Hi i was just wandering when a 2 stroke engine is stroked to a higher cc what are the different ways of doing it.
Thanks
Full Version: How a 250 is stroked to a 270 ?
Increasing the engine displacement is done in one (or both) of two ways - increase the bore or the stroke. The bore is the diamater of the piston. The stroke is how far it travels.
To increase the bore:
Typically the engine cylinders are sleeved. That is, there's a tube of metal slipped into the block, and the piston travels within this tube. That means the piston won't wear the inside ot the engine - just this tube. To increase the capacity, then, you can "overbore" the engine, by running it with a wider piston mated to a tube with thinner walls.
Benefits - Increased displacement / power.
Hazards - Make sure the engine can still disperse the heat of the engine. Not only do you now have less material there to soak up the heat, but you're burning more fuel so you're generating more heat. Also be aware of the weight of the new bigger pistons. If they're heavier, and you're revving the engine high, you'll be putting a great deal more force on the con-rods, which make an unpleasant "ping" noise when they snap...
To increase the stroke:
Much less trivial. Basically this involves changing the profile of the crank so that the points that the con-rods attach to moves further in each rotation. That means fitting a bigger crank. Of course, that alone (whilst not being a simple job) means that you'll slam the pistons straight into the head, so you must replace the con-rods with shorter ones. And that means ensuring that the piston won't be drawn too far into the crank on each stroke.
Well, that's the basic overview of how it's done. What you can get away with comes down to what parts, tools, time and money you have on your hands. A Ducati 748 can be increased to a full litre, but the motor in that is the same as the 996, only sleeved down and the crank reduced - they've taken the above principles and run them the other way to create a new model in a smaller (and more sellable) size.
If you want to do it, study your mechanical engineering a bit. The sleeves must be an "interference fit" in the block, and the pistons + rings should be an interference fit in the sleeves. The piston mustn't get too close to the head (aside from banging valves you need to worry about your compression rising too far and blowing seals), and it mustn't go too far the other way as it needs to stay in the sleeve to keep the seal. The theory is simple enough, but the practise is hard and expensive.
Ah - just realised you can ignore the bit about banging the valves for a 2-stroke. But it'll still slam the head pretty hard. And you have to be sure that the pistons will mask the inlet and exhaust ports correctly. Especially if you're increasing the stroke.
To increase the bore:
Typically the engine cylinders are sleeved. That is, there's a tube of metal slipped into the block, and the piston travels within this tube. That means the piston won't wear the inside ot the engine - just this tube. To increase the capacity, then, you can "overbore" the engine, by running it with a wider piston mated to a tube with thinner walls.
Benefits - Increased displacement / power.
Hazards - Make sure the engine can still disperse the heat of the engine. Not only do you now have less material there to soak up the heat, but you're burning more fuel so you're generating more heat. Also be aware of the weight of the new bigger pistons. If they're heavier, and you're revving the engine high, you'll be putting a great deal more force on the con-rods, which make an unpleasant "ping" noise when they snap...
To increase the stroke:
Much less trivial. Basically this involves changing the profile of the crank so that the points that the con-rods attach to moves further in each rotation. That means fitting a bigger crank. Of course, that alone (whilst not being a simple job) means that you'll slam the pistons straight into the head, so you must replace the con-rods with shorter ones. And that means ensuring that the piston won't be drawn too far into the crank on each stroke.
Well, that's the basic overview of how it's done. What you can get away with comes down to what parts, tools, time and money you have on your hands. A Ducati 748 can be increased to a full litre, but the motor in that is the same as the 996, only sleeved down and the crank reduced - they've taken the above principles and run them the other way to create a new model in a smaller (and more sellable) size.
If you want to do it, study your mechanical engineering a bit. The sleeves must be an "interference fit" in the block, and the pistons + rings should be an interference fit in the sleeves. The piston mustn't get too close to the head (aside from banging valves you need to worry about your compression rising too far and blowing seals), and it mustn't go too far the other way as it needs to stay in the sleeve to keep the seal. The theory is simple enough, but the practise is hard and expensive.
Ah - just realised you can ignore the bit about banging the valves for a 2-stroke. But it'll still slam the head pretty hard. And you have to be sure that the pistons will mask the inlet and exhaust ports correctly. Especially if you're increasing the stroke.
And there really wouldn't be a lot of advantage in moving up from 250 to 270 so it's not really worth the time, money, and effort!
What they said 
Take the 250cc two-stroke twin engine like the Suzuki Hustler, Super Six and Yamaha RD250. They had the 'classic' square bore and stroke figures of 54mm x 54mm. In other words the piston measured 54mm diameter and moved 54mm up and down the barrel.
The actual volume using Pi x r2 x h was 123 and a bit cc per cylinder giving you 247cc. If you took them out to the manufacturers maximum oversize piston available - usually plus 1 mm - that would give you a tad over 128cc per cylinder or 256cc total.
Any further than this would mean you'd have to get pistons specially made or adapt them from another model. You also run the risk of failure of the bottom of the liner that sticks down into the crankcase mouth as you'd be shaving a lot of metal off it.
Take the 250cc two-stroke twin engine like the Suzuki Hustler, Super Six and Yamaha RD250. They had the 'classic' square bore and stroke figures of 54mm x 54mm. In other words the piston measured 54mm diameter and moved 54mm up and down the barrel.
The actual volume using Pi x r2 x h was 123 and a bit cc per cylinder giving you 247cc. If you took them out to the manufacturers maximum oversize piston available - usually plus 1 mm - that would give you a tad over 128cc per cylinder or 256cc total.
Any further than this would mean you'd have to get pistons specially made or adapt them from another model. You also run the risk of failure of the bottom of the liner that sticks down into the crankcase mouth as you'd be shaving a lot of metal off it.
Oh granpa what a big picture you have said Lille Red Riding Hodd.......
QUOTE(BikerGran @ Apr 9 2010, 09:05 PM) 
Oh granpa what a big picture you have said Lille Red Riding Hodd.......
Is that Little Red Riding Hood's dyslexic sister?
Yeh, sorry about that
Couldn't find a decent pic showing the liner clearly
QUOTE(fastfitter @ Apr 10 2010, 12:42 PM)
Is that Little Red Riding Hood's dyslexic sister?
No silly, it's the French version!
Erm, sorry Roysten, you have have your thread back now!
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