Not head gasket... holed pistons!

[Fixxxer]

Here's my original thread to fill you in on what's happened (thought I had a blown head gasket): http://www.thegsresources.com/_forum/showthread.php?

So I got the head off today without breaking any bolts or anything, and I have a gasket set on the way from Dennis Kirk, so I figured that after it got here on Monday I could have it back together and ready to ride by the 4th. However, once I got the head off I noticed that the gasket looked fine... strange. Then I actually took a gander at the pistons... Pistons 1 and 3 (the cylinders with no compression) had holes in the tops of them. Damn...

Now, why would this happen? I noticed that these pistons had NGK spark plugs (BR9ES) while the other two had Champion plugs (N4C). I thought this might have something to do with it but according to the conversion chart the NGK 9's and the Champion 4's are in the same heat range, and my Clymer manual actually recommends NGK 8's, which are hotter than what I had in it.

The other thing I found through some searching is that running too lean can hole the pistons. I also read that it can cause exhaust popping on decceleration, which I was noticing before the pistons went. So could this be my problem?

I want to figure out what the problem is before I replace the pistons so I don't just have the same thing happen again. I found OEM pistons on bike bandit for $40 for just the piston or $66 for the "Piston set" which I'm assuming has the rings and everything already installed. Would I have to get the set or could I reuse everything else if I just replaced the actual piston? Also, I'm guessing I need to replace all four, but, well, do I? Or could I just replace the holed pistons?

Thanks for all the help.

[renobruce]

What color were the plugs? Sounds like it ran way too lean. Any intake or exhaust leaks? Open air box?

[brveagle]

Holy Crap... Can you get some pictures of the pistons? How much city driving do you do?

[Fixxxer]

Here's what they looked like. The Champions are on the left, the NGKs from the cylinders with the holed pistons are on the right. There are some exhaust leaks (holes in the pipes). The breather hose was not attached to the breather cover... I don't see how that would have caused it though.

OK, I just went out to check the air filter for the first time since I bought the bike a month ago... NO AIR FILTER! Could this be it? I feel like an idiot for not checking this stuff before I started riding it...

[renobruce]

That would definitely make it run lean.

[petelardo]

Man, I HATE reading about holed pistons; a lot of people simply don't believe that things like this happen...

The link you posted is defective; this is the link to your original thread:

http://www.thegsresources.com/_forum/showthread.php?t=110621

GET NEW RINGS for the new pistons... FALSE ECONOMY is NO ECONOMY, and this is NO time to try to save money on a repair...

Would you take some photos of your holed pistons, then post them here, so others can SEE what a lean condition can do? Far too many think it only happens in racing...

I never saw your original post, but if I had, I'd have told you to expect the worst... NOW, you have to make SURE that none of the piston shrapnel has fallen into the bottom end of the engine... If any fragments block an oil passage, you'll be replacing the entire engine REAL soon, so check carefully BEFORE you buy ANY parts...

I wish you the best of luck with this repair; update us when you can, and please post some photos so everyone can see what you're dealing with. I really wish I had an engine to give you, so you could still ride while you inspect the damaged one...

[relic-rider]

Sorry to hear that. I had a feeling they were holed when I responded to your last post. An all to common problem for myself in dealing with old sleds. 2 strokes are especially prone to holing with air leaks. I've burned up plenty pistons. As suggested, make sure your bottom end is cleaned up of all the aluminum schrapnel.

[tconroy]

1 and 3 must have been running lean in the first place and then with no air filter that just added flames to the fire! How do your pipes look on those 2 cylinders? Are they different from the other 2? Are they blue?

[DimitriT]

I got a hole in #1 on my 550 about a month ago.

A little research revealed that holed pistons are usually caused by
preignition.

That's when the charge ignites at the wrong time. This could
happen because the spark plug is the wrong heat range and overheats.
The overheating would likely happen when the mixture is lean.

Sometimes the preignition happens when there's some carbon on the piston
which gets too hot and starts glowing. Again, a lean condition which fails
to cool the piston could cause this.

In my case I was running WOT for an extended time. I later found out I
have a faulty petcock with constricted flow. It's possible my carb ran dry
and leaned out.

You will replace the two bad pistons. While you're in there you probably want to replace the rings on all of them. Also, check the valves on the two blown pistons. On my bike, the aluminum bits which sand-blasts the cylinder got blown into the exhaust and messed up the valve. I got a deal on both exhaust and intake on steal-bay so I replaced them both. I also found some roughness at the top of the cylinder and some aluminum fused into the cylinder wall. I tried rubbing this out and I hope it did no more harm than good.

I was also worried about the bottom end so I pulled the oil pan. It was filled
with aluminum bits. I cleaned out the strainer and everything else. Who knows what all that crap did to the bottom end. I rode the bike for another 20 miles after it blew.

You're looking at a good outlay for all this. You may want to check for a replacement motor.

[Fixxxer]

Here are the pics of the holes... the first pic is of #1 and the second of #3. So, yeah, this really sucks. I've been trying to get the cylinder off but the base gasket is holding on for dear life... I tried a hammer and a block of wood, but just splintered the wood. I think I heard it laugh mockingly when I tried a rubber mallet. Anyway, I sprayed some WD-40ish stuff all around it... hopefully that will help loosen it? Any suggestions?

So I'll definitely have to take apart the lower end to check for the piston shrapnel? This just gets better and better. What would be really nice is if I ever get this thing back together... before today the most work I've done on a bike was changing the oil and filter. Thankfully the clymer manual has been fairly straightforward thus far, and you guys are always a big help. Thanks again.

So, assuming I replace the pistons... After replacing the air filter, do I need to do anything else to correct the lean condition? It seems like there must be something else, but I don't know. Will I need to do something with the carbs?

[Fixxxer]

I went back and looked at the exhaust pipes... all four header pipes looked the same and none of them had any noticable bluing...

[DimitriT]

I left the bottom end alone. They're supposed to be bullet proof.
We'll see.

At the very least I would open up the carbs and make sure they're
clean.

So are those spark plugs stock? I thought the plug to use was the NGK B8ES.
Maybe you just had the wrong heat plug.

[Fixxxer]

So are those spark plugs stock? I thought the plug to use was the NGK B8ES.
Maybe you just had the wrong heat plug.

They're not stock; the previous owner apparently installed them. The don't match, either... the two on the cylinders with the holed pistons were NGKs and the other two were Champions, although they are the same heat range according to the conversion chart. The thing is, the NGKs are BR9ES, and the Champions are the equivalent, which means they are colder plugs than stock. So how would cold plugs alone burn a hole in the piston... must have been the lean mixture, right?

[Fixxxer]

Would the pistons from an '82 550L work? There's some on Ebay with no bids yet...

[DimitriT]

Check the part #s on bikebandit.com to make sure but I think the pistons from an 82 will fit.