250SX / 300 head swap

**Rajunrick** · 01-06-2007, 02:10 AM

The other day I was using my dad's TRX300 to unload and stack firewood at my house when I made a discovery. His 300 motor and my 250SX motor look identical! So now I'm wondering, can you do a cylinder/head swap between a 300 and a 250SX? Anyone ever tried this? I looked around and found a 300 motor that is seized up and is fairly cheap. If no one has tried this then I'll get the motor and tear it down. Got another 250 motor that I can use as my lab rat.

Give me some feed back on this.

**p26575r16** · 01-06-2007, 03:39 AM

I am not completly sure, but I had a 250x that the top end went bad on. I then bought a 250sx in order to use the topend off of it. I rode the sx around a little and discovered that it was probably the funnest atv(c) that I had ever rode, so I never did the conversion. I do know that the 300\250x heads have four valves and the sx has 2. As far as the conversion I think it would work but don't quote me on it.

**bigreddaddy** · 01-06-2007, 04:45 AM

Very interesting. I would like more info myself since I have two 250es big red's.

**Jason T** · 01-06-2007, 11:56 AM

Hi,
The 4 valve 300 engine is only in the 300ex, the TRX 300 engine has 2 valves and is very similar to the 250sx engine!!
Check out the following links for our conversion:

http://www.3wheelerworldforums.com/s...ad.php?t=37678

http://www.3wheelerworldforums.com/s...ad.php?t=38932

http://www.3wheelerworldforums.com/s...ad.php?t=46728

hope that helps

Jason

**p26575r16** · 01-06-2007, 01:16 PM

Originally Posted by p26575r16

I am not completly sure, but I had a 250x that the top end went bad on. I then bought a 250sx in order to use the topend off of it. I rode the sx around a little and discovered that it was probably the funnest atv(c) that I had ever rode, so I never did the conversion. I do know that the 300\250x heads have four valves and the sx has 2. As far as the conversion I think it would work but don't quote me on it.

Sorry, I meant trx 300ex\trx 250x.

**Rajunrick** · 01-06-2007, 04:02 PM

Thank you Jason T!!! You didn't answer my question completely, but did give me a new idea. Tell me, how does that thing run with that 300 motor? Better? Worse?

**Jason T** · 01-07-2007, 10:47 AM

Hi,
It runs really well

You can't compare it to a 350x, but it is a good improvement over the 250.
It was a lot of effort to get it finished, but learning from our mistakes it would go a lot smoother I'm sure

I will try to get some videos of it in action, and reply to your PM ASAP

Jason

**Rajunrick** · 01-11-2007, 01:33 AM

I think I may have answered my own question. I was stuck in front of this computer all weekend with a bruised knee (ouch) so all i could do is look at the net. Jason T has already established that a TRX300 motor will bolt into a 250SX frame. What I found is that a 250SX has a bore and stroke of 74 by 57.3mm. A TRX300 and a 300EX both have a bore and stroke of 74 by 65.5mm. I need to do some more research and see if the 300 rod has the same crank dia. as the 250SX. If so, then all I would need is the head, jug, rod, and a 300 timing chain to turn my motor into a 300. I'll post what I find.

**Howard** · 06-05-2007, 06:19 PM

please let me know what your status is on your swap?

Thanks, Howard

**tapper190** · 02-13-2010, 05:47 PM

so, has anyone done this swap yet with just the top ends?

Thanks
Tapper

**Louis Mielke** · 02-13-2010, 06:03 PM

The top ends should swap but except for the piston. The piston should be the same size but the wrist pin location will be different between the two bikes. YOU WILL NOT get a performance increase by swapping the top ends. They are the same sized piston. The difference is in the crank shaft. If you could get a crank from the trx300 it would effectively be a stroker crank for a atc250sx. It will only get you to about 285cc because the trx300 is not a true 300cc engine.

**Rajunrick** · 02-20-2010, 04:26 AM

Wow, thought this thread had died long ago, lol. My project got shoved to the back burner for awhile now but I am finally back on it. Since then I have had alot of time to think about it. Instead of coverting the 250 to a 300 I decided to completely use the 300...... but I have a new question in my mind to answer. If a 300 engine will fit, will a 350 engine fit? Not a 350X, but a 86-89 TRX350 engine. Been looking at them online and they look identical to a 300 engine except for the exhaust ports ( has 2 ports like an X ), the angle drive, and the left side crank case cover ( has a recess in it that the front drive shaft passes through ). After studying it alittle I believe the cover can be swapped with either a cover from a 250 or 300. The angle drive from the 250 looks like it will bolt right up. The only parts I'm unsure about are the dimensions of the engine and will the upper mounts bolt up the same as the 250 and 300. The biggest problem is finding one. If I'm lucky I might have a complete lined up if the person is willing to sell. Lol, the thought of it is what make me foam at the mouth, a ATC 350SX ( if the conversion works ). Shaft drive with reverse, 5 forward gears ( believe the 350X has 6 because there is no reverse ), and a 350. Another thought is will the transmission from a 350X fit inside the TRX350? Lol, have a shaft drive 350X. And yes, I've had too long to think about this, lol.

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250SX / 300 head swap