250SX engine conversion to a 350 update

[Rajunrick]

Haven't been on here in awhile, but I have slowly made progress on my engine. I have taken an 86' TRX250 engine and tore it down to the cases. I found and bought an 86' TRX350 4x4 for $50, the selling point being the engine. Though it has been missing the leftside crankcase cover for X amount of years and the guy didn't know what was wrong with it, when I kicked it over by hand (auto decompression valve, love it!!) I was able to hear the intake stroke and the exhaust stroke. SOLD!!!! Took the 350 engine out and tore it down to the cases. The differences between the two is the mag flywheel for the 350 is larger, the kickstart shaft has the components needed for the auto decomp on the end of the shaft, the final drive and the transmission spined shaft are separate pieces, and the auto clutch has 5 pawls vs the 4 that the 250 has. The auto clutch is the same dia. as the 250 and the clutch basket is the same dia. and has the same number of discs. I took the 350 cyl. and tested to see if it would fit the 250 cases before I tore the 250 apart and it fit like a glove!! I've researched the crank bearings and they have the same part numbers. So this is where I'm at right now, I have the 350 crank and counter balancer out. I plan to clean up the 250 cases and get them ready for assembly. The plan is to put the 350 crank, counter balancer, timing chain and guides in, and reuse the 250 transmission, clutches, and flywheel. I will have to change the cyl. studs out because the 350 studs are longer. I will also have to deal with the upper engine mount on top of the head because it isn't in the right location for my frame. The finished results should be an upgrade to the over looked 250 engine. And yes, I plan to use the oil cooler and fan also. Any comments or tech advice will be appreciated.

[swifty]

sounds like a good start, keep us updated

[TheRealFatShady]

Can't wait to see it done! I know one guy on Highlifter.com was putting one in his quad, but that required cutting the frame.

Another master machinist on there used the TRX 300 (very similar to 250sx/es) engine with Rancher??? sleeve and 350x/trx 350 head. Not sure what he needed to do, but I believe it was a lot of work with a 350 crank or stroker crank.

And be careful not the crack the block when removing studs, I needed lots of MAPP gas.

I'm working on a 250sx engine swap with a TRX 300, 4 valve 300ex head. Got it mostly done just need gaskets. If that works, I can always buy a big bore sleeve to get 350+cc's.

Good luck. Just make sure the 350 engine isn't too tall for the frame. Maybe you can make a smaller top engine mount and it will work, not sure.

[Mickey Dunlap]

Well I hate to bring bad news but there are some things you will need to look at. #1 is the 250sx (and the trx300)motor spins backwards to turn the drive shaft the right way. The 250sx (and the trx300) has the cam chain tensioner in the front of the cylinder to take up the cam chain slack in the way the motor turns. Then the big one,this took me about nine months to get right,and that is building a cam to run backwards from a cam core out of the 300ex or 250x 4 valve head.

IMO it is better to use your 250sx crank and stroke it,use a 300ex or 250x head and do a big valve port and flow job with a big bore piston kit like the one in the pic to make a 380 or 390 out of it. You use your stock cylinder and you have your cam out of the 300ex to match the head and have it welded up and re-ground. The reason we go this way is the 300ex head with big valves will flow almost as good as a 350x,but you don't have to have a heavy big dome piston to get the compression up to run a big cam,infact the 390 piston is lighter the the stock 350x piston. This makes a faster and quicker motor then going the way you talked about.

118011

[TheRealFatShady]

Thanks for the info - I had to get a modded head where the cam had been re-ground and rocker arms had to be made. Hoping it works out; the piston does not seem to touch the valves at all, and it is all timed up properly. I did have to swap the output shaft from the 250sx engine into the TRX300 for it to line up, but I haven't had it installed/running yet.

Do you use bigger valves that stock for the 250x/300ex heads on your 4 valve conversions? And are those 380/390 kits semi-reliable? (And do they come off the TRX300 utility or 300ex?) Sounds like you've done a LOT of work in this area.

[Rajunrick]

I didn't even think about the engine rotation. Thought they all turned the same. Will have to check the rotation arrows on both flywheels out when I get home. But as for the chain guides, they both looked the same, just the 350 guides are longer. Head studs I wasn't going to exert too much effort on. If I can get them out great, if not I'll buy new ones. And the top mount, if it's too tall I can either dimple the frame, machine the mount off completely, or modify the frame to accept the new mount position. Ready to get home now so I can look at some of these things!!!!!!!!!!! Thanks for the info!

[Mickey Dunlap]

Thanks for the info - I had to get a modded head where the cam had been re-ground and rocker arms had to be made. Hoping it works out; the piston does not seem to touch the valves at all, and it is all timed up properly. I did have to swap the output shaft from the 250sx engine into the TRX300 for it to line up, but I haven't had it installed/running yet.

Do you use bigger valves that stock for the 250x/300ex heads on your 4 valve conversions? And are those 380/390 kits semi-reliable? (And do they come off the TRX300 utility or 300ex?) Sounds like you've done a LOT of work in this area.

It may look like it is time right,but when you crank the motor over it still going to turn backwards. The ex will be sucking in,and the intake with be blowing out. Bottom line is it won't work.

Yes we put bigger valves in the intake side only. The other good thing about using these heads is the ex pipe bolts right up,just like stock. All my motors are reliable,that's why we buy four strokes, isn't it.I build all my kits to run pump gas.

[Rajunrick]

And Mickey was right....... the rotations and cam lobe profiles are different. So I am stalled for a moment while I figure out my options. But on the goodside I put the engine together (no internals) for a mockup in the frame and it clears.... barely.

[Mickey Dunlap]

And Mickey was right....... the rotations and cam lobe profiles are different. So I am stalled for a moment while I figure out my options. But on the goodside I put the engine together (no internals) for a mockup in the frame and it clears.... barely.

Sorry,but look at it this way,it save you a lot of time and money.

[3Wheelingfool]

IMO it is better to use your 250sx crank and stroke it,use a 300ex or 250x head and do a big valve port and flow job with a big bore piston kit like the one in the pic to make a 380 or 390 out of it. You use your stock cylinder and you have your cam out of the 300ex to match the head and have it welded up and re-ground. The reason we go this way is the 300ex head with big valves will flow almost as good as a 350x,but you don't have to have a heavy big dome piston to get the compression up to run a big cam,infact the 390 piston is lighter the the stock 350x piston. This makes a faster and quicker motor then going the way you talked about.

118011

Ok, sounds good Mickey. I live just up the road from you so I'll just drop my 250 BigRed off next week and you can get started...:p

[jays375]

That would just be to cool to have a big bore 4 Stroke-Tech motor in your bike.Good thing he doesn't live near me.

[TheRealFatShady]

http://forum.highlifter.com/dual-300-build-up-m4342332-p6.aspx

There's a thread where a guy did what the threadstarter wants to do....took a lot of work and I believe the cam came from megacycles, trx350 cylinder with different sleeve, 350x head, not sure what the crank/rod/flywheel were.

Mickey - Have you gotten the 300ex heads to work on the 250sx or trx 300? That's the combo I am trying to use myself, the seller said he had it on his sx for 6 months but had to cut valve reliefs in the piston. Doesn't seem necessary for the trx 300 engine, though???

[Mickey Dunlap]

Ok, sounds good Mickey. I live just up the road from you so I'll just drop my 250 BigRed off next week and you can get started...:p

Yea I don't think that's going to happen any time soon.My arm isn't up to it!:(

[Mickey Dunlap]

http://forum.highlifter.com/dual-300-build-up-m4342332-p6.aspx

There's a thread where a guy did what the threadstarter wants to do....took a lot of work and I believe the cam came from megacycles, trx350 cylinder with different sleeve, 350x head, not sure what the crank/rod/flywheel were.

Mickey - Have you gotten the 300ex heads to work on the 250sx or trx 300? That's the combo I am trying to use myself, the seller said he had it on his sx for 6 months but had to cut valve reliefs in the piston. Doesn't seem necessary for the trx 300 engine, though???

Yea Megacycle sent me one of their cams,and like that guy said they didn't get the line-up right,but that wasn't the real problem as far as doing it right. If he would have degreed that cam in he would have found out it was a mess as far as opening and closing numbers,plus the lobe centers were way off. This is what took 9 months to get right. Cam grinders are not motor builders. Each time you give the changes to make they try to understand where to make the changes but the don't alway get it right. And each time they charge you to weld it back up and re-grind it. I had $2200 into getting mine right,then I couldn't reall run that cam because it had been weld too many times so it just became a model.

I don't have any heads right now. The 250sx and 300trx are the same heads and cylinders. The stroke is different and the rods are different but my 380 and 390 kits are the same for the 250sx,250x,300trx and the 300ex. I have to dish the piston to keep the compressing down on all 4 so I don't have to have valve pockets.

Oh and my cam grind can be put on the 350x to if someone wants to go that way.