14.2:1 Piston on 250F

Create New Tag

1/9/2019 5:18 PM

Who here has experience running a piston with a 14.2:1 compression ratio on 250F?

I’m having second thoughts about putting a CP 14.2:1 piston into my brand new 17 kx250F. I’m aware of the extra operating expenses; I don’t have a problem running VP T4 or the more frequent service intervals. I’m planning on rebuild top end every 30 hours. Bottom end at 60. This has worked fine for me in the past with similar mods. I want to maintain or improve the KX250F’s free revving quick/light feel. Ive always ran a 13.8:1 compression ratio and I’m thinking maybe it was ideal.
Something is telling me that the higher compression will result in more pronounced engine braking. I bought the CP piston for cheaper than a new OEM replacement and I know it’s a great product, I just am thinking I may not like the additional compression. I’ve searched and have come up empty handed with any good reads.

For what it’s worth, The bike will also be equipped with a FMF, ECU tune, Hinson clutch, and ISF/REM & Cryo treated transmission. All other engine components will remain stock for at least a couple rebuilds. Later on I may upgrade to a more durable Carrillo rod


1/10/2019 1:01 AM

I'm pretty sure ktm comes stock with 14.1:1 but if it were me I'd go for it especially with all of the other mods I think it will go great together and worse case scenario next top end put the 13.8 back in


1/10/2019 2:12 AM

Ive got a hc piston from cp in my 09 kx250 and 450. Honestly didnt even notice additional engine braking. Durability doesnt seem to be hindered either. Bike still starts first kick every time.


1/10/2019 2:29 PM

Ran a JE 14:1 last rebuild now running a CP in our 2017 KX250f. Pretty much stock other than that & the head cleaned up some. Does have a bit more low end power but seems to vibrate a bit more than our other kx250f which is still completely stock? Very high quality product though. Run 50/50 race an non-ethanol premium. Everything looked great when I pulled the JE out I just rebuild ours every 30 hours or so since it's my son's nike and he is a faster B rider.........


1/11/2019 12:20 PM
Edited Date/Time: 1/11/2019 12:20 PM

4 tenths of a point isn't going to make or break the combo. Put some good fuel in it and maintain it just like your used to.

On compression alone I doubt there is even a .5hp difference between the 13.8 and 14.2 pistons.


Race Bike: 2018 KTM 350SXF

Other Bikes: 1985 CR80R, 1990 CR250R, 1998 PW80, Specialized Fuse Comp 29.

Sold: 2016 YZ250F, 2012 CRF250R

1/11/2019 4:31 PM

I’d run it. It will be fine. I think you will notice a slight increase in bottom end torque like when you come out of corners.
When I raced late 2000s CRF250R s I would Run the cp 14:1 or something in that range without any problems with it.