Anyone else tried this ?

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, and others.
I am going to drop one of these heads on a OBR piston port billet case in the next day or so. As far as I can tell the km head will be a direct bolt on but you will have to have a modded piston or original from OBR because the case has less open space for a stock piston to complete a full cycle.
 
i applied epoxy to help with the free port and so far haven't had and issue. in running bolt on reed cage with .014 squish. but i only have about 7 tanks of gas through the motor so far.
 
BertR said:
I am going to drop one of these heads on a OBR piston port billet case in the next day or so. As far as I can tell the km head will be a direct bolt on but you will have to have a modded piston or original from OBR because the case has less open space for a stock piston to complete a full cycle.
Click to expand...
Does obr sell just the piston an when i say original do u mean un modded or an obr original mod piston
 
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.
Click to expand...

I do a lot of modding. In fact I don't run anything stock. I just swapped out the 2 bolt engine i modded to a 4 bolt. I had a wonky coil and it wasnt till i tried it on the 2nd engine I realized what it was. Any how it was replaced and my four bolt is just being broken in. It is a heavily modded cyf270 top end on the km bottom end. The km bottom end has a slightly lower deck height and it put my squish right at .5mm so it was a great match. Already the engine looks KILLER!! It should have awesome acceleration and rev like snot.
 
rovanmadlunatic said:
I do a lot of modding. In fact I don't run anything stock. I just swapped out the 2 bolt engine i modded to a 4 bolt. I had a wonky coil and it wasnt till i tried it on the 2nd engine I realized what it was. Any how it was replaced and my four bolt is just being broken in. It is a heavily modded cyf270 top end on the km bottom end. The km bottom end has a slightly lower deck height and it put my squish right at .5mm so it was a great match. Already the engine looks KILLER!! It should have awesome acceleration and rev like snot.
Click to expand...

Nice! Now We're talking!!!
:celebrate::celebrate:
 
I found that when the intake is open I have about 2mm on the exhaust side open to the case.. Hell you can see light pass between the ports. Maybe if the skirt was longer on the piston it would have a chance but it looks like they went way overboard with the base of the exhaust port. I should have hit the pistons I have with the caliper to find one that will close the gap but Hey not one of my finer moment's lol.
Looks like I have more farting around to do tonight...keep ya posted!
 
BertR said:
I found that when the intake is open I have about 2mm on the exhaust side open to the case.. Hell you can see light pass between the ports. Maybe if the skirt was longer on the piston it would have a chance but it looks like they went way overboard with the base of the exhaust port. I should have hit the pistons I have with the caliper to find one that will close the gap but Hey not one of my finer moment's lol.
Looks like I have more farting around to do tonight...keep ya posted!
Click to expand...


It's called overlap and think of it as 360 degrees and the intake and the exhaust are finishing and starting to work on each other's time and this is one of the reasons that a turbocharger will not work on a two stroke motor
The intake closes first and then the exhaust that's why a pipe is so important to these motors
 
Is this common to have such a gap between the bottom of the piston skirt and the bottom of the port on the exhaust. I have seen them close but never with this much space to stick a key into the case from the exhaust port.
 
If it's a mod motor who knows
Each builder has what he thinks is the best
Some boat guys have some extreme mods they do to there motors for top speed
But these don't do well for cars in my opinion and they break more often
You can't set up a motor for top speed and think it will live for very long in a car to much heat and up and down throttle for them [emoji389]
So if it's stock then it's normal
 
Last edited:
nitrocat said:
If it's a mod motor who knows
Each builder has what he thinks is the best
Some boat guys have some extreme mods they do to there motors for top speed
But these don't do well for cars in my opinion and they break more often
You can't set up a motor for top speed and think it will live for very long in a car to much heat and up and down throttle for them [emoji389]
So if it's stock then it's normal
Click to expand...


I hear what your saying.. This head came off a 5b I picked up on the cheap
With this ported head stock and ran flawless for the last 5 months. This morning I put it back on the original case and has the same gap so that's not the reason why it won't fire up.. To be honest it should run better on the OBR billet case so now to swap back to the billet and put on a spare coil , kill switch and 990 to eliminate the possibility of those 3.. I thank you for your input you have given.
 
First thing I would check is the carb and clean it with some motor spray and the pumper on the bottom of it
Just make sure the little flap on gasket is right and pulse line is lined up right it will start then
If the fuel you rune has methanol in it it will stop you're carb in no time
When mine won't start its my first stop
 
I got it fired up.. It was my coil! I will say that km ported head is a dog compared to a worn out OBR prostock lol. I don't run methonal or ethonal in my gas. I am going to pull the whole engine that came with this new addition to the fleet and drop in the losi full mod 7hp for a sand rail
 
BertR said:
Is this common to have such a gap between the bottom of the piston skirt and the bottom of the port on the exhaust. I have seen them close but never with this much space to stick a key into the case from the exhaust port.
Click to expand...

There should never be a gap between the skirt and exhaust port floor. Its a defect. It will let spent fuel from your pipe into the fresh charge in the crankcase or let the fresh charge escape out through the pipe, or both.The motor is built for low end power and have very low timings,but some where along the line they goofed.
 
Back
Top
✖
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock    No Thanks