350X Top End on 250SX

[nstyle73]

Either this swap is something that I've just never read about and missed it, or there is a glaring reason that it won't work. I slipped the cylinder from a 350x right on the bottom end of a 250SX. Granted, I don't have a piston to see if case machining would be necessary, or a head. I'm assuming there would be some work involved in timing the valves, but does anyone know if this has been done and what issues stand in the way? Sounds like a winter project to me.

[TheRealFatShady]

To be honest, I've heard that the TRX300 (similar to ES/SX engine) has been modded with a 4 valve 300ex or 250X head. I know those engines are similar to the 350X (somewhat, and from quads).

So I recall talk on it, but people seemed turned off by the idea. I'd like to find out as I want to run dual pipes and a 4 valve head of my big bore 376cc Big Red (whenever it's ready....not for awhile)

[Dirtcrasher]

The SX cam chain adjuster is in the front, the 350X is in the rear. That alone would make me think the engines rotate different, the adjuster is usually opposite the drive or pulling side.

Thats just scratching the surface.

Call me a spoiler but I highly doubt the 350X top end bolts on for simple and complex reasons neither of which I feel like looking into; Like hitting the starter, bolting, cam chain, oil pump for 4 rockers instead of 2 etc etc etc.

But have a ball if you wish, you'd be a hero :D

We've been swapping parts for 15 years now, I can't imagine that being missed..........

[TheRealFatShady]

Also, if the 350x top end is like the 250x and 300ex, there are 4 little relief ports on the piston, as opposed to 2 on the SX. I'd look into oversized valves as I BELIEVE the 350X goes the opposite way (cam/timing chain wise) than the SX, so if you made your starter work backwards you'd, theoretically, be able to make it work really fast in reverse???

Maybe I'm wrong, just going by the info I read awhile ago when looking into the EX head swap on the TRX300 engine for my Big Red or SX.

If you really want performance, put in 450 Foreman gears to a TRX or SX engine, get a 350x or 400ex carb, and then you'll have more speed. Or a gear reduction if you want more torque.

[Dirtcrasher]

If they run different directions the one way clutches would work opposite of how they should...........

[nstyle73]

LOL, I just had a 350x cylinder laying on a shelf, and a 250sx bottom end laying there, and to conserve shelf space slid the cylinder onto the bottom end, and thought....huh....I bet someone already thought of this. 376CC? Foreman 450 gears? Very interesting possibilities. The Foreman gears in the tranny or the rear end? I have to split the cases on that bottom end sometime. Might as well upgrade while I'm in there.

[TheRealFatShady]

Gear reduction or faster gears go in the engine/tranny as opposed to the rear end.

Here's a gear reduction info/picture thread:
http://forum.highlifter.com/tm.aspx?300-gear-reduction-pics&m=2868399&high=300+reduction+300ex&mpage=1

Basically I believe the concept is the same, only with an adverse affect for the Foreman gears. EDIT - Actually in the thread they are just called 450 gears, not sure exactly which model. Here is a thread:
http://forum.highlifter.com/tm.aspx?m=2372108&high=TRX300+Gear+reduction+best