Head rebuild
- Thread starter Scruffy
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crankestein
Well-Known Member
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- 546
No, excessive blow-by.
The ring and piston is toast.
I bet you went through hell trying to tune it and was running gurggly a lot.
How are the walls of the cylinder head?
Shiny or dull? Does it have the cross hatch, scuffs?
Can you take a few better pictures? Set it on macro and take them under a lamp, no flash.
On most cases the bottom end is safe, its the top end who suffers. Try to remove the wrist pin. When attempting to remove the clip do so inside a large plastic bag or in the bathtub. They usually go out flying and it will save you time and frustration looking for the clip.
Congratulations on tearing up the engine.
The ring and piston is toast.
I bet you went through hell trying to tune it and was running gurggly a lot.
How are the walls of the cylinder head?
Shiny or dull? Does it have the cross hatch, scuffs?
Can you take a few better pictures? Set it on macro and take them under a lamp, no flash.
On most cases the bottom end is safe, its the top end who suffers. Try to remove the wrist pin. When attempting to remove the clip do so inside a large plastic bag or in the bathtub. They usually go out flying and it will save you time and frustration looking for the clip.
Congratulations on tearing up the engine.
crankestein
Well-Known Member
- Messages
- 546
I saw the pictures from my mobile phone, and looked very bad. I had the chance to look at them one more time and the piston walls don't seem that bad, and the same goes for the cylinder head. Check for scratches, do the typical nail test, and the same with the cylinder walls. As long as there are no rough sections, missing metal, it should be salvageable. Get some sand paper, 240/350 or so, and cross hatch the cylinder walls, using wd-40 or something similar. Finish it off with 600 sand paper, just enough to remove the shiny surface, make it dull so the new ring will break in properly.
Its your call, a CH kit from Aliexpress is $28.00, so its cheap, but at least will get you running until the parts arrive. I would put a new ring, and bolt everything back in, and keep bashing it, just so you know a rebuild is in the future. There will always be some vertical lines, as long as they are not too deep, its fine. Do the same steps for the piston, but with 600 or higher paper, and wd-40, go along the surface, just enough any rough edges are eliminated, the lines, its no big deal. Test that the piston slides easily all the way in without getting kinked inside the cylinder head.
Its a full floating piston, they will dance on the wrist pin, its meant to be that way. If you can, remove the wrist pin clip, check that the wrist pin and bearing are fine, smooth, and that they slide inside the piston hole/channel (whatever is called). These little engines can sustain a lot of porting, sanding, but on the same side, a minor air leak, and it goes out of tune very quick.
Why the ring failed? Who knows, not enough oil in the pre filter element, an air leak and sucked in very fine dust, initial bad tuning creating premature wear on the ring. It does not seem to be from bad mixture or low quality oil. IMO, the top end can be recovered and a few more gallons can run through it.
I have had similar damage on mine, and have been able to get some more run time. Parts are cheap, thats why I usually buy them from Ali in bulk, one of my last purchases was just that, two top ends for 58 dollars, included cylinder head, piston, wrist pin and bearing, clips with the gaskets. if you want to play it safe, take the new piston to a machine shop, or a friend that has one, and tell them to remove .005 off about 1/4 below the piston ring, helps minimize such damages, and the typical vertical lines which show up on most two stroke engines.
Its your call, a CH kit from Aliexpress is $28.00, so its cheap, but at least will get you running until the parts arrive. I would put a new ring, and bolt everything back in, and keep bashing it, just so you know a rebuild is in the future. There will always be some vertical lines, as long as they are not too deep, its fine. Do the same steps for the piston, but with 600 or higher paper, and wd-40, go along the surface, just enough any rough edges are eliminated, the lines, its no big deal. Test that the piston slides easily all the way in without getting kinked inside the cylinder head.
Its a full floating piston, they will dance on the wrist pin, its meant to be that way. If you can, remove the wrist pin clip, check that the wrist pin and bearing are fine, smooth, and that they slide inside the piston hole/channel (whatever is called). These little engines can sustain a lot of porting, sanding, but on the same side, a minor air leak, and it goes out of tune very quick.
Why the ring failed? Who knows, not enough oil in the pre filter element, an air leak and sucked in very fine dust, initial bad tuning creating premature wear on the ring. It does not seem to be from bad mixture or low quality oil. IMO, the top end can be recovered and a few more gallons can run through it.
I have had similar damage on mine, and have been able to get some more run time. Parts are cheap, thats why I usually buy them from Ali in bulk, one of my last purchases was just that, two top ends for 58 dollars, included cylinder head, piston, wrist pin and bearing, clips with the gaskets. if you want to play it safe, take the new piston to a machine shop, or a friend that has one, and tell them to remove .005 off about 1/4 below the piston ring, helps minimize such damages, and the typical vertical lines which show up on most two stroke engines.
Scruffy
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- 590
- Thread starter
- #6
The cylinder walls and the piston both feel smooth as silk. Im just gonna sand everything and clean and install new ring and give it a try. The crank bearings feel good too. Smooth with no slop in them. Could the oil leaking inside the clutch housing have been cause from the blow-by? We will see how it goes I guess 
crankestein
Well-Known Member
- Messages
- 546
Not bashing on the KM/Rovan engines, but many of them may have imperfections from factory. Similar story with nitro mills, including expensive ones, its always recommended to tear them up and check for any notches, rough edges. May have had left over material from the production line.
The oil leaking near the clutch bell, is at the same time an open gap (air leak) to the engine, and possibly some fine dust got into the engine. It could have been enough to eat up the piston ring (creating the excessive blowby), burning the left over dust during combustion. It may seem contradictory as one may think it will make the engine accelerate as if going lean. IMO, because it was not a lot, may have just sucked air in during expansion, or during very harsh acceleration. The good thing is you caught it on time, and that top end is still like new. If the surfaces are dull, then don't even mess with them, if anything, clean up with gasoline and/or alcohol. Just sand it enough so the shine goes away, emphasize more on the cylinder head, the piston can be polished, and then let the new ring do its job, new gasket, new seals.
The oil leaking near the clutch bell, is at the same time an open gap (air leak) to the engine, and possibly some fine dust got into the engine. It could have been enough to eat up the piston ring (creating the excessive blowby), burning the left over dust during combustion. It may seem contradictory as one may think it will make the engine accelerate as if going lean. IMO, because it was not a lot, may have just sucked air in during expansion, or during very harsh acceleration. The good thing is you caught it on time, and that top end is still like new. If the surfaces are dull, then don't even mess with them, if anything, clean up with gasoline and/or alcohol. Just sand it enough so the shine goes away, emphasize more on the cylinder head, the piston can be polished, and then let the new ring do its job, new gasket, new seals.
crankestein
Well-Known Member
- Messages
- 546
Make the wall look like in this picture.
It was all glossy, just go easy, don't push in into the surface, follow the surface, spray, sand, wipe, and do it all over again.
Above is the cross hatch from a new top end, for reference.
Good luck
rager33
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- 113
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- santa clara CA.
one thing I noticed is the carbon trail in the transfer ports. This usually is a sign of running to high octane. I will try & explain. these engines have little stroke & high rpm. so what happens at high rpm is when the piston is at tdc & starts downward in it's stroke the fuel is burning and expanding to slow(high octane) & is still burning when the transfer & exhaust port open and ignite the incoming fuel. this is why the piston is scorched on the transfer ports side. speed secret here if you grind a flat spot on top of the piston about the size of a dime and run white gas(very low octane) you will gain rpm.
myrcbud
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- Astoria, Oregon
myrcbud
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- Astoria, Oregon
Would you happen to have a pic of you piston mod rager33?
crankestein
Well-Known Member
- Messages
- 546
IMO, it should be fine. If it was my engine, new ring and done.
I am 99% sure it will be fine.
How does the cylinder walls look in the area where the piston has the marking?
What oil and mix ratio you use? Is the ignition coil and flywheel magneto in good shape?
I am 99% sure it will be fine.
How does the cylinder walls look in the area where the piston has the marking?
What oil and mix ratio you use? Is the ignition coil and flywheel magneto in good shape?
crankestein
Well-Known Member
- Messages
- 546
Lets say you dodge the bullet on this one.
The Lucas oil is good stuff, engine internals are good as new.
I was going to comment about the fuel, but seeing that he has run 11 gallons through it tells me that is not the issue, but something that just surfaced in the last few runs, which in this case the oil leak near the clutch area. All that clutch dust got sucked in into the engine and ate up that piston ring, its amazing it still ran fine, testament those CR290s are the real deal.
The Lucas oil is good stuff, engine internals are good as new.
I was going to comment about the fuel, but seeing that he has run 11 gallons through it tells me that is not the issue, but something that just surfaced in the last few runs, which in this case the oil leak near the clutch area. All that clutch dust got sucked in into the engine and ate up that piston ring, its amazing it still ran fine, testament those CR290s are the real deal.
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rager33
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- santa clara CA.
This is a piston I modified for one of my boats. This flat spot is bigger(size of a penny). A little to much for a car. The intake side of the skirt has been shortened to give it 164 degrees intake timing. the inside & transfer openings have been milled out. piston now weighs 18g it was 29g. the top been turned .0001
rager33
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- 113
- Location
- santa clara CA.
sorry I can't seem to get the pics to work; can someone tell me how to get pics from my pc to post. I can't remember how I did it last time.
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