I just had my 98 GP 1200 bored .025 as part of rebuild. How is displacement affected, and by how much?

Convert your 0.025 to mm, add to total original diameter of bore, then use PI X D X H where PI is 3.14, D is cylinder diameter and H is stroke. Have fun.

I'm sorry I asked.

25.4001 MM PER INCH

Chris --- if I did the math correctly, then displacement should increase roughly 7.6%, from 1136cc to 1222cc.

Of course, if I did the math incorrectly ...

Actually the formula is Pi x R^2 x Height
pi = 3.14
r = radius (half your bore)
eight = stroke
if you use diameter you will get an incorrect value.

Also remember that bore and stroke are expressed in mm while displacement is expressed in cm. You will have to knock the answer down a couple decimal places to account for this...

bore x bore x stroke x .7854 = displacment

84.5x84.5x68x.7854= 381 x3 = 1143 c.c.

.025 inch overbore is an odd size.
Are you sure you don't mean .25 mm which is a standard overbore size. (roughly .010 in.)

GrandPaRules --- you are correct. It is 0.25mm overbore. Me so stupid.

Well you are from Tallahassee...it must be that Seminole math . You know down here in Gainesville we don't have any trouble with that math stuff...

yA. wHAT Sirhc 7897 said. if we are talking .25mm then :
stock bore - 84mm
stroke-68mm
pi 3.14
Radius is half diameter, -42mm. add 1/2 of added bore: .125
so: pi R squared H = 3.14 X [42.125 X 42.125] X 68 [then move the decimal to show cc!]
= 378.9 cc X 3 cylinders = 1136.7 cc
Up 5.7 cc from stock 1131cc

Next.....NO there will be no appreciable performance gains here. Overbreing 2mm will show something! But stroking 1mm will show more!

Thanks everyone. I have the answer I was looking for --- no appreciable gains from this action alone.
Bob

This is something I'm curious about:

how would the extra 1mm of travel create more power than 2mm of extra bore? Is it because that extra mm of travel creates more inertia (torque) wih the same amount of material than the extra 2mm of material moving over the same distance?

Just trying to soak up some understanding...

Ya. it is a torque thing.

A longer stroke means the piston is moving faster and further per the same revolution--as if Lance Armstrong went from to a 180mm crank from a 175mm; for a bike, that's about equal to switching to the next lower gear, e.g., about like adding about 2 extra teeth to the cog: great for going uphill, and the load on a boat's engine is like it's always going uphill. Imagine the piston speeds on the FX140's engine at 11K rpm if the stroke was increased.