Anyone else tried this ?

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Throttle Switch said:
i bought a zenoah piston for my km 30.5 it is taller after lapping the top of the cylinder with the stock gasket im at .014 squish. i had to surface grind the washers to get them to fit think i had to take about .004 off the washers to make them fit nicely. im also running a reed block so i added windows and lightened the piston a bit.

the plating on the cylinders suck but that didnt stop me from trying to make some extra pony. im on my third motor build with the km 30.5 now.
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looker2756 said:
30.5 cc engines do have the +2mm crank shaft - Unless the KM is not catagorizing the engine correctly, which could be the case as well.
I would look at a +1mm to see if that would allow the port to stay closed & see how that would work.
Lots of options in these little engines, but messing around with a sub par build like this seems like a waste of time and money to me.
It is your project though, and as long as you are happy doing it, More power to you !
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A 1mm crank will make the problem worse The farther up the piston goes the bigger the opening. I tried a stock zenoah to see if the Km piston had too short of a skirt. None of the standard pistons I tried would fix it. The plating in mine looks much better than the one on Km's website. Maybe not last as long as a zen but I expect it will hold up for a while.If it blows I haven't really lost anything. I was stuck with the parts anyway so the money has already been spent. I personally don't consider it a waste of time cause its what I enjoy doing.Anyone can go plunk down 700.00 for a modded motor. I get much more enjoyment outta doing it myself, especially if i,ve had to think out side the box a bit to get there:cool:
 
quentrm250 said:
I have been doing a bit of research and it is hard to find exact spec's on the Rovan motors. Here's what I think I have figured out with ALL the different brands of engines.

Most of the engines orignally came with a 28mm stroke and a couple of different bores (IE: 23cc and 26cc)
Over time the engines got bigger and in order to make more cc you either increase the bore or stroke or both.

If stroke is increased and everything else stays the same it will hit the top of the cylinder. There are 3 solutions: 1. shorten the rod 2. Make the cylinder taller 3. Move the wrist pin higher in the piston

If the bore is increased you can leave everything else alone and just change the internal diameter of the cylinder and the diameter of the piston (easiest way for the manuf. to make bigger engines).

I believe there might be a misunderstanding of the word "stroker". A stroker is taking an engine and modifying it to have a longer stroke which would give more CC. Some hobbyist have done this to their engines. Some of the engine's being produced by the factories are just duplicates of what the hobbyist were doing. For example a cy29 motor is a 30mm stroke engine which is +2mm stroked over the original design of the cy23.

I have been unable to find out if ALL of the factory rod's are the same length on the cy, g, ky, rovan, and zen motors (excluding the new 32cc and above motors). If the rod's are all the same length then basically I believe all motors with 30mm strokes should allow for us to choose a 36mm 30.5cc cylinder and upgrade our cy29 or cy-f270 to 30.5cc with just a piston and cylinder. Caveat is that we cannot use the g290rc cylinder because the crank in that motor is only 28mm stroke so the cylinder will not have the extra 2mm of height built into it.

Am I thinking correctly on this???

Below is a nice chart that funbob posted (I added motors on the end):

32mm bore + 28mm stroke = 22.5cc cy23rc g240rc
32mm bore + 29mm stroke = 23.3cc
32mm bore + 30mm stroke = 24.1cc
34mm bore + 28mm stroke = 25.4cc cy26rc
34mm bore + 29mm stroke = 26.3cc
34mm bore + 30mm stroke = 27.2cc cy-f270
35mm bore + 28mm stroke = 26.9cc cy27rc
35mm bore + 29mm stroke = 27.9cc
35mm bore + 30mm stroke = 28.9cc cy29rc
36mm bore + 28mm stroke = 28.5cc g290rc cy290f
36mm bore + 29mm stroke = 29.5cc
36mm bore + 30mm stroke = 30.5cc cy31rc
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You are kinda right. If your engine has a 30mm stroke than it is using a stroker crank no matter who puts it in the engine. The standard stroke on almost all of these motors is 28mm and the standard rod length is 50mm. Too my knowledge the only motor(other than possibly your rovan) that was made with a +2mm stroke is the cy f270. (not including anything with a bore under 34mm since i don't pay attention to those sizes) The standard squish on most if not all stock motors is approx 1mm. So when dealing with a 1mm stroker you can add a slightly thicker gasket and end up with a squish of .5mm but a 1mm longer stroke. With a 2mm its much trickier because raising the cylinder by adding a thicker gasket also changes the port timings, so depending on what they were to start with you may throw them too far outta wack.Using a thinner gasket on a stock motor will drop your squish and raise compression but will also decrease port timings so again depending on the design of the motor it may negate any advantage to raising the compression.There are other concerns with using a 2mm stroker in other engines as well so it may not be something you really want to get into. It all comes down to what you want. If your looking to replace your engine you just need to decide if you are looking for something that revs it,s nutts off or something with tire shredding torque, or just in between. Then find what fits the budget.:)
 
rovanmadlunatic said:
You are kinda right. If your engine has a 30mm stroke than it is using a stroker crank no matter who puts it in the engine. The standard stroke on almost all of these motors is 28mm and the standard rod length is 50mm. Too my knowledge the only motor(other than possibly your rovan) that was made with a +2mm stroke is the cy f270. (not including anything with a bore under 34mm since i don't pay attention to those sizes) The standard squish on most if not all stock motors is approx 1mm. So when dealing with a 1mm stroker you can add a slightly thicker gasket and end up with a squish of .5mm but a 1mm longer stroke. With a 2mm its much trickier because raising the cylinder by adding a thicker gasket also changes the port timings, so depending on what they were to start with you may throw them too far outta wack.Using a thinner gasket on a stock motor will drop your squish and raise compression but will also decrease port timings so again depending on the design of the motor it may negate any advantage to raising the compression.There are other concerns with using a 2mm stroker in other engines as well so it may not be something you really want to get into. It all comes down to what you want. If your looking to replace your engine you just need to decide if you are looking for something that revs it,s nutts off or something with tire shredding torque, or just in between. Then find what fits the budget.:)
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I have a brand new Rovan 27.5cc with bad spark plug threads. I am hoping it is a clone of the cy-f270 so I can put that cylinder on it or a 30.5cc (+2mm) cylinder and piston on it. Will hopefully know more when I get it apart this weekend.
 
Ya the f270 cylinder should work fine. It has larger transfers than most of the other cylinders i,ve seen, and any videos i,ve watched they seem to rip. For 25.00 for a complete top end you cant beat it. I have one i,ve ported but I haven't finished cleaning and assembling it yet. Hoping it will rock. Bummer about your threads. It would be nice if it would get replaced for you like it should but i am doubtful.Kinda blows big ol chunks.
 
rovanmadlunatic said:
Ya the f270 cylinder should work fine. It has larger transfers than most of the other cylinders i,ve seen, and any videos i,ve watched they seem to rip. For 25.00 for a complete top end you cant beat it. I have one i,ve ported but I haven't finished cleaning and assembling it yet. Hoping it will rock. Bummer about your threads. It would be nice if it would get replaced for you like it should but i am doubtful.Kinda blows big ol chunks.
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Bought it that way at a discount. I knew it before I bought it.:D
I ordered that $24.00 pulled set and am just hoping it is correct. Both are still on the way. For $24.00 I can put it on the shelf for a spare later. Hard to beat that price.
 
quentrm250 said:
Bought it that way at a discount. I knew it before I bought it.:D
I ordered that $24.00 pulled set and am just hoping it is correct. Both are still on the way. For $24.00 I can put it on the shelf for a spare later. Hard to beat that price.
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Oh my bad. I thought it was just sent like that. Ya like I said i bought two of those kits. They are complete with pin ,bearing, ring and gasket. Awesome deal.I cant wait to try my modded one. I think it will haul...
 
I just picked up one of these heads to put on a TR/OBR billet case I have to see where it falls between 30.5 stock and a 30.5 prostock. I am going to use the piston that came with the Obr so i wont have issues with case volume.I'm curious to see what the end result will yield.. I will give my findings by the weekend.
 
The Rovan 27.5 has the same problem fyi. I put on a CY f270 complete topend and the problem is fixed. It also has higher compression as a bonus (less quench).
 
If you use a .040 gasket what would the squish be seems that this size gasket would make up the difference that is needed unless the combusrion chamber was machined
 
I just bought a 28mm cranshaft that should fix the the problem it will bump me down to 29cc but be less of a headach in the long run i feel
 
Motor

Seems like a bunch of you folks are on my wavelength so I wanted to ask if anyone ever had issues with swapping cylinders and cases as far as port matching on the transfer ports?? I've been running these motors for years but never got into heavily modding them. While I've been servicing and modifying motocross engines (mostly 2-strokes) since the 90's and love doing so I see a lot of good experimenting that could be had with these little cheap motors!! Do any of you guys get into porting? There is a lot that can be done with how the charge is directed through the transfer ports into the cylinder.
 
Yeah im gonna put it togther an run it till it blows lol i checked the squish it was between .020 to .025 with the paper gasket i also smoothed transfer ports an case matched it lol im gonna fire it an use it as a back up engine
 
Cycletech said:
Seems like a bunch of you folks are on my wavelength so I wanted to ask if anyone ever had issues with swapping cylinders and cases as far as port matching on the transfer ports?? I've been running these motors for years but never got into heavily modding them. While I've been servicing and modifying motocross engines (mostly 2-strokes) since the 90's and love doing so I see a lot of good experimenting that could be had with these little cheap motors!! Do any of you guys get into porting? There is a lot that can be done with how the charge is directed through the transfer ports into the cylinder.
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I just gave it a try for the first time i case matched it with a dremel an as far as the ports they seem opened up pretty good i feel that might b the isdue with the free port on the exhaust end the piston tht came with the kit was cut down 3mm plus it looked lightened but im goin to replace it eigh a zenoah i polished the exhaust port up im just waiting to assemble an try it
 
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