Atc300x aka 300ex in 350x

[oldskool83]

Here's my 300x.

Years ago I was told yeah 250x 300ex motors swap right in. Never saw one in real life or on a forum however. Since I hate job I needed to get my mind off things.

300x project started

YES a trx250x / 300ex motor a FITS in.
NO a trx250x / 300ex motor does NOT bolt in.

The front lower 350x motor mounts and rear lower mounts line up and bolt in 100%

The head stay mounts are not used.

The middle lower bolt between frame rails lines up and bolts in however different width spacers are needed.

The rear upper motor mount lines up but does not bolt in. You need an extra 20mm on that bolt. 350x is 140mm so you need 160mm long bolt. Belmetric or McMaster-Carr have longer bolts.

The rear motor mount that bolts onto the frame needs to be formed opposite (mirrored) of the 350x oem mount. The oem length spacer needs to be made longer.

You could mill half the 300ex rear mount off on opposite side of motor just as guys did on 350xs. But that is not the correct way of doing it. Just make a new mount or you will crack cases. Any metal forming / laser cutting place can do this for you.

I have the specs to produce the correct spacers for the swap.

Lastly here is the start of my seat latch base. The sheet metal was junk. Most of them will be by now. I had laser cut this piece to oem specs. However between 3 frames not one had the same dimensions. The old frames were just build +/- 1/16" at least it seems.

If doing this swap please fully gusset your frames, if you think nearly 40 year old paper think frames are strong I'd suggest not attempting this type of build.

Stay tuned. This should help people put.

[shortline10]

Nice job Aaron , 300ex parts are cheap and easily found .

[jasong_10]

Thanks Aaron, this is great info.

[ATC King]

Thanks for making the thread. I'll keep notes.

[oldskool83]

Cleaned up clarke tank, converted to yamaha atv petcock. Just sealed nicer than old clarke style.

[oldskool83]

Built the front hub tonight.

[BarnBoy]

I like it. Keep it coming....

[oldskool83]

Spacers made & longer bolt came in.

Motor fits & bolts in 100% now. Attached are my drawings you need to produce the spacers for the swap. OD / ID of spacer is not very critical.

I used:

1" OD for rear mount with .425" ID
7/8" OD for the front lowers with .425" ID

OD means outside diameter
ID means inside diameter

Next will be to line chain perfectly for conversion sprocket hub spacer. There are more cool detailed things to come.

[oldskool83]

Front hub and caliper rebuilt

[oldskool83]

I chose not to use the 350x oil cooler & guard.

Two reasons:

#1 - 300ex spark plug coil fits in perfectly at the bottom of the guard mount.

#2 my cooler did not seem to fit nice on OEM mounts, figured something was tweaked.

All other mounts will be shaved off but lower guard mount. I had a laser cut piece made to copy the 300ex sheet metal mount. It connects to the head just as it would in a 300ex frame.

Engineering takes the most time. Being picky helps things fit as Honda designed it. So far it's not a hard swap. The wiring harness might be the worst part. I'm at that point shortly.

At the same time there is a 300ex build going for my daughter too one day.

[oldskool83]

Axle came today. It's width assembled was 43.5" which is .40" narrower then the stock width 350x width at 43.9" thinking of installing a small wheel spacer on the rear or maybe trying tusk extended hubs. Little more mock up is needed until I decide.

So there it is...300ex axle in 350x.

Will check chain alignment once the coil mount it welded to the frame which will be finished by end of this week.

[oldskool83]

Finished up some small assemblies. My loft is stashed with parts in process. These pins came from forum member I've personal met multiple times. I bought about 7 atv/atc's off him this past winter. The front fender got new OEM rubbers and collars. Tonight I hope to shave everything off the frame which is not needed at this point.

[oldskool83]

More. Electronics positioned and will be welded in place tonight with photos.

Heres the coil holder. This cost me $1.45 to be laser cut. It's very cheap, I sub everything out now than have it milled. Anyone can use the attached drawing if doing the the same swap.

Louis came thru again with new front axle and spacers.

[oldskool83]

Coil mount per my drawing.

Coil mount welded in location to coil wire does not hit head at all.

CDI mounted, used 300ex mount but cut it up. Needed to weld a .750" OD x .500 thick boss on frame. It had a M6x1.0 threaded thru hole in it.

Lastly I need to have a laser cut connector holder made. I did not have an extra frame to cut that off. Everything clears nice so far.

[oldskool83]

Spend 2 hours messing with wiring routing. PITA - After all that time still need to see if the shock fits in. I needed to take photos so I remember a few months from now when it goes back together. As of now looks like 4 things need put on the frame to hole connectors properly. I have 3 more things to draw up to get made.

[oldskool83]

Scored another 300ex and 400ex i robbed parts off than sold. Here is the full 300ex brake assembly on the 350x swinger. Not sure I will keep this setup or use my yz426 frobt caliper and 250r tiny rotor setup. I tried 400ex but it was loose on the 300ex/350x carrier. Plus the 400ex caliper angles way back so the carrier bolt was interfering. Either the 400ex carrier is just a tad larger od or the aluminim 400ex brake stay was evenly worn. I'd rather use steel anyway.

Mounted a 300ex shock which I like better. Ill make some spacers to check alignment...chain alignment seems to be off .25" which some spacers will fix that.

Lastly I took off a oem 400ex muffler.

If i choose the 300ex shock I'll buy an elka. The 350x one I fully have rebuild and relaved but I'd almost rather have a longer travel shock like the 300ex.

[oldskool83]

Switched back to 350x shock... I did drill and tap frame for 12mm bolt so if I want to change to 300ex shocks in future I can. As of now I went back to the 350x shock and adjusted it a bit more. Bought new front fender hardware, powder coated and polished up triples...now to put the stem back in. So much massage this or that. Turns out I need a 300ex CDI box. The black back on the harness was only for reverse. Time to go find another 300ex to buy rob parts off or then sell.

[patriot1]

Awesome thread. Thank you

[oldskool83]

38mm oem 400ex carb i had...yeah this don't fit worth crap. Barely ckears frame. Oem 300ex carb boot won't work. Cant take 32mm boot and stretch to 38mm...however a billet aluminum adapter and radiator hose works. I would recommend oem 300ex carb that way you have splices from air filter to carb. If using 300ex carb and air boot air box moves back an inch, which is nicer anyway. Ill sleep on what I want to do. If not ill buy another 300ex for parts!

[oldskool83]

Acquired another 300ex for cdi. Designed cdi bolt on bracket and cutting up two oem cdi holders from 300ex.

Next issue is 300ex valve cover breather needs rotated 30 degreed since it directly hits 350x frame. This build is basicly a full design job. The details can get you in trouble so its constant mock ups. Hopefully this helps future 300x copiers

[oldskool83]

Battery stuff all found a home. Not fun. However nothing is in the way after 20 mock ups. I'm about tired of this project!

[Dirtcrasher]

What was the deal with the 300EX rear shock?? Same overall length eye to eye just different diameter?? Same spring weight?? Resi or no resi?? Just curious... I think guys use the 300EX fronts on the 250SX rears.

As far as your chain, you probably centered the motor based on the couple of mounts that lined up.

If it's only 1/4" you can probably space it out, I'm sure you know Louis makes the 350X to 300EX sprocket adapter which could be trimmed down but I think you like making your own stuff. If you just need an aluminum ring I can help, not sure if HRE is still around to help.

[oldskool83]

Hre is doing his own thing. I'm doing not adding to his 6 month back log of work. I work laser fab shops and work at a machine shop too. Been building on my skills by trial and error.

Oem 300ex shock has no res. It's also about same length but will mount different.. It does make the machine ride way softer which is great if your under 150lbs. Currently I sold the project to a new buyer who wants to complete the swap. I saved the rear end to use on my 300ex I'm building too. I'm happy to stop messing 3 hours a night dreaming up things to build this. I wanted to spend more time with my daughter and less time in garage. I have 3 300exs now just from parting 12 machines out this year. I learned alot.

Off to build a 300ex trail machine now. I honestly don't have 1 machine I really want lol...but hey I'm a great dad.

Who knows what I'll buy next. I've bought 6 machines in 4 weeks...I kept cool parts from them.

For now I feel like the obsession stopped. I needed to break up with the build.

[jasong_10]

Sorry to see the project end but understand the reasoning completely, they only grow up once.

[ATC King]

Thanks for the input otherwise.

Mine is still summer hibernating in back of a project pickup, leaking the diesel I filled the engine up with. Maybe this winter I'll pick up the torch.