New CY 36cc teardown and blueprint.

Polar_Bus

Well-Known Member
Messages
98
Location
New Hampshire USA
This thread will be helpful to the guys that maybe are looking to dive a little deeper into engine teardown and simple part fit assessment and reassembly and gain some real world knowledge about how to keep your weed wacker engine in top running condition. . I was not sure I needed to do this until I removed my engine and gave it a close inspection and immediately spotted some concerns. The primary concern was why the factory added Threebond type gasket sealer goop to brand new fresh gaskets. Never seen this done in any engines i've delt with. This immediately tells me that CY has NO trust in their assembly quality. There are 2 primary reasons 2 stroke engines fail : #1 is Too lean of a air/fuel mixture causing a seizure , and #2 is dirt ingestion past your air filter and usually trashing the crank bearings. To add to engine failure #1 is air leaks. Air leaks are common at the crank rubber seals and the cylinder base gasket. Engines can also suck air through the case halves but this is less common. To absolutely ensure reliable engine life and experience consistent carb tuning you need to have a AIR TIGHT engine. All the engine hop-up's in the world won't do you squat if you have an air leak(s). Obviously CY has issues with air leaks on new engines and don't want the engine blowing up the first outing for warranty consideration. Don't be afraid to tear them down and learn ! In this thread i'll cover the issues I found and simple fixes.

Issue #1) Upon a full engine disassembly I observed the flywheel inboard rubber seal was distorted. Looks like some type of bushing seal driver was used and too much force was applied and mashed one half of the seal. It was actually cocked in the bore slightly. This without a doubt could cause an air leak. You can see in the pic (sorry for slightly blurry) where half the seal is distorted. The seal is a "double lip" so chances are it would be ok but the seal probably would have worn out quickly from misalignment. Why risk all the headaches over a $3 seal ?? New crank seals on the way, and an installation tip I practice is to smear some synthetic grease inside the seal lips to help keep the seal lips from tearing upon installation , and the grease adds some sealing protection and lubrication of the rubber.




Issue #2) The crank case surfaces out of flatness. This is an area where a little elbow grease lapping surfaces pays off and helps seal surfaces. The center mating surfaces where the crank halves mate was a good fit. I gave a quick lapping on my granite surface plate using 600 emery paper and all is good. However the case surfaces where the cylinder bolts was NOT flat. If you look at the pic the dark shades in the red circles are "low spots" as much as -.0015" . This is not a lot but remember when aluminum heats up it moves around. I'll bet some of these fan cooled engines run pretty hot especially in the deep south and when crud plugs up the recoil housing intake air vents. Took me about 20 min of gentile lapping and got the surfaces flat to within .0004". Good enough. The second pic you can see a more uniformed shiny surface indication good flatness. You can do this at home if you have a granite counter top or a ceramic stove top. These surfaces are usually very flat. Use some 400 or 600 emery paper and work lightly, slowly and in "figure 8" directional motion.

.



That's all for now, i'm waiting on my engine gasket /seal kit. I'll update this thread as I finish assembly.
 

Z.hb71

Well-Known Member
Messages
2,942
Location
Manvel, Texas
This thread will be helpful to the guys that maybe are looking to dive a little deeper into engine teardown and simple part fit assessment and reassembly and gain some real world knowledge about how to keep your weed wacker engine in top running condition. . I was not sure I needed to do this until I removed my engine and gave it a close inspection and immediately spotted some concerns. The primary concern was why the factory added Threebond type gasket sealer goop to brand new fresh gaskets. Never seen this done in any engines i've delt with. This immediately tells me that CY has NO trust in their assembly quality. There are 2 primary reasons 2 stroke engines fail : #1 is Too lean of a air/fuel mixture causing a seizure , and #2 is dirt ingestion past your air filter and usually trashing the crank bearings. To add to engine failure #1 is air leaks. Air leaks are common at the crank rubber seals and the cylinder base gasket. Engines can also suck air through the case halves but this is less common. To absolutely ensure reliable engine life and experience consistent carb tuning you need to have a AIR TIGHT engine. All the engine hop-up's in the world won't do you squat if you have an air leak(s). Obviously CY has issues with air leaks on new engines and don't want the engine blowing up the first outing for warranty consideration. Don't be afraid to tear them down and learn ! In this thread i'll cover the issues I found and simple fixes.

Issue #1) Upon a full engine disassembly I observed the flywheel inboard rubber seal was distorted. Looks like some type of bushing seal driver was used and too much force was applied and mashed one half of the seal. It was actually cocked in the bore slightly. This without a doubt could cause an air leak. You can see in the pic (sorry for slightly blurry) where half the seal is distorted. The seal is a "double lip" so chances are it would be ok but the seal probably would have worn out quickly from misalignment. Why risk all the headaches over a $3 seal ?? New crank seals on the way, and an installation tip I practice is to smear some synthetic grease inside the seal lips to help keep the seal lips from tearing upon installation , and the grease adds some sealing protection and lubrication of the rubber.




Issue #2) The crank case surfaces out of flatness. This is an area where a little elbow grease lapping surfaces pays off and helps seal surfaces. The center mating surfaces where the crank halves mate was a good fit. I gave a quick lapping on my granite surface plate using 600 emery paper and all is good. However the case surfaces where the cylinder bolts was NOT flat. If you look at the pic the dark shades in the red circles are "low spots" as much as -.0015" . This is not a lot but remember when aluminum heats up it moves around. I'll bet some of these fan cooled engines run pretty hot especially in the deep south and when crud plugs up the recoil housing intake air vents. Took me about 20 min of gentile lapping and got the surfaces flat to within .0004". Good enough. The second pic you can see a more uniformed shiny surface indication good flatness. You can do this at home if you have a granite counter top or a ceramic stove top. These surfaces are usually very flat. Use some 400 or 600 emery paper and work lightly, slowly and in "figure 8" directional motion.

.



That's all for now, i'm waiting on my engine gasket /seal kit. I'll update this thread as I finish assembly.
Great information. Hopefully you won't have problems like other members with engine failure now.
 

lorraine

Well-Known Member
Messages
95
Location
new york
See now you done it !!! there is no more room for the Threebond to sit in anymore!!!!.......lol
Just a heads up, I had a few of those 36s come in with 0 crankshaft end play ! We had to repress the crank webs to get them back in spec. Those cranks are all over the place and two are hardly ever the same.
 
Last edited:

Seandonato73

Well-Known Member
Messages
1,199
It's fairly normal in the small engines 2 or 4 stroke to have some sort of gasket sealer on them from new. They need to cut cost somehow less machining processes = less cost.
 

lorraine

Well-Known Member
Messages
95
Location
new york
It's fairly normal in the small engines 2 or 4 stroke to have some sort of gasket sealer on them from new. They need to cut cost somehow less machining processes = less cost.
You do know the china man makes our r/c cars out of our empty beer cans we return here in states. They buy the scrap cans and BINGO a r/c car!! Now we got to get the brewers to start canning with 7075-t6 cans and in a years time we will have some nice RTRs, So drink up and support the cause........
 

Polar_Bus

Well-Known Member
Messages
98
Location
New Hampshire USA
It's fairly normal in the small engines 2 or 4 stroke to have some sort of gasket sealer on them from new. They need to cut cost somehow less machining processes = less cost.
I'm actually mulling over ditching the crankcase center gasket and sealing the case halves with "Yammabond" sealant. More bottom end stability with the case halves bolted together tight IMO, but the downside is now im altering all the dimensional fits without the added thickness of the gasket.
 

lorraine

Well-Known Member
Messages
95
Location
new york
I'm actually mulling over ditching the crankcase center gasket and sealing the case halves with "Yammabond" sealant. More bottom end stability with the case halves bolted together tight IMO, but the downside is now im altering all the dimensional fits without the added thickness of the gasket.
dry fit the cases together and check crank end play then see if the bolt spread for cylinder mounting is still doable, no gasket sometimes opens up a can of worms
We found the RTR motors have just plain crap bolts/screws, after a few heat cycles they are no longer at torque, You kiss them back up and a few runs latter the same thing. they stretch like silly putty.
DDM has metal reinforced gasket sets and they work wonders for the RTR motors and crush down and stay put and the dimensions stay the same through the heat cycles and don't wander around like paper.......
 
Last edited:

Seandonato73

Well-Known Member
Messages
1,199
I'm actually mulling over ditching the crankcase center gasket and sealing the case halves with "Yammabond" sealant. More bottom end stability with the case halves bolted together tight IMO, but the downside is now im altering all the dimensional fits without the added thickness of the gasket.
Check clearances. I've tried to ditch the gasket on several of my saw builds to no avail. No end play. I haven't had the need to take the engine apart on my dbxl as of yet. But it's an obr so bbn I would hope it's good to go.
I didnt realize ddm offered metal gaskets. Will they need a thin coat of hylomar to help seal or do they have a sealant strip bedded in it?
 

Polar_Bus

Well-Known Member
Messages
98
Location
New Hampshire USA

Z.hb71

Well-Known Member
Messages
2,942
Location
Manvel, Texas
Yea, i'm kinda' thinking it's best to run a center gasket. My engine actually bolts together without the gasket and the cranks spins smoothly but i'll still measure and verify all the tolerances. My gasket sealer of choice is Permatex Copper. Been using this stuff for the past 25 years with excellent results :

https://www.permatex.com/products/gasketing/gasket-sealants/permatex-copper-spray-a-gasket-hi-temp-sealant/
unrelated question but, was thinking about using that stuff on my gaskets for the 70cc, do u think i should?
 

Polar_Bus

Well-Known Member
Messages
98
Location
New Hampshire USA
GOOD THING I HAVE PATIENTS AND PERSEVERANCE !!!

this build is making me loose what little hair I have left on my friggin' head ! My deformed clutch side crank seal dilemma now gets worse. What I was seeing upon a close inspection was the fact when I would assemble the crank and attempt to close the case halves together there would be an uneven gap, in other words the cases would not fit flat against each other. After depth mic checking all my crank axial end play I was supposed to have about .028" of gap. This is a pretty large clearance, the spec is supposed to be around .010" . Took the cases and crank to my work where I have all the precision instruments to be able to check flatness , runout and concentrically. I was able to figure out that the clutch side bearing (same side with the deformed crank seal) had the bearing slightly cocked in it's bore. So when everything was assembled a "pinch" was being forced upon the crank and not allowing any side to side play. Crank still rotated smooth. I used a press to remove the misaligned bearing and tried pressing it back in and no luck. The alignment was a little better, but still not right. Now i'm getting pissed off because I know that this engine was screwed up right from the factory and for sure I would have had a blown engine in short time. I had one last idea and this was to freeze the bearings, freeze the crank assembly then heat the cases to about 275F . I would then slip the bearing on the crank and quickly assembly everything by hand and let the crank establish the bearing alignment within the cases. To my absolute delight this worked like a charm ! With frozen bearings and the cases heated to 275F the bearings fell right into the bores and I assembled and bolted the cases together and let everything cool down. To verify I have the proper alignment I checked for the crank end play and with a gasket installed I have .012" of axial end play. Perfect, i'm done dicking around with this bottom end. Sorry for the long post, but now you can see how attention to details, spotting problems and checking tolerances pays off HUGE when dealing with garbage from China.

Pic of checking the bore alignment :



Here's a pic after my final assembly alignment. When I first took the engine apart the two round "ears" for the bearing "C" clip were actually bent at an angle which also indicates that excessive force was applied to the bearing when pressing into the bore. I straightened the clip and now everything is seated nicely.



Here's a pic of depth checking the bearings to caculate the axial end play. Calculate both bearing depths on each case half, and add them together plus the thickness of your center gasket, then subtract the width of the crank journal surfaces and you have your axial end play dimension :



And FINALLY I have a true running bottom end. Crank spins very smooth, and I can feel the axial play by slight and SMOOTH movement of the crank side to side.


To be continued.....
 

Z.hb71

Well-Known Member
Messages
2,942
Location
Manvel, Texas
GOOD THING I HAVE PATIENTS AND PERSEVERANCE !!!

this build is making me loose what little hair I have left on my friggin' head ! My deformed clutch side crank seal dilemma now gets worse. What I was seeing upon a close inspection was the fact when I would assemble the crank and attempt to close the case halves together there would be an uneven gap, in other words the cases would not fit flat against each other. After depth mic checking all my crank axial end play I was supposed to have about .028" of gap. This is a pretty large clearance, the spec is supposed to be around .010" . Took the cases and crank to my work where I have all the precision instruments to be able to check flatness , runout and concentrically. I was able to figure out that the clutch side bearing (same side with the deformed crank seal) had the bearing slightly cocked in it's bore. So when everything was assembled a "pinch" was being forced upon the crank and not allowing any side to side play. Crank still rotated smooth. I used a press to remove the misaligned bearing and tried pressing it back in and no luck. The alignment was a little better, but still not right. Now i'm getting pissed off because I know that this engine was screwed up right from the factory and for sure I would have had a blown engine in short time. I had one last idea and this was to freeze the bearings, freeze the crank assembly then heat the cases to about 275F . I would then slip the bearing on the crank and quickly assembly everything by hand and let the crank establish the bearing alignment within the cases. To my absolute delight this worked like a charm ! With frozen bearings and the cases heated to 275F the bearings fell right into the bores and I assembled and bolted the cases together and let everything cool down. To verify I have the proper alignment I checked for the crank end play and with a gasket installed I have .012" of axial end play. Perfect, i'm done dicking around with this bottom end. Sorry for the long post, but now you can see how attention to details, spotting problems and checking tolerances pays off HUGE when dealing with garbage from China.

Pic of checking the bore alignment :



Here's a pic after my final assembly alignment. When I first took the engine apart the two round "ears" for the bearing "C" clip were actually bent at an angle which also indicates that excessive force was applied to the bearing when pressing into the bore. I straightened the clip and now everything is seated nicely.



Here's a pic of depth checking the bearings to caculate the axial end play. Calculate both bearing depths on each case half, and add them together plus the thickness of your center gasket, then subtract the width of the crank journal surfaces and you have your axial end play dimension :



And FINALLY I have a true running bottom end. Crank spins very smooth, and I can feel the axial play by slight and SMOOTH movement of the crank side to side.


To be continued.....
Now this folks is why when u buy a rovan rc and you don't know as much or as handy as this guy, throw it right in the trash and buy a zenoah. Great work @Polar_Bus !! I admire that you haven't lost your sh!t and threw it across the room, cuz I would have 😂. hope this works out for u.
 

lorraine

Well-Known Member
Messages
95
Location
new york
Now watch ! that rovan motor will like to run at 375 and undo your nifty work. I would of gave up right then and there and took it out and put it up on our death post and take my Henry 45-70 and take it back apart the easy way, Done a few this way and it opens them rovans up in a jiffy
 

Polar_Bus

Well-Known Member
Messages
98
Location
New Hampshire USA
Now watch ! that rovan motor will like to run at 375 and undo your nifty work. I would of gave up right then and there and took it out and put it up on our death post and take my Henry 45-70 and take it back apart the easy way, Done a few this way and it opens them rovans up in a jiffy
You got a lot of hostility bottled up. If there comes a sad day when I paint a bullseye on the CY engine it will be with my S&W 460 mag revolver :devilish:(y) . Trust me there won't be nothing left but shards of aluminum... But for now it's a good learning experience and only cost me $12 in gaskets and seals.




Ever see a handgun with a bipod ? Now you have. LOL
 
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