Boring?

[EXe3]

[size=7][color=blue] Hello,I have a 1997 Polaris Scrambler 4x4,and recently the piston skirt shattered,also knocking a hole in the top of the motor.The mechanic said he repaired the hole,and bored the piston-cylinder 25 over,with new piston,etc. My question is,what size engine do I have now,after boring? I know it is bigger than a stock 400,but how much bigger? Also,how much power(speed) increase can I expect?I have not yet picked it up from the shop,because there are a few more small things I want done,just want to know what to expect when I do get it back.any help would be appreciated.Thanx, Exe3

Cant be sure on the actual cc's from the info you offered but its not a whole lot more (maybe a 405 etc) and with some models there are so many overbore's avail its hard to even know what piston he installed.

As far as a increase in power this would be more related to how poorly the cylinder produced compression before (how worn out it may have been etc) and how good a job is being done now because there would only be a small difference between tow similar engines with one having the slightly larger piston unless there were changes made to increase either compression or air/fuel flow into the engine.

So chances are since there was a problem before you are going to feel a pretty decent change in power, but much of it could have been lost long before the rebuild.

[ridnwarrior]

Not much of a difference.
Might not even be a 405. Probably only a 403.
You shouldn't really notice too much difference.
Especially after not riding it for awhile while it was in the shop.

:rider:

[rpyfz450]

Each time you bore a 2-stroke you slightly change the tuning of the engine. If you're really "in tune" with your ATV's performance, you might notice a difference, but as 400ex4me said, you'll think there's a performance increase if you're comparing a fresh cylinder's performance to a worn cylinder's performance. I would say you will definitely notice a change in performance from a cylinder at it's stock bore versus one that's been bored out to it's service limit.

Roof angles present in each of the ports means that as the cylinder bore gets increased, the ports heights get slightly lower. This means that the "port time area" is reduced and they become slightly less effective. On the plus side, lower port heights mean the length of the compression stroke is slightly increased.

Avoiding this situation is one area where a plated bore has an advantage. Ideally, a plated cylinder never needs to be bored and you simply change the piston with another of the same size.

Here's a website that will allow you to calculate the displacement increase, but you need to convert your bore and stoke measurements to inches. The results will be in cubic inches, so you'll want to convert that back to cubic centimeters.