I am looking to bore my 1984 KDX80, they sell pistons in .5mm and 1.0mm over-sized. What are the risks of running it with a piston 1mm larger than stock? Am I more prone to blow something or would I have to re-jet the carb? Thanks

 

Rejetting the carb is a distinct possiblity. Usually the aftermarket pistons are only offered in sizes the original cylinder can handle reliably. Where you MAY run into issues is if you go high on the compression. The other thing is if the 1mm is the max overbore, you will be looking at resleeving the cylinder sooner rather than later. You don't gain much power just from a small overbore, so I would stick with the minimum needed to get it freshened up.

 

I don't think there are any "risks" if it's done right. I would expect you'd have to rejet- even if it's only 1 mm over stock.

 

How would I raise my compression level? Or are you saying maybe a straighter shaped expansion chamber would lower/even out the compression for the 1mm overbore? I suppose just run it extra rich to keep those cylinder walls longer (N plated?) You see every bit of power counts, it's on a drag racing miniature lawn mower, I even want to make exhaust changes but that's a whole different thread.

KDX80 Engine Racing Modification Help

KDX80 Engine Identification & Modification Help - ThumperTalk

Well looks like I need to figure out my jetting then, if it's safe then why aren't they bored to maximum size from factory? Did they want even numbers?

 

2 strokes are designed to be rebuilt, so you are able to rebore a few times with the original. Through design a 2 stroke basically eats itself but that is what happens to a race designed engine. If that was not possible resleeving every time would basically put the manufactorers out of business cause nobody would be buying a couple sleeves a year to keep their bikes running. An expansion chamber would have no bearing on the compression. And boring by itself on a 2 stroke won't give you any more noticible power.

 

to change (raise) your compression ratio- you can buy high crown pistons, shave the head, use different gaskets, replace the crank with a longer one.....

 

They don't even make any of that stuff that I can find of for this engine, they do have aftermarket connecting rods but they aren't longer.

 

just shave the head a few thousandths of an inch and you'll raise your compression-

I'm sure they make a high comp piston for that motor...

Raising that comp ratio will make that motor run hot- you'll need to adjust your plug, gas octane, carb settings and possibly tune your exhaust pipe.

 

I'm going to keep my compression ratio, the engine runs in too confined of a space to run any hotter.

 

I agree, don't mess with compression.
I think the stock jetting will be fine.
I highly doubt your cylinder is nikasiled. I've only seen a .025 mm bigger pistons for machines w/nickasil cylinders. You do not bore out nickasiled cylinders.
What size? Look at your cylinder. If normal wear is present, the .05 mm is what I'd get.
If heavily gouged, the 1.0 mm oversize. The person doing the boring can advise with more presision.