how many ponies can you push?

is it possible, if you build a ktm sx 300 (bigbore)to its limit, like boring and stroking aswell to increase displacement, to achieve in upwards of 70 hp?

the reason i am asking is because i dont think that the big bores will be around when i get old enough to ride them, and i still want the experience of riding a massive dirt bike a few times.

is it likley that the maico 700 will be around for a long time? because i know you can get alot of power from the 700 two stroke for sure.

I haven't been able to find any info on increasing KTM 300 displacement to 400cc or more. It seems to me like such a mod would have a fairly large potential market these days.

Here's a 2008 300 dyno chart. I'd be happy with 5hp more everywhere myself, which should be super easy to get from 400cc:

I think you'd want at least 500cc to get 70hp with a wide powerband. You might be able to squeeze 70hp peak from a 300 at 9,500 rpm or so, but the resulting motor might be a little too exciting to be fun, if you know what I mean. :thumbsup:

I think the Service Honda open-classers will be around for a while, and it seems like quite a few of them pop up for sale in excellent used condition as well.

I'd definitely want to ride - or at least start - a 700cc Maico before I bought one.


If you could build the "right" pipe and make it fit the bike with stock cylinder it would pump out a lot more peak power - but it would not be fun to ride..

MX bases kart 250cc engines push well over 60 horses - close to 70 seems to be upper limit.

just turbo a 300. put like, 8 pounds boost. and youll probebly pass the 500's in a strait.

or just go ride a 1000cc triple snowmobile to feel big bore 2 stroke power.

never seen a ktm 300 so i cant speculate what its power potential is. just because you have 300cc doesnt meen you get 70hp. some cylinder designs are capable of more power than others. the old ktm big bores (500cc) were some of the best foundations to start from. tripple port exhaust with 8 transfers and huge reed cages. alot of potential in the old ones, not sure about the new ones. i prefer non power valve design. making a 300 or any bike for that matter run at the ragged edge costs alot of money and special fabricated parts. it would be useless except for drag racing in straight line at redline rpm.

70 hp on a 250lb bike on dirt?!?!? I push a whopping 135lbs and a 250 smoker is all i need. Anything bigger than that is asking to get hurt.

I'd definitely want to ride - or at least start - a 700cc Maico before I bought one.


The local bike mechanics had a maico 700 in the shop for a service, so when they finished they tried to start it, no good, way too much compression to even move the starter!!!

So the owner came in to pick it up and they asked him to start it----

He replied that he has to roll start it!!!!

Switch to methanol. Never used it myself but apparently it boosts HP by approx 20%, but goes through a tank twice as fast on the account the jets need to be about twice the size as when you use petrol.

The local bike mechanics had a maico 700 in the shop for a service, so when they finished they tried to start it, no good, way too much compression to even move the starter!!!
The 700 has a compression release in the barrel, does it not?

There's a guy claiming 70hp from a KTM250EXC based road race bike.

Most modern 2 strokes can reliably push around 300hp per litre. Some do it stock.

Whether they can do it while making an acceptable power delivery is another issue....

400hp/litre was commonplace from a 500GP bike, and the 125 and 250 GP bikes bettered that.

The real issue is that the big bore two strokes aren't about peak power. Their real appeal lies in the way that made their power, including the not so good bits like the vibration and difficult starting...

You can fit a 380 top end onto a 300, which would have to be the starting point if you wanted a modern big bore.

I think the real answer to you question is a 500AF.

do the people at honda af still build these cr 500 engines or are they scavenged.

a question about building a sx 300, i dont understand what you guys mean about being happy? like the engine becomes less reliable or the powerband is to strong?

like i have the desire to have a modern big bore when i get like out and grown, and im concerned that there wont be any. i know that they pushed around 70 hp, and i was wondering if that is achievable. like i just want the exitement of a bike that can lift its front end in any gear by openin the throttle... pure power i guess. the 125 dosent do it for me lol. still fun though :smirk:

You can play all day with the big bores..

The KTM 300, nice design, I have done 60HP without stroking or boring..stock feel bottom end pul with super midrange and hard long pull.. @ 8200 it started to fall off..

Yes it had a hit since the power rush was so great. Light Crank, Sx flywheel and balanced, domed light piston, Factory Fatty, Boyseen reed block, 38mm carb.. vortes ignition (Sx style curve spread out). VP4 racefuel... mild port and polish..

Since it is detuned to around 52-54HP and woods friendy for a Expert rider.. can hang with any 450 with same gearing..

I have experience with one stroker motor, I lost touch with the project as the guy went in another direction with the build by overboring etc.. At the point of a 5mm stroke, lighter balanced crank, SX skirt trimed domed piston with a 1mm shrunkin rod, spacer gasket it ran OK, pull a funny short power band to only get 53+ HP at 7300 or so..

After some port correction, the boyseen block, some epoxy in the cases, SST exhaust/silencer, XC ignition, 39mm oval carb had it back to the high 58 hp with loads of torque .. added a flywheel and then... project took a turn to a stock rod and stock cylinder...blah blah...

IMO that combo with some port welding and work woul have been cool for a desert screamer..

over 60 hp is very possible but the power will be narrowed and in the upper range..

You can also get a 380 and start some cool work on that puppy, heavier cylinder and head but some nice possibles...

Just need time and money

Can a 300 be punched out to a 400 safely? Or maby stroked aswell?

i hate to bump this but id like for someone else to help me clarify it.

bore and stroke of a modern 300 is 72x72 mm, a square engine. by what i have read, the bore and stroke of a 500 is close to 85x85mm. that means it is square as well, so the bore and stroke must be increased by 13 mils. isnt that a feasible number? or could a displacement of only 400 ccs be created by boreing and stroking 7 mils? someone please help clarify me.


Engine Type: 2-Stroke Single

Displacement: 685cc / 616cc

Bore and stroke: 700: 100 X 87mm

620: 95 X 87mm

Cooling: Liquid Cooled

Compression ratio: 700: 10.9:1

620: 10.3:1

Horsepower: 700: 78HP

620: 76HP

Carburetion: 39mm Keihin

Ignition: SEM 130w

Transmission: 5-speed

Starting system: Kick

Fuel capacity: 2.5 Gallon

Wheelbase: 58.5-inches

Seat height: 37.2-inches

Ground clearance: 14.0-inches

Front wheel travel: 12.0-inches

Rear wheel travel: 12.6-inches

Front brake: 260mm Rotor

Rear brake: 220mm Rotor

Dry Weight: 238lbs.

MSRP $12,995

for those that toomuch is never enough:thumbsup:

a 79x79 mm will result in 387.39153257975414 cc, so just a bit shy of your projected 400. but the real limit to how much you can bore and stroke any engine is the size limits of the barrel and the cases. you can only make the cylinder so thin before its not strong enough to handle the pressure. as far as the limits on a ktm 300 case and barrel i dont know exactly how far they can safely be overbored. you might want to check with a company that specialises in big bore kits and stroker cranks to see what the safe limit is. or check out that atk. boy i would love to through my leg over a 700 2stroke just once!

The 89 sx500 in my pic is kicking 80hp but that bike was not to fun to ride it was all buisness, stock it was 62hp with the recamended 50:1 fuel mix ratio

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