Losi 5T Multiple Cylinder/Piston Failures

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Rockfootball47

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I’m at a loss and could really use some input. I’ve had my 5T for about 9 months. At the end of last season, with probably 120hrs of driving time, I was having a lot of difficultly starting it and found it had low compression (60psi). Tore it down and found the piston ring groove pin had come out damaging the piston and cylinder. I replaced the cylinder, piston & ring and broke it in following Losi’s break in instructions this spring. Shortly after break in, it stalled suddenly. Checked compression and of course it was low again. Tore it apart and found damage to the piston and ring, with a small chip in the side port inline with the piston damage. I ground the edge at the port making sure it wasn’t raised since this didn’t extend further into the cylinder at all, completely disassembled and cleaned the crankcase, thinking maybe a small piece of debris was leftover from the first failure and somehow got sucked up causing this damage.

I get everything put back together and break it in again. I did notice some oil buildup on the throttle servo. At first I didn’t think much of it assuming it was just overspray from the exhaust. Now I’m wondering if it was coming from the exhaust gasket since I reused the original and it had some chips out of it and likely didn’t have a good seal. I didn’t think much of it and the next day when I was out driving it suddenly died again after twenty minutes. Low and behold low compression again and by far the most cylinder and piston damage I’ve ever seen.

It wasn’t overly hot any of the times it died. The engine always spun freely without binding both before and after it died. What the hell am I overlooking?
 
Are you rebuilding these engines or are you buying long blocks if not what are you replacing when you rebuild? The way it sounds and I hate to say it but its something your doing, oil ratio in the fuel, poor tuning and its running very lean, not letting it warm up enough before you go hammer on it.
 
XJman said:
Are you making sure the arrow is facing the exhaust?
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Yes, it definitely has both times I replaced the pistons. The original piston that came in the motor didn't have an arrow, but I can only assume it was pointing the correct direction.
 
Rep732 said:
Are you rebuilding these engines or are you buying long blocks if not what are you replacing when you rebuild? The way it sounds and I hate to say it but its something your doing, oil ratio in the fuel, poor tuning and its running very lean, not letting it warm up enough before you go hammer on it.
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Nope, just replacing with standard size oem Cylinder and Piston. No modified stroke or anything. For the first rebuild, I replaced Cylinder, Piston, Piston Roller Bearing, Pin, Spring Clips and Cylinder to Crankcase gasket. I also made sure to properly seal the two side port covers with RTV making sure not to have too much.

Second rebuild was just piston and ring since all of the other parts were just replaced and seemed fine. I also fully tore the engine down to the crankcase and cleaned everything. I made sure to rebuild with 2-stroke oil on all moving parts.

I'm using a mixing bottle and doing my best to stay between 1:30 & 1:25 since the difference in the lines is so small. I'm running non ethanol fuel and Castor 927 oil. Besides one bottle of VP Racing premix when I first got it, that's all I've ran through it.

I also cleaned the carb during the winter, after the initial cylinder/piston replacement. I am certain that the HI/LO needles are set correctly. I always let it warm up for 2-3 minutes before I start driving and I take it easy for the first few minutes.

I've honestly wondered if driving style after break in had caused the most recent issues, but since I followed Losi's break-in instructions and then just drove it the same way I always have after, it didn't seem to make sense. I was just doing laps in my backyard with full throttle 30% of the time.

Could a leaking exhaust gasket have anything to do with it?
 
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Rockfootball47 said:
Nope, just replacing with standard size oem Cylinder and Piston. No modified stroke or anything. For the first rebuild, I replaced Cylinder, Piston, Piston Roller Bearing, Pin, Spring Clips and Cylinder to Crankcase gasket. I also made sure to properly seal the two side port covers with RTV making sure not to have too much.

Second rebuild was just piston and ring since all of the other parts were just replaced and seemed fine. I also fully tore the engine down to the crankcase and cleaned everything. I made sure to rebuild with 2-stroke oil on all moving parts.

I'm using a mixing bottle and doing my best to stay between 1:30 & 1:25 since the difference in the lines is so small. I'm running non ethanol fuel and Castor 927 oil. Besides one bottle of VP Racing premix when I first got it, that's all I've ran through it.

I also cleaned the carb during the winter, after the initial cylinder/piston replacement. I am certain that the HI/LO needles are set correctly. I always let it warm up for 2-3 minutes before I start driving and I take it easy for the first few minutes.

I've honestly wondered if driving style after break in had caused the most recent issues, but since I followed Losi's break-in instructions and then just drove it the same way I always have after, it didn't seem to make sense. I was just doing laps in my backyard with full throttle 30% of the time.

Could a leaking exhaust gasket have anything to do with it?
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The exhaust leak won't cause that. I have vintage 2 stroke chainsaws that you can see the piston looking in the "muffler". Essentially an open port with a bit of protection from large debris getting into the cylinder. If your tuning and oil ratio are correct, it could be an air leak. Does your engine have a stock plastic intake?
 
jeepfreak1972 said:
The exhaust leak won't cause that. I have vintage 2 stroke chainsaws that you can see the piston looking in the "muffler". Essentially an open port with a bit of protection from large debris getting into the cylinder. If your tuning and oil ratio are correct, it could be an air leak. Does your engine have a stock plastic intake?
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I upgraded to this DT1 Air Filter shortly after I bought the car. How exactly would an air leak cause this? Just debris getting inside?

I should clarify that each time I’ve had an engine issue causing me to replace the cylinder/piston, there’s been severe damage to either the cylinder wall or piston in one or two single places. I’ve also found non aluminum metal debris either in the top or the piston or in the top of the cylinder cavity. They are the shape of the dents/dings in the piston.
 
Rockfootball47 said:
I upgraded to this DT1 Air Filter shortly after I bought the car. How exactly would an air leak cause this? Just debris getting inside?

I should clarify that each time I’ve had an engine issue causing me to replace the cylinder/piston, there’s been severe damage to either the cylinder wall or piston in one or two single places. I’ve also found non aluminum metal debris either in the top or the piston or in the top of the cylinder cavity. They are the shape of the dents/dings in the piston.
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Air leaks cause the engine to run lean. If its sucking air anywhere between the carburetor and cylinder it will run lean. Same thing if its sucking air past a crankshaft seal. My Zenoah 32cc started showing lean signs once warmed up. I immediatly recognized the symptoms and stopped running it before any damage was done. The plastic intake was the culprit. I replaced it with an OBR aluminum unit. Problem solved.
 
jeepfreak1972 said:
Air leaks cause the engine to run lean. If its sucking air anywhere between the carburetor and cylinder it will run lean. Same thing if its sucking air past a crankshaft seal. My Zenoah 32cc started showing lean signs once warmed up. I immediatly recognized the symptoms and stopped running it before any damage was done. The plastic intake was the culprit. I replaced it with an OBR aluminum unit. Problem solved.
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I highly doubt an air leak could leak enough to let debris in that large to have chunks on piston top. Considering to get to piston it must travel through the crankcase first.
 
smokinwheel said:
I highly doubt an air leak could leak enough to let debris in that large to have chunks on piston top. Considering to get to piston it must travel through the crankcase first.
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I wasn't referring to debris. My assumption is the debris is from the damage itself, not causing it. An air leak big enough to allow debris in wouldn't even let th engine run to begin with in my opinion.
 
Rockfootball47 said:
Nope, just replacing with standard size oem Cylinder and Piston. No modified stroke or anything. For the first rebuild, I replaced Cylinder, Piston, Piston Roller Bearing, Pin, Spring Clips and Cylinder to Crankcase gasket. I also made sure to properly seal the two side port covers with RTV making sure not to have too much.

Second rebuild was just piston and ring since all of the other parts were just replaced and seemed fine. I also fully tore the engine down to the crankcase and cleaned everything. I made sure to rebuild with 2-stroke oil on all moving parts.

I'm using a mixing bottle and doing my best to stay between 1:30 & 1:25 since the difference in the lines is so small. I'm running non ethanol fuel and Castor 927 oil. Besides one bottle of VP Racing premix when I first got it, that's all I've ran through it.

I also cleaned the carb during the winter, after the initial cylinder/piston replacement. I am certain that the HI/LO needles are set correctly. I always let it warm up for 2-3 minutes before I start driving and I take it easy for the first few minutes.

I've honestly wondered if driving style after break in had caused the most recent issues, but since I followed Losi's break-in instructions and then just drove it the same way I always have after, it didn't seem to make sense. I was just doing laps in my backyard with full throttle 30% of the time.

Could a leaking exhaust gasket have anything to do with it?
Click to expand...
Its either running really lean, or your mix is off. Try using a mixing funnel like a ratio rite https://ratiorite.com/

What is losi's break-in? I've literally broken engines in and just sent them out of the box and the difference in life is minimal honestly. When I do break an engine in, sometimes in the past but definitely the last few, i set the needles per the carb spec and take both about 1/8-1/4 turn rich. let it idle until the temp stabilizes then shut it down. Let it cool a few hours, then repeat.

After that I let it warm up for about 5 minutes and do an easy couple laps some WOT, then I beat the absolute poop out of it.

I am not easy on my 1/5 scales really, nor do I baby the engines. I literally just spent about an hour om my g320 wide open throttle blowing the tires to the belts in a single outing.

I'm thinking you've got a AFR problem. check the gaskets for the intake, thats usually the most likely failure. However if you haven't rebuilt the bottom you could have leaks around the crank seals.
 
Circlips often have little ears so you can squeeze them. They can break off. The ears need to be pointed in the direction the rod travels. Otherwise they flex at the top and bottom of every stroke. Fatigue eventually allows the ears to snap off.
 
Ok, I just looked up a genuine Zenoah G320 top end kit. Those circlips have no ears, which is a good thing. Not sure if you have a real Zenoah or not and are using genuine Zenoah parts. If you are, it should eliminate that as a failure if seated properly when assembled. This is odd...
 
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