Not head gasket... holed pistons!

[Fixxxer]

Hot damn, it might be my lucky day... well, that's definitely an overstatement, BUT... the part numbers on bikebandit match... But for there are actually two parts named "piston set": 478142-001 and 478143-001. However, they are the same for both the '82 and the '78. What is the meaning of the two parts? Anyone know?

Now, I'll ask the owner about the set on ebay, but if they're not damaged would it be a good idea to snag them and replace all four pistons in my bike? Or is there some reason that I would really want to get new pistons from bikebandit, and spend $268?

[first timer]

buy the used pistons and get your self 4 new ring kits then just lightly hone the bores and you will be all set, well except for geting you fuel mix right.

[Nessism]

I've seen lots of NEW 550 pistons on ebay for cheap so don't buy some old junk if you don't have to.

http://cgi.ebay.com/ebaymotors/NOS-1977-1981-Suzuki-GS550-Piston_W0QQcmdZViewItemQQcategoryZ35581QQihZ010QQitemZ200097207666QQrdZ1QQsspagenameZWD1V

http://cgi.ebay.com/ebaymotors/SUZUKI-NEW-OLD-STOCK-PISTON-GS550_W0QQcmdZViewItemQQcategoryZ35595QQihZ017QQitemZ270129635641QQrdZ1QQsspagenameZWD1V

http://cgi.ebay.com/ebaymotors/NOS-1977-1981-Suzuki-GS550-Piston_W0QQcmdZViewItemQQcategoryZ35581QQihZ010QQitemZ200098168613QQrdZ1QQsspagenameZWD1V

My understanding is that the pistons have not changed over the years. And yes, get new rings for all four holes and hone the cylinder - if you don't hone the rings will not seat.

Regarding the holed pistons, your pistons are dead clean which is very surprising. The speckled surface on the piston crown definetly looks like detonation damage. The bike must have been running lean for some reason. You best find the cause or you may have the same problem again.

Good luck.

[chef1366]

Holy sheet. I've heard of holed pistons but that's the first time I've actually seen one.
Bummer. :?

[49er]

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...

Man, those 550 pistons have thin crowns!

You have had an extreme lean condition on all four cylinders. The only reason that 2&4 pistons are not holed as well is that they had very cold plugs fitted. Higher numbers on champion plugs means the plugs are hotter. This is the opposite to the numbering system on NKG plugs. If you look closely at the left hand plugs, the porcelains are very white. They were also close to burning holes in pistons.
Besides the missing air filter, you should double check for intake leaks, that the carbs are jetted correctly and that the ignition timing isn't over advanced.
Make sure that all the debrie is thoroughly cleaned out of the internals before fitting up replacement pistons.
You may find it cheaper and less hassle to find a replacement engine!!
Good luck.

[Road_Clam]

The pics of those holed pistons are not from running too lean. Too lean of a mixture heats up the whole crown, and will melt the top of the piston, and cause a siezure in the bores near the top of the crown. Those dime sized holes are from detonation. A combination of bad fuel, and running the bike under hard, prolonged acceleration, will "dime a piston". Could also be your ignition timing is way off.

[Suzuki mad]

I've never seen a set of GS pistons with holes in like that before. If I didn't know better I would have said it was a tuned 2 stroke gone bang!

Suzuki mad

[Nessism]

The pics of those holed pistons are not from running too lean. Too lean of a mixture heats up the whole crown, and will melt the top of the piston, and cause a siezure in the bores near the top of the crown. Those dime sized holes are from detonation. A combination of bad fuel, and running the bike under hard, prolonged acceleration, will "dime a piston". Could also be your ignition timing is way off.

I agree about the detonation. Ignition timing can definitely cause this to happen, along with using cheap gas and lugging the engine. For some reason I thought that running the engine lean will exasperate the detonation tendency. Wrong?

I had one of the first generation Gpz550's and it detonated like a bitch. I eventually sold the bike because of it (yea, stupid). Timing was not adjustable as I recall and octane boost didn't help. Reliving my past (in my mind), I was wondering if the header I installed messed up the mixture enough to cause the problem? Not sure but maybe relevent.

[first timer]

do those ngk plugs even have the little tab that the spark jumps from the electrode to? it looks like they don't, the spark jumping from the electrode to the piston directly would also cause holing. ether way get some newwer pistons with rings, new air filter, check timing and clean your carbs and set back to factory settings and you should be fine.

[Fixxxer]

do those ngk plugs even have the little tab that the spark jumps from the electrode to? it looks like they don't, the spark jumping from the electrode to the piston directly would also cause holing.

They have the tabs, it's just a crappy pic. The NGKs and the champions are in equivalent heat ranges, though... and cheap gas? I was running 87 octane, the same stuff I use in my car... does that really make a difference? And as for "hard, prolonged acceleration," I was riding fairly hard, but no where near redline... definitely didn't seem like I was abusing the engine.

[80GS750]

after reading all this, im gonna go pull my plugs and make sure mine dont look like that....

the last thing i need is holed pistons after getting my bike just running again...

[petelardo]

Though I agree that the visible damage to the pistons was most likely caused by detonation, I'm worried that we will overlook the CAUSE of the detonation. The lack of an air filter allowed too much air to enter the engine, relative to the amount of fuel being delivered by the carburetors... A lean condition can also cause detonation; in fact, detonation occurs at a LOWER temperature when a lean condition exists. If the piston is holed as a result, then obviously, the damage occurs BEFORE temps inside the cylinder can go high enough to show the effects of the lean condition. IOW, bad fuel, engine lugging, and improper ignition timing are NOT the ONLY reasons why detonation can occur.

The stock air filter and airbox are DESIGNED to be restrictive; this allows smaller carb jets, and (presumed) better fuel economy. The smaller jets used under those conditions make it easier for the OEMs to satisfy EPA requirements, which ISN'T so good for the engine... as we can all see, this one now has a very nasty migraine...

I've seen similar piston damage caused by detonation alone, and I've seen similar damage caused by the combination of a lean mixture AND detonation. If the problem was ONLY a lean condition, then I would agree that the entire piston crown would be likely to exhibit damage, but I'd also expect to see some other effects of super-heated gases in the combustion chamber, such as burned / damaged / scorched exhaust valve(s)... I believe the detonation knocked the holes in the piston crowns BEFORE the engine got hot enough to do the damage we associate with a lean condition, but that does NOT mean that a lean condition did NOT exist here. The absence of the air filter caused a lean condition (among other factors); the lean condition allowed detonation to occur at a lower-than-normal engine temperature; the damage done to the pistons by the detonation prevented the cylinders from getting hot enough to cause the damage we associate with the presence of a lean condition.

Nessism and some others are correct; running the engine lean DOES increase the odds of detonation occuring. If anyone would like to test that for themselves, feel free to remove your air filter and ride without it, as some PO did to this GS550. It might take a while, or the engine might fail for another reason before the lean condition causes severe damage, but you WILL damage your engine. Those sparkplugs were long overdue for replacement; MANY factors contributed to this problem, so let us not dismiss or otherwise overlook the EXTREME lean condition that contributed to this. The HOLES certainly do look to be the result of detonation; don't ignore the contributing factor provided by the lean condition.

I sincerely hope that the OP will soon post some good photos of the OTHER two pistons, the ones that are currently intact, so we can do an online autopsy, based on the visual evidence available. I also would like to see photos of the valves in the cylinder head, so we can check them for any obvious signs of damage.

Fixxxer, I do wish you the best resolving this. Do NOT rush into a decision on how to handle this; I hope you'll use the collective wisdom represented here to help guide you in deciding what to do next.

[49er]

Though I agree that the visible damage to the pistons was most likely caused by detonation, I'm worried that we will overlook the CAUSE of the detonation. The lack of an air filter allowed too much air to enter the engine, relative to the amount of fuel being delivered by the carburetors... A lean condition can also cause detonation; in fact, detonation occurs at a LOWER temperature when a lean condition exists. If the piston is holed as a result, then obviously, the damage occurs BEFORE temps inside the cylinder can go high enough to show the effects of the lean condition. IOW, bad fuel, engine lugging, and improper ignition timing are NOT the ONLY reasons why detonation can occur.

The stock air filter and airbox are DESIGNED to be restrictive; this allows smaller carb jets, and (presumed) better fuel economy. The smaller jets used under those conditions make it easier for the OEMs to satisfy EPA requirements, which ISN'T so good for the engine... as we can all see, this one now has a very nasty migraine...

I've seen similar piston damage caused by detonation alone, and I've seen similar damage caused by the combination of a lean mixture AND detonation. If the problem was ONLY a lean condition, then I would agree that the entire piston crown would be likely to exhibit damage, but I'd also expect to see some other effects of super-heated gases in the combustion chamber, such as burned / damaged / scorched exhaust valve(s)... I believe the detonation knocked the holes in the piston crowns BEFORE the engine got hot enough to do the damage we associate with a lean condition, but that does NOT mean that a lean condition did NOT exist here. The absence of the air filter caused a lean condition (among other factors); the lean condition allowed detonation to occur at a lower-than-normal engine temperature; the damage done to the pistons by the detonation prevented the cylinders from getting hot enough to cause the damage we associate with the presence of a lean condition.

Nessism and some others are correct; running the engine lean DOES increase the odds of detonation occuring. If anyone would like to test that for themselves, feel free to remove your air filter and ride without it, as some PO did to this GS550. It might take a while, or the engine might fail for another reason before the lean condition causes severe damage, but you WILL damage your engine. Those sparkplugs were long overdue for replacement; MANY factors contributed to this problem, so let us not dismiss or otherwise overlook the EXTREME lean condition that contributed to this. The HOLES certainly do look to be the result of detonation; don't ignore the contributing factor provided by the lean condition.

I sincerely hope that the OP will soon post some good photos of the OTHER two pistons, the ones that are currently intact, so we can do an online autopsy, based on the visual evidence available. I also would like to see photos of the valves in the cylinder head, so we can check them for any obvious signs of damage.

Fixxxer, I do wish you the best resolving this. Do NOT rush into a decision on how to handle this; I hope you'll use the collective wisdom represented here to help guide you in deciding what to do next.

IMO, a good analogy. I'm not surprised that many 550 owners have experineced "dime detonation". The thinness of those stock crowns means that the pistons would be extremely prone to this type of damage. This thin piston crown design with it's large thin valve pockets, allows little chance for good heat disapation, should the ideal operating conditions not be maitained. Any lean condition would cause concern along with the associated detonation.

[jimcor]

When you put that bike back together use the correct NGK B8ES plugs in it.

[KEITH KRAUSE]

The damage is done and it seems there's different opinions on what exactly happened or if it's a combination of things.
All you can do is check/clean out the motor and replace the parts.
The thing to do is re-tune everything and it shouldn't happen again.
Use the correct NGK plugs...period. Replace the filter and verify the box isn't modified. Verify jetting. Check timing and advance action.
The plug pics aren't the best but the two left side plugs look like the insulators are a tan/brown color. Nothing very lean to me, just dirty/old plugs. The two right side are carbon stained.
A severe lean condition can cause piston holing as you've all said. But the plugs would normally look snow white and the insulators glazed over under that kind of heat. On the other hand, a rich condition or poor spark can lead to excessive carbon deposits and they will start to glow and build up this kind of heat. The two plugs do look carbon stained but a holed piston will obviously compromise combustion and plug reads are useless (for determining correct mixture at the time).
I think the excess heat is related more to poor ignition timing and possibly the plug rating than related to the lack of a filter, though the lack of the filter certainly worsens things. Even though the damage was at cylinders 1 and 3 (correct??) and being fired off different coils set at possibly different timing, I still think this is more electrical related than jetting related. I think the other two pistons weren't far from holing too.
Doesn't matter now if all basic tuning is done right and the correct parts are used.