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Thread: 300EX engine in a 350X?

  1. #1
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    300EX engine in a 350X?

    Is it a bolt-in swap?

    With all the talk of Goki starters recently, if the 300EX engine is a direct swap, BAM, OEM e-start and be done with it.
    The story of three wheels and a man...

  2. #2
    wild200x is offline At The Back Of The Pack Arm chair racerFirst time rider
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    I have heard this is true. Also that you could put the 350x top end on? Not sure. Just what I have heard.

  3. #3
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    350x top ends wont work on 300ex. you can use a 350x head but its a bad idea as they dont flow as much oil. Use 300ex motor, all of it and spend the money to get the motor some good HP. it can be done. I am doing a 300ex motor in a 350x right now. I have a 11.5:1 CP piston in my motor, head will get a cam and porting with a 5 angle valve grind....a nice exhaust and it will move just like a 350x.

  4. #4
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    Thanks.

    I've got a line on a 300EX that is about to be junked. I don't have a use for it at the moment, but don't want to see it just tossed.

    If I could find a 350X roller at a bargain price, the e-start and reverse 300EX engine would be bee's knees.
    The story of three wheels and a man...

  5. #5
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    Other than the top engine mount and finding a spot for your battery ( I just use the battery box from the 300ex) it's a direct bolt in.
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  6. #6
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    here 9s mine for those who need to see pics. Ive know about this swap 15 years but still never saw one.
    Attached Thumbnails Attached Thumbnails 20210312_224541.jpg   20210312_224638.jpg   20201230_225023.jpg  
    Last edited by oldskool83; 03-31-2021 at 12:26 PM.

  7. #7
    patriot1 is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    Oldskool83:

    Were you also able to use the 300EX swingarm? axle?

  8. #8
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    300ex swinger is 3/4 "shorter in length and wider in the width between frame. It also does not have a rear shock mount like the 350x does. I am using a 350x swinger and 350x axle. You want to use 1986 350x swing arm as it has the gussets and rear brake stay pin. If not you can use 1985 but it needs to be gusseted and have the pin added. Not a big deal either way.

    The chain won't line up as 300ex drive sprocket is in different location than 350x sprocket. I put an order in for a laser cut adaptor to line everything up. It's not a really big deal

    I have both year swingers. You also use 86 350x / 300ex carriers with are 100% the same. You will not use 1985 carrier, they are junk with the finned adjustment tabs.

    I had a 300ex swing arm a cut at work cut in half with a band saw. I didn't ask him to do it but he miss understood me. I had a bad 1985 swinger with a carrier someone cut in half I spend the next few days grinding apart the 300ex swinger for its carrier and added it to the 350x. my 1985 swinger is about 1/8" longer then factory now. Nothing to brag about.

    Next i had the chain guide mounts cut off and 300ex ones welded on since you can not get new 350x guides. There was a guy selling them for $90 JR trike sports but he stopped also. I had a TM design one and figured I'd just use it. 300ex stuff should be around another decade at least.
    Attached Thumbnails Attached Thumbnails 20210411_125133.jpg   20210411_125139.jpg   20210411_125150.jpg   20210406_184026.jpg  
    Last edited by oldskool83; 04-11-2021 at 12:59 PM.

  9. #9
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    Quote Originally Posted by ATC King View Post
    I've got a line on a 300EX that is about to be junked. I don't have a use for it at the moment, but don't want to see it just tossed.
    Did you end up with the EX as a start to future project?

  10. #10
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    I just need to go get it.

    I had put it out there for someone else to use, but I'm going to keep it for myself and make a long term project out of it. Long term, because I'm not throwing mad money at it, so I'll have to wait until that right 350X donor trike comes my way.
    The story of three wheels and a man...

  11. #11
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    I bought all my 350x parts independently from people just wanting to blow stuff out. Then I sold off what I didn't need. My 350x has cost me nearly nothing. Not paying these 4K prices for clamped out machines needing full overhauls. Until I'm done I might have a 1990's used prices into my machine.

  12. #12
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    Finding a good frame is going to be the hardest part.

    I don't want some rust bucket that'll need major repairs, or anything with more cracks than solid tube. Without a jig, keeping everything true during major repairs is questionable, and without a good and straight frame to start with, building a proper jig would be virtually impossible, unless Honda has a 350X frame jig squirreled away somewhere. I couldn't afford it anyway.

    No hurry either, because I've got to make space for the project before I can really get it going. I've got to get ride of a couple rides to do that, no way around it.
    The story of three wheels and a man...

  13. #13
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    My frame was all broken up..they will be if used good. Your going to need to gusset the heck out of certain area anyway. I paid $180 or so for my frame shipped. Bought a gusset kit and new foot peg plugs. Welded it up.

    Working now on the horrible seat latch area. You wont find a frame not needling work. Plus there are item's not needed on the 350x frame to run 300ex motor and you'll need to weld stuff onto it also I'm sure.

    You want a frame needing work as your going to make it better, nothing build in 1985is going to be like todays designs. All we have done is make things stronger to last.

    I titled my frame too. Ill make a build thread one day how to do the 300ex swap.

    However I do feel people need to think to build what they want, no one has told me tricks. I've figured it out myself with out asking a single question.

  14. #14
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    I'm setting the frame goal high, so I have wiggle room.


    Gussets were already on the list and a battery box and such bits; already considered. I don't think I'll be able to slide in to a frame for that $180 range unless luck is on my side. It's a virtual trike desert around here, and any of the good stuff is hidden away, collecting dust in some geriatric's garage.


    "nothing build in 1985is going to be like todays designs"

    For sure. Even the welds look ten times better nowadays.
    The story of three wheels and a man...

  15. #15
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    I just finished my new seat latch area for the frame. That's one area nearly all frames split. I may offer kits as a fix. Depends how busy my job keeps me and another issue i have at hand. Each day i do somthing, atv atv or home rekated... btw i got my frame and triple tree off a facebook group. I noticed people in groups really want to sell the paets vs the clowns on your local marketplace who just are day dreaming.

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