Boring How Many CC does it add

[Ka0sX]

Ok i have had a talk tonight that boring out a bike does not add any extra CC to the engine.. i was told by many of people when younger when a cylynder was bored out it added 25CC to the engine because of the oversized piston and rings that had to go in it..

am'i correct or wrong that everytime you bore the cylnder that it adds another 25cc to it ????

Please let me know

cheers

[intensem1rider]

Yes boring the cylinder add's cc's to it, but by how much, no one knows until you measure it. You could take off a tiny tiny bit and add just a few cc's, or you can bore it a bunch and add 40cc.

But I hope your not planning on doing this since you will then have to re-plate the cylinder, and if you bore too much then you weaken the cylinder too much etc.

[nickmell]

yes and no ...yes boring will add some cc but it depends on how much you bore it... my yz125 is bored 2mm over and that makes it a 124 while you can get some big bore kits for 450s that will take them to 580s so it just depends on how much you bore it

[sporthog]

There is a mathmatical formula for caluclating cubic inches or cubic centimeters.--- 1/2 of the bore Squared x the stroke (the amount the piston travels from top to bottom) x 3.14 (pi) times the number of cylinders. Anytime a cylinder is bored larger there is an increase of cubic inches/cc, it is in relationship to the stroke though. The smaller the stroke the less total increase you will get.

[klxd]

(bore2 x bore2 - bore1 x bore1) x stroke x 3.14 /4

[Smacaroni]

the current bore, vs the new bore combined with the stroke (which I'm assuming is the same) will determine the actual volume change.

Boring a YZ80 .080 over will net a lot fewer ccs over the current set up than boring a XT600 .080 over, for starters, the 600 is a much larger piston, so .080 over adds many times difference the area of the cylinder and IIRC, the 600 stroke is a good bit longer than the 80s.

The seat of the pants increase may be the same though, since the XT is a four and the YZ is a 2t.

[Smacaroni]

(bore2 x bore2 - bore1 x bore1) x 3.14 /4

You have to figure the stroke in, your only going to get piston area that way.

[klxd]

Oops, edited equation.

Thanks

[Smacaroni]

Something doesn't look right still.

You have a 1 unit wide piston and want to bore to 1.25 units, so your equation will return the volumn of a piston .25 units @ what ever stroke.

I think you have to do it the hard way:

v=PIr^2h right?

so volume difference would be

a = old r

b = new r

PIa^2h - PIb^2h = delta v?

So maybe I'm wrong, but it should be 1/2 bore and the subtraction should be done after the volume calculation?

Not trying to be a math nazi. Really, I'm not.

[ToBadBrad]

most, if not all motorcycles that have borable cyl. are under there stated size. a 250 might be 246cc stock, and when it has it's max oversized pistion it will be 250cc.

[Nosebag]

Something doesn't look right still.

You have a 1 unit wide piston and want to bore to 1.25 units, so your equation will return the volumn of a piston .25 units @ what ever stroke.

I think you have to do it the hard way:

v=PIr^2h right?

so volume difference would be

a = old r

b = new r

PIa^2h - PIb^2h = delta v?

So maybe I'm wrong, but it should be 1/2 bore and the subtraction should be done after the volume calculation?

Not trying to be a math nazi. Really, I'm not.

:nitpicking: That's right, but delta = final value - initial value. No big deal, just change the sign, but I've revised the equation to avoid confusion.

r = 1/2 x bore

(pi x rfinal^2 x h) - (pi x rinitial^2 x h) = delta v

Delta v will be an increase in displacement. And since bore and stroke is measured in mm, your answer will end up in mm^3, so just divide by 10 to get cm^3

[XR650L_Dave]

OK, but by how much does it raise the compression ratio?

Bwuhahahahaha...

Dave

[Smacaroni]

you're funny Dave, you sick, sick man.

We forgot to mention how this impacts your power band(s), you may want to replace them after boring your machine.

[Smacaroni]

:nitpicking: That's right, but delta = final value - initial value. No big deal, just change the sign, but I've revised the equation to avoid confusion.

r = 1/2 x bore

(pi x rfinal^2 x h) - (pi x rinitial^2 x h) = delta v

Delta v will be an increase in displacement. And since bore and stroke is measured in mm, your answer will end up in mm^3, so just divide by 10 to get cm^3

I wish it was as easy as you said the first time, or the second time.

circular area/volume calcs usually give me headaches.

[RMSpeed6]

you will then have to re-plate the cylinder, and if you bore too much then you weaken the cylinder too much etc.

Not true. LA Sleeve put in = no plating needed.

[zig06]

Ok a 2004 RM125 has a bore and stroke of 54.0 and 54.5 and that equals 124.8cc's.

.030" equals .762mm (CC's is metric...), and that needs to be added to the bore.

~ so ~

A 2004 RM125 bored out .030 equals a bore of 54.762mm, and that equals 128.4cc's.

A 2004 RM250 has a bore and stroke of 66.4 and 72.0 for a displacement of 249.3cc's. A .030 bore increase will bring that up to 67.162mm's for a total displacement of 255.1.

The bigger the original bore the larger the increase in CC's will be.

[klxd]

Change in displacement equals new minus old.

r squared equals diameter or bore squared over four

Pi and stroke in the numerator and 4 in the denomenator are common to the new and old displacement calcs so I factored them out.

I fink I got it right.

[Ka0sX]

Thanks guys and no iam not boring out a bike it was just a discussin we had last nite thats all i was just curious as to how it all worked out

Thanks for all the replys much appericated

Cheers

[Rip Wheeler]

Ok i have had a talk tonight that boring out a bike does not add any extra CC to the engine.. i was told by many of people when younger when a cylynder was bored out it added 25CC to the engine because of the oversized piston and rings that had to go in it..

am'i correct or wrong that everytime you bore the cylnder that it adds another 25cc to it ????

Please let me know

cheers

Not much. But this formula will work.

.7854 X Bore X Bore X stroke (X number of cylinders, in your case, 1, so skip this step)

[ktmgary185]

the calculation is quite simple for anything to figure out cc or cubic inches of any engine, it is just a matter of first geting the square # from a round diameter measurement then multiplying in the stroke. the equasion is Y x Y=R x .7854 x STROKE= total cc, for instance on the 125cc were talking about say the bore is 54mm the stroke is 54.5. first to remember is we are tryng to figure cubic centimeter so we have to convert from mm, and in bore/stroke we are dealing with millimeter measurment so the cm bore is 5.4 (54mm) so 5.4 x 5.4=29.16 this times .7854=22.90cm(2290mm), this is the square cm/mm of the cylinder now simply multiply that by the stroke 5.45cm (54.5mm) and we get our cc --124.805 or 125cc

same would go for lets say 350 small block chevy it has a 4inch bore and 3.48 stroke so 4x4=16 x.7854=12.5664 times 8 for 8 cylinders=100.5312 then times stroke 3.48 =349.848 cubic inches or a 350

let say its a ktm 250sxf like I have 76mm bore 55mm stroke, first convert to cm so 7.6cm bore and 5.5cm stroke 7.6 x 7.6=57.76 x .7854=45.364 this times the stroke 5.5mm = 249.502cc or 250cc

so if your boring the 125 cylinder 2mm over lets do the math ,with the same stroke, 54.5mm, remember we have to conver to cm for cc so

5.42 x 5.42=29.3764 this times .7854=23.0722 this times our stroke of 5.45cm =125.743cc total a gain of about 1cc

the cylinder square equasion Y x Y=R x .7854 gives you the square size of the round hole and the stroke multiplied gives the cubic part of the equasion.

hope all this math stuff helps, thought I tried to make it easy.

if you still have trouble just put up the bore and stroke specs and ill do the math for ya.