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Author Topic: LT500R to 400cc engine mods to make NEEDT Legal  (Read 582 times)

Offline Scott270R

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LT500R to 400cc engine mods to make NEEDT Legal
« on: February 25, 2019, 10:28:21 am »
Hello,

The NEEDT rules have recently changed for TT and Short Track racing in 2019 allowing 400cc 2 strokes to run against 450cc 4 strokes.

Has anyone tried running the 70mm LT250R crank in an LT500R? Or the LT500R rod on LT250R webs? Or would it be better to sleeve the stock cylinder down and stick with the stock 86mm stroke? I was thinking a shorter stroke would allow the LT500 to rev out further more like a big bore/stroker  Honda TRX250R.

Interested in what others have tried or would recommend to get the displacement down to 400cc legal. I would like to convert my 250 to an LT250R/500R Hybrid to run legally at the Nationals.

Also, would the latest generation LT 500 pipes still work well with the decreased displacement?

Thanks for the help, I know many on here are experts in modifying LT500 engines.

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Offline ZillaFreak

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Re: LT500R to 400cc engine mods to make NEEDT Legal
« Reply #1 on: February 25, 2019, 02:44:42 pm »
I doubt you will find anyone that has done this.
It might be easier to sleeve down as less work on the bottom end, just need to figure out the piston (maybe a custom piston) and get it to work.
If you know anyone with a machine shop, maybe make a custom crank that is shorter stroke. If you do this, keep us updated, this is very interesting.

Offline Motoman991

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Re: LT500R to 400cc engine mods to make NEEDT Legal
« Reply #2 on: February 26, 2019, 01:21:50 pm »
I've never heard of anyone doing something like that.  Seems like a substantial amount of time and money to get it to work.  What about using a KTM 300 engine and boring/stroking it or something?

Offline Q2W

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Re: LT500R to 400cc engine mods to make NEEDT Legal
« Reply #3 on: February 27, 2019, 08:05:52 am »
The honda 330 piston is 76mm.  That with the 86 mm stroke of the 500 is around 390cc.

Not sure how you would sleeve it.  I bet a company like LA sleeve could help.

Probably end up being cheaper to run a 250r motor with a 370 kit.

Offline ZillaFreak

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Re: LT500R to 400cc engine mods to make NEEDT Legal
« Reply #4 on: March 04, 2019, 08:04:33 am »
The honda 330 piston is 76mm.  That with the 86 mm stroke of the 500 is around 390cc.

Not sure how you would sleeve it.  I bet a company like LA sleeve could help.

Probably end up being cheaper to run a 250r motor with a 370 kit.
Could do this, not sure if the wrist pins are the same, but a custom rod is nothing.
Just would need to check to make sure the rod doesn't hit the piston skirts and it could work.

Offline Motoman991

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Re: LT500R to 400cc engine mods to make NEEDT Legal
« Reply #5 on: March 05, 2019, 10:38:47 am »
KTM made a 380cc two stroke. 

Offline RUn2it

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Re: LT500R to 400cc engine mods to make NEEDT Legal
« Reply #6 on: March 20, 2019, 06:17:39 pm »
I'm a little late but what a nightmare, smaller bore, imagine the port work to a thick iron sleeve, what about cooling through all the iron to cooling jacket, and headwork. Flip side short stroke means different crank and now you have all the open area in bottom from smaller crank throw, and pipes are designed for displacement (volume), rpm, and if you found someplace to fabricate the parts..... What would it cost for HPR to build and test 1 pipe. I won't say impossible but improbable just from the $ point. Just my opinion. I would read the story if you do try it though.

 

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