442cc atc 250sx.

[350xmerk]

Well have been getting parts made and customized for a honda 300 4x4 but decided to put the motor in a atc 250sx. Finally got all my parts.

Crank is from a 86 trx350d machined to fit the 300 honda cases. It's +2.5 stroke over 300 crank.

Next I used a 350x head that I had highly ported and polished.

The cam is a mega cycle performance cam for a atc 350x but built to opposite rotation because the 300 motor spins backwards this corrected that.

I had the cases bored out to accept a Wiseco 91 mm piston 10.5:1 compression for a xr500.

HD valve springs and hardend rocker arms

400ex carb for it. And ebc dirt racer clutch.

Custom cometic jug spacer and 116 length timing chain and cylinder was o ringed along with a copper head gasket. Cylinder is off a 350x.

Any opinions on how his engine will run in a 250sx a fun machine?

[350xmerk]

I'm also thinking of throwing a 450 foreman clutch basket and primary gear for 15% higher gearing

[slashfan7964]

Man that thing is gonna have some balls when you are done with it!

[NeverLift]

That's a thing of beauty. I love the custom motor builds. Can't wait to see it come together.

[350xmerk]

Ya that's the plan lol. I've built a couple with stock 300 honda motors that were fun in sure this one will haul with upped gearing. I'm really big into custom 300 honda 4x4 motors for mudding but already have a 477cc engine like this that has alot more goodies than this engine so I figured we will throw this one in a 3 wheeler lol

[jakep53]

That thinks gonna haul @$$ when its done!!!

[jakep53]

what jug are you using?? is it a 300 jug with a xr500 sleeve??

[350xmerk]

350x jug, 500 sleeve

[Stonewall]

This is gonna be awesome! This is very similar to what I want to build to put in a Big Red for mud boggin. What work had to be done to the 350D crank? How much machine work does it take to fit the 350X jug on the 300 case?

[JasonB]

DUDE! This things going to be sick! that would be my ideal 250sx, real power and geared to haul balls!

[Ghostv2]

Any plans to strengthen the bike up for that kind of motor? Gussets? Extra welds?
Or do you think you won't need it?

[350xmerk]

The 350d crabk was turned down on the fly wheel side so the fly wheel off the 399 will go.into it and it had to be shortend slightly. Putting a 350d jug on a 300 case requires no machine work but since my sleeve was so large the cases had to be bored larger

[Stonewall]

The 350d crabk was turned down on the fly wheel side so the fly wheel off the 399 will go.into it and it had to be shortend slightly. Putting a 350d jug on a 300 case requires no machine work but since my sleeve was so large the cases had to be bored larger

So are you runnin the 350 or 300 crank balancers? If I'm readin your first post right you had to use a spacer to get the jug to the right height? Thanks for answerin all my questions lol, I can't wait to see how this engine turns out!

[Zach]

Wow, this is an awesome build I really cant wait to see how it turns out. Sounds like you really had to work some numbers to get everything right, just awesome!

[big specht]

As for the spacer why not use the 500 jug and head ? Or is the 500 jug to tall? and how much longer is the 350 crank? and the clutch side is all the same ?