank. I didn't want to have to purchase a new single pull throttle cable so I only removed the oil tank and added a premix only sticker to the tank. Then blocked off the inputs and outputs of the pump with rubber hoses and clamps. I've run the 87 Trail Boss about 10 to 12 hours since the rebuild and removal of the oil injection tank. This was not all idling but is a mixture of idling, cruising, and very short bursts of WOT after the break in period.

My questions:
  1. Is it too late to go back to oil injection on the 87 Trail boss? i.e. Is the oil pump still ok to use if set properly?
  2. If I do go back to oil injection in the 87 Trail boss is there anything special I need to do?
  3. Regarding not using oil injection and premix at the same time. I've done this after rebuilds until I confirm the oil injection system is working. Generally one full tank at 40 to 1 and then to straight premium gasoline. I was actually advised to do this for the first tank. Is doing this a real bad idea after a rebuild?
Thanks!

Edit - Spelling and clarification
 

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You have not done everything wrong - capping both the inlet and outlet of the oil pump prevents the pump from running dry - it traps oil in the pump better yet would be a loop of oil filled line so the oil would simply circulate from the outlet to the inlet and go ahead with running premix.

Some manufacturers recommend premixing along with oil injection for break-in - you did nothing wrong there.

If you decide to reconnect the oil injection, you only need to bleed the air out of the system, but you don't have to go back, just prevent the pump from running dry.

You did just fine
 

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Thanks Latebird. I think I will remove the pump and spend the money for a block off plate and a single throw throttle cable and stick with premix at 24 to 1.
 
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