Boost port matching

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4/12/2018 11:54 PM
Edited Date/Time: 4/13/2018 2:40 PM

Should the boost port match have a smooth transition through the case? Or not that important. My transition ports needed only a light touch up to be nice. Photo


4/13/2018 5:54 AM

Port matching between the cases and cylinder DOES add more performance due to smooth airflow. Doesnt have to be that way but its definitely better.


4/13/2018 6:43 AM

Allways looks better if it all lines up but over the past few years there is some good evidence that the smaller boost port is better going by later KTMs...


4/13/2018 1:13 PM

100% yes, what yr CR125 is that? I can help you do a few mods that will get more out of it, post back- J


4/13/2018 4:09 PM

Cheers guys. It's a 99 cr125 freshly plated at 139cc. Pj38 carb, rad valve set up.
The transfer ports needed a small grind to match nicely so that's good, but that boost port is way off and I was hesitant to open it up to match as it needs a fair bit taken out to suit.


4/13/2018 4:23 PM

aha great motor,,,match the boost port for sure, and roll all those sharp edges over the crank bearing areas so the gases flow and roll over them easier..also..I will try to photo edit the pic and post something..


4/13/2018 4:45 PM
Edited Date/Time: 4/13/2018 4:47 PM

OK, so Photo

here are the areas I would address, the boost port on the cyl should be ground back so the "overhang" is removed..the other areas circled and then highlighted in blue should all be "rolled" over heavily so they are round...the intake where the cyl and the cases meet at the end of where the reed block would be,(bascailly the circle closest to the piston), should get a heavy roll so the gases have a promoted "up" path. Hard to describe but you wil lsee in there.

Also, ANYTHING that gets fresh fuel should be roughed up REAL HARD, prickly to the touch, hammer, punch, tooling, etc, very coarse feeling. Anything that sees burnt gases should be polished, head, pv flapper..

The back two rear boost ports can get a little epoxy and profiled so the "hook" is removed and so the port appose each other. When the gases come up, they will shoot across the piston crown at each other to cool the piston crown and retain more charge..This is very important, the gases serve two functions, combustion yes, but cooling the crown! You will need a right angle porting tool to get in there after the epoxy dries.

The spark plug lig can be nipped down by .75mm so the plug sits even with the bowl, chuch it up in a lathe and nip away at the lug.

Your exh height shoudl be right around 27.5mm and a finished squish clearance of no less than 1mm.

If I look through my notes I will see if I can find more info, post back! looking good!


4/13/2018 8:32 PM


Kinda like this...

4/14/2018 4:05 AM

yes that area looks good, the boost port window could use some work as well as some of the other areas outlined. All those surfaces can be real rough like a hi speed pointed jack hammer was used to texture the surface.


4/14/2018 1:31 PM

JBone knows his stuff and is pointing u in a great direction, he's also very good at doing head work, had him do a couple of ktms


4/14/2018 1:45 PM

thank you!!!


4/14/2018 6:15 PM

Yeah pretty cool of him to do it. I've already got started on it.
Josh. What are your thoughts on main bearings, would you run ceramic hybrids? Much cheaper are also polyamide caged bearings that I'm leaning to.


4/15/2018 3:33 AM

I love this stuff and always love to see people diving in, thats how you learn!

As for the bearings, I have used the ceramic coated before on a few builds, they DEF made a diff in how easy the crank spins over, feels like "buttah", (You have to say that like you are from Boston, my neck of the woods lol).

As for the polymide, I havent heard of them?


4/16/2018 2:18 AM

Polymide are a plastic like cage in the bearing so it's a touch lighter, guessing spins up and decels easier.
Saw them on a kart website. Going to stump up for the hybrids. I'm in deep enough what's a bit more hey?