Oversized pistons??

[asdfg]

If one of my pistons has cracked badly and I want to get a new one do I need to get an oversized one? im not worried about performance just want it to go cheap as possible. will a new standard sized piston fit in without boring the cylinder, cheers

[FilthyLuka]

is the cylinder burred or rough? if so you will need to hone it, measure it and consult your manual. The bore that is...

[asdfg]

the cylinder is fine as far as i can see, i just gave the piston i good beating when i was trying to open the crankcase. i was just wondering why people bore out cylinders an if a standard piston will fit ok in my old cylinder, cheers

[xwhatsit]

Why people bore out cylinders? Bores do wear out over time, unevenly, and if your piston lets go or something hard gets inside your cylinder, it can gouge it or scratch it up. It's like sanding back the paint on your bike to bare metal; sure, you lose a tiny bit of metal off the tank when you do it, but you're getting a smooth surface. Pistons are usually produced in 0.25mm increments above the original bore size; by boring out 0.25mm from what it used to be, that's usually enough to get back to a good clean surface. I think on my bike you can go all the way to +1mm, but those are for the dirt bike version of my engine, and they do suffer a lot of wear (plus the lack of an oil filter).

Of course that's just normal `maintenance' overboring. Makes no real difference in performance over stock. If you want to bore it out for performance reasons, you'll have to go a hell of a lot bigger than just +0.25mm (or even +1mm). Wiseco et al do oversized (and higher compression) pistons for a huge variety of bikes. I would guess that it's often more trouble than it's worth, though; possibly re-jetting carburettors, stuff like that. Reduced reliability. Plus, if it blows up again, you've got less meat to work with if you need to bore it.

[Buddha#81:Me]

Try Tony He's well priced and a good buggar. http://www.motoequip.co.nz/

[F5 Dave]

. . i just gave the piston i good beating when i was trying to open the crankcase. . .

So how damaged are the other pistons & what of the rod if you managed to crack a piston? So was it seized or do you just randomly beat machinery to failure?

I see you have an 86 Honda. Informative.

[TonyB]

Thanks Mr Buddah....though I must say I can not recall being buggered by you. You haven't been using those date rape drugs again have you?

What bike have you got asdfg?

To be honest it REALLY puzzles me when people will go to all the trouble of buying a new piston kit, ripping the engine down, getting new gaskets and all the rest, and then won't spend $40 to $60 getting the thing bored and honed for an oversized piston. When buying from me, an oversized piston will cost the same as a standard one. The ONLY extra cost is the cost of boring and honing, plus a couple of days waiting while its done. You will get better performance, and the engine will last a LOT longer if you bore it out. Think of it as minor short term pain for significant long term gain.

[surfer]

To be honest it REALLY puzzles me when people will go to all the trouble of buying a new piston kit, ripping the engine down, getting new gaskets and all the rest, and then won't spend $40 to $60 getting the thing bored and honed for an oversized piston. When buying from me, an oversized piston will cost the same as a standard one. The ONLY extra cost is the cost of boring and honing, plus a couple of days waiting while its done. You will get better performance, and the engine will last a LOT longer if you bore it out. Think of it as minor short term pain for significant long term gain.

+1 what he says.

[Ixion]

Well, most machines have only a limited number of rebores. Use them up too quickly (especially on a two stroke) and you may end up looking at the cost of a new barrel. Depending , of course, on why the piston is being replaced. But I have known pistons to be significantly scored, or worn in the lands (or, on a two stroke, have a honking great hole in the top!) while the bore is still in quite good nick.

[Wired1]

Interesting reading. The XT550 motor I recently gained had a rusted piston which I managed to free after a few days soaking. So the question is, do I need to rebore it to get back to clean metal or will a cleanup with an emery flail and a real light hone do the trick? I'm guessing that if this worked I could use the existing piston with new rings for no loss in performance.
Or should I buy the first over piston and rings and take the barrel along to my local machine shop and get it bored to match? What kind of dosh is a piston, rings and a rebore anyway?

[FilthyLuka]

Interesting reading. The XT550 motor I recently gained had a rusted piston which I managed to free after a few days soaking. So the question is, do I need to rebore it to get back to clean metal or will a cleanup with an emery flail and a real light hone do the trick? I'm guessing that if this worked I could use the existing piston with new rings for no loss in performance.
Or should I buy the first over piston and rings and take the barrel along to my local machine shop and get it bored to match? What kind of dosh is a piston, rings and a rebore anyway?

Dunno for your specific bike, but it will be round $120+/- for the piston kit, and im not sure for the rebore

How bad is the bore? you may get away with a light hone. Can you post up a pic?

If your not sure you can always go oversize to be on the safe side

[Wired1]

Here are some horrible photos, it's worse than I thought.
I'll ask the Yamaha shop what a piston & rings is worth and go from there.

[Ixion]

Yes, even if it appeared to clean up with a hone, the surface would be porous, pitted with tiny holes. Which would lead to high oil consumption and rapid wear.

The other thing you'll need to check is, if that much water has reached the bores, it is possible there may be rust pitting on the big ends (or mains). Even if they move freely, if there is evidence of rust, they will wear out very fast. They need to be black (with oil) and shiny

[Wired1]

Yes there was water in the crank case and the crank shaft has signs of rust on it. I was planning on new main bearings, I guess I will have to get the crank split and do the conrod too then bugger it. The gearbox is good, there was enough oil at this end to prevent any corrosion. Well I'll call it oil, it looked more like cutting lubricant with the amount of water in it!
So it's a piston, rings, conrod, main bearings, clutch, idler gear ... maybe it's not such a good idea afterall.

[Wired1]

Update to that above, the main bearings are clear of the rust - the water in the crank didn't reach them so that's the good news.
The bad news is the local Yamaha shop just quoted me $300 for a first oversize piston and rings, too much for this boy.
Anyone know any alternative sources of pistons & rings?