'81 Honda 250r - What piston? OEM or Wiseco

[jaykup]

Picked up a old '81 250r recently (was only $650) but has very low compression... so low it won't start anymore. I'd like to bore it over one size, and put a new piston/rings in it.

I can get an OEM piston kit from eBay for $30-$40 shipped, or a Wiseco piston for ~$100 shipped.

This bike has no other mods, and probably never will. I mostly do trail riding (a lot of high/full throttle for long distances).

Is the Wiseco piston worth the extra $$ for this application?

[RedRider_AK]

The Wiseco (Pro-Lite) has additional holes in the crankpin area for extra oiling, and it's much beefier in general construction. I'd go with that.

[3Razors]

Go with the OEM cast piston if you can. A properly set-up and maintained cast piston will outlive a forged one in most applications. Make sure to set the piston clearance to spec.

[jaykup]

Go with the OEM cast piston if you can. A properly set-up and maintained cast piston will outlive a forged one in most applications. Make sure to set the piston clearance to spec.

I'm not sure I follow here. I was under the impression that a forged piston was stronger and would last longer than cast, which seems brittle at times?

What is your reasoning behind it? I'm interested now :)

[3Razors]

Yes a forged piston is stronger than cast, forged pistons are good for modified engines running stroker crank, Nitrous, superchargers, high compression, ect. They handle stress much better. BUT they need more piston to bore clearance to be setup right. This extra clearance along with the molecular makeup wear characteristics of the forged piston is not so good for long term high use/milage engines.

[jaykup]

Here is the piston in question, just pulled it out.

This happened at full throttle, probably about 5 minutes of it. Is this a result of overheating? or running too lean? (gas/air or gas/oil mixture?)

http://img137.imageshack.us/img137/1710/piston1ia1.th.jpg (http://img137.imageshack.us/my.php?image=piston1ia1.jpg)http://img137.imageshack.us/img137/4493/piston2br3.th.jpg (http://img137.imageshack.us/my.php?image=piston2br3.jpg)http://img137.imageshack.us/img137/3512/piston3dv0.th.jpg (http://img137.imageshack.us/my.php?image=piston3dv0.jpg)http://img137.imageshack.us/img137/9255/piston4xk0.th.jpg (http://img137.imageshack.us/my.php?image=piston4xk0.jpg)

[Dirtcrasher]

Honda makes some great OEM parts. Pistons and clutches are just a couple of OEM items I will always try to buy first.

[TeamGeek6]

Something shoved the piston sideways, so much to grind both the rings and piston down. Im guessing someone left a wrist clip off?

Id Go OEM for that cost difference , if youre not going racing Id not spend the extra money.

[jaykup]

Yeah, I'm pretty set on the OEM cast piston. The one I pulled out was a Wiseco, and both wrist pins were on...

I was full throttle down a trail, about 25 F out, when it happened. First it started to crackle a little, very faint, so I let off the throttle, after it went away, hit it again. Few seconds later, same noise, so I let off again... and it went away again. Hit it a little past 3/4 throttle, and the noise came back, and in an instant, the engine lost all power. No unusual noises other than the engine not firing. Kicked it, felt like no compression, and wouldn't even pop. Took it home, and that's what the piston looks like. I was running 20:1 non synthetic 93 octane fuel.

Any ideas what happened?

Mostly stock motor (it had been bored 0.5mm over) with an aftermarket UNI style foam filter with no air box at all. It had a 145 main jet (Too lean I'm sure)


Honda makes some great OEM parts. Pistons and clutches are just a couple of OEM items I will always try to buy first.

DC, do you also purchase OEM brake pads? I've heard they are usually better than aftermarket...

[TeamGeek6]

Oops, that the exhaust port side of the piston, and where the piston is damaged is near the exhaust port. Wonder if the port wasnt chamfered and caught a ring?

[nate b]

go with oem, and don't hold any motor wot for 5 minutes, unless you like doing this

[jaykup]

go with oem, and don't hold any motor wot for 5 minutes, unless you like doing this

I ended up getting the OEM piston and rings 1.00 over (last bore?)

I was actually wondering how long you can hold an motor at WOT, provided it is not over its "redline". Proper gearing makes all the difference. I read this post here recently, that says it should be OK to run full throttle for long periods of time.

3WW post (http://3wheelerworldforums.com/showthread.php?t=57341)

What is your reasoning behind it? I'd like to prevent this, but I think proper jetting and oil mixture would have prevented it even at full throttle.

[3Razors]

That piston looks pretty bad. Usually if the wear is on the exhaust side it is due from being too lean at high rpm. You need to take it very easy on a forged piston during break in. Also if you have a clamp-on filter with stock carb jetting this can cause the motor to go very lean. Like the Geek said make sure the ports are properly chamfered too...

Also while the cylinder is off you should flush the crankcase area out with some gas and compressed air to get rid of any aluminum particles down there which would add more wear your crank bearings.

[jaykup]

This piston was broken in by the previous owner, at least I think it was. He didn't mention a rebuild.

The ports still have a decent chamfer, and it doesn't look like a ring caught a port. If you look closely at the pictures (Its hard to see) it looks like the aluminum was actually melted over the rings.

[TeamGeek6]

It might look like a good chamfer, but it only takes one tiny spot that grows with heat. Running that long at WOT is really going to heat both the piston and exhaust port and make them both expand. If the chamfer was good, then the only assumption left is that the piston just got too hot and expanded into the sleeve.

Engines run at high speed and long duration need different builds than stock, they need bigger clearances to take up expansion. Just look at the Daytona time trials yesterday, the giant Toyota that makes great cars cant keep an engine together in NASCAR.

Also, it may not have been a piston failure, on that old an engine, a crank or rod bearing could cause that if it stuck for an instant. The piston is top heavy and if the rod stops, the crown will dig into the cylinder wall. Turn the crank and rod by hand and feel for rough spots or change all the bearings.

[CoeShow]

In one of your posts you mentioned that the outside temperature was around 25 deg. when things went sour.

Cold temperatures have a HUGE impact on jetting! Plus, when temps are that low it takes even longer for engines to get to normalrunning temp if they even can. An engine that is to be used hard in very cold temps needs to be jetted MUCH richer than compared to Summer jetting, and the bore size should be a bit looser as well. 20:1 oil mix will also LEAN out the air/fuel mixture vs. higher numeric oil/gas mix ratios. This has historically been an issue for some to realize.

2 strokes are notorious for "cold seizure". Even properly set-up and tuned 2 cycles will cold seize on a damp morning if not warmed enough prior to hard running conditions. I have seen it happen dozens of times. Very few if any engine will run at peak performance in all weather conditions, and air cooled 2 cycles are very sensitive.

In any circumstance, a FORGED piston is superior to a cast piston. You just must be certain it has been set up properly to keep from repeating what just happened.

So, unless you set-up your engine to match the environment, you can still have a "cold seizure". Forged OR cast construction will fail if the machine hasn't been altered to match ambient outside riding conditions and then subjected to aggressive full throttle riding.

Good luck and ride safely.

[TeamGeek6]

20:1 oil mix will also LEAN out the air/fuel mixture vs. higher numeric oil/gas mix ratios. This has historically been an issue for some to realize.

.


Yup, it makes a mathematic difference.

In reality, it makes NO difference. Heres why:

Here are the numbers at 20:1 F/O ratio and based on stoich. AFR of 14.7:1

Air= 14.7
Fuel = 0.95
Oil = 0.05
AFR = 14.7/0.95 = 15:47
AOR=14.7: 0.05 = 294:1


Now change from 20:1 to 40:1, which cuts oil in half. The fuel will change, yes: (rounded to two D.P.)

Air =14.7
Fuel = .0975
Oil = 0.025
AFR = 14.7/0.975 = 15.08
OFR = 14.7 / 0.025 = 588:1

The net difference in AFR going from 20:1 to 40:1 is --

0.038:1

And there are NONE of you that can either measure a change that small, or tune to it.


The change in OFR IS BIG, and is 294.

Coe, it sounds good, but from an engineering standpoint, utter NONSENSE.

IT takes more than that to make a noticeable change in spark plug porcelain color and as extremely rich as most of you run plugs (brown and black) youd never see the difference.:crazy:

There is ONLY ONE REASON a 2 stroke, or any other engine seizes, and thats lack of lubricant.

But I love this BBS lore, its good for a laugh.