GT550 2 holed pistons

[crazyeights]

I recently purchased a 1972 GT550 and I was getting ready to get it running for the first time. I looked down the sparkplug hole on the right cylinder and I saw a huge hole in the piston. I then took off the head and then saw another hole in the center cylinder. What would have caused this? What is the best source for some new pistons? Any input appreciated.

[ConnerVT]

Aluminium moths.

The pistons likely melted due to pre-ignition/detonation,from one of the following (most to least probable):

-- Vacuum leak
-- Timing too advanced
-- Carb jetting too lean
-- Wrong spark plugs installed (too hot)

So which one caused this?

Let's make the assumption that after blowing up the engine, the bike was parked, and nothing (or very little) was done to it.

Working from the bottom to the top of the above list, check that the correct spark plugs are installed (NGK B7ES or equiv for the '72).

Carb jetting is likely stock, as stock exhaust is installed. But the carbs could of been rebuilt with a non-OEM kit, so it *is* possible that the wrong parts are there.

As long as the engine still turns, you can check the ignition timing, to see if it is close to how it should be set up, or completely messed up.

Which brings us to an air leak. That two pistons were holed makes it more interesting. Is the SRIS system installed and intact? If SRIS is removed, are all of the openings plugged? Check condition of carburator intake boots. If no smoking gun has been found so far, then an engine leak down test to confirm crank seals and other gaskets are still good.

[yeadon_m]

Conner,
Kudos, a great post.
What is particularly interesting is TWO pistons seemingly holed in the same way. That surely took admirable application on the part of the rider way back when, eh!?
The size of the holes suggests they went at different times, but oh no, he was not backing off just because it was running on two
The holes look to be from erosion, bit by tiny bit, from one or more of the causes you list.
What might also be interesting as a possible way to rule some things out or to demote them in order of probability is to confirm jetting in the carb on the intact side, and likewise timing. Best therefore to check timing on all cylinders, and not assume whatever it is on one pot is representative.
Gets popcorn
Cheers,
Mike

[Craig380]

In terms of fixing the problem, see how the cylinders look. Smeared aluminium can be cleaned off carefully using a little acid.

The real problem is that molten bits of piston get everywhere. I'm sorry that there's no easy way to say this, but it's likely the engine would need a complete crank rebuild, as the bits will be stuck in the big end, in the seals and more.

Good luck with it and keep us posted.

[tz375]

The holes are both from detonation which as Conner and Mike Y pointed out is caused by excessive heat in the combustion chamber.

Why two? Probably burned out one on a run and the PO opened the throttle more to make up for the loss of power and holed the center as well. The root causes could be a combination of all the above plus possibly blocked exhaust baffles which can lead to heat build up. I had a 72 for a few months and it had a similar shot left cylinder which turned out to be a completely shot left crank seal. The center of the seal had torn out.

Shot seals would be at the top of my suspect list. Left and right are easy to check. Pour a cup of kerosene of diesel into the bottom end and see if it pours out of the end seals. On the left remove the alternator cover and you can see it pour out. The right side leaks into the primary drive case (clutch cover).

Jets could be wrong for that year of bike but more likely that they are just clogged - partially at least. Points could be too wide or too far advanced and that needs to be checked. It's a good plan to measure everything as the motor is being torn down. Try to keep accurate results of every thing you find. Hand written notes are fine, or create a WORD Document and take lots of pictures as you start to remove parts.

Send the crank to Bill Bune in MN- or your local 2 stroke crank expert for a rebuild with new seals first. Because that cost is high and if the crank cannot be rebuilt for some reason, upfront would be a good time to find that out.

[Coyote]

The best source for OEM pistons is Paul Miller. He has kits on ebay with everything you need to rebuild the top end. Pistons and rings in stock bore, .5 over and 1.0 over' You get 3 pistons, 6 rings,3 wrist pins, 3 top bearings and 6 pin retaining circlips. All Suzuki stuff. On eBay, just search "Suzuki GT550 pistons". the cost is around $300.. There is another seller out of Japan / China that sells reproduction kits for a lot less than one third the cost of OEM. Some guys have used them with good results. Others wouldn't touch them with a 10 foot pole. If the bike is a keeper, use the good stuff. If just fixin it up to sell, consider the cheap route. Cost for that reproduction kit with the same parts is $75.00.

[crazyeights]

Is there any other places to get oem pistons? I personally don't like to buy from Paul Miller because I see many wrongly advertised and listed parts and then when I question it, he is pretty mean about it.
What are the drawbacks to the aftermarket pistons? About new pistons- What measurements are necessary to determine if a rebore is needed? The cylinders look good with no smeared aluminum

I got into the shop today so the carbs, cylinders and and the pipes came off. The upper right pipe is full from a mouse nest I still don't know if it the was the issue. I'm also convinced the upper left pipe is NOS or unused because the baffle looks brand new and there is no carbon throughout the pipe whatsoever. Also the left pipe has spring loops for the crossover unlike the other two pipes. Is this correct? The pipes on the blown piston cylinders have some heat rash compared to the good cylinder.

[Coyote]

I'm not certain, but I believe the exhaust springs started in 74. But it wouldn't fit without an adapter. 72 and 73 pipes were the same. 74-77 ware the same, but slightly different at the rear mount than the earlier pipes. They can be interchanged, but not without fabricating an adapter for the rear hanger.
No Paul Miller eh? Everyone has issues with him at one time or another, and yes he is cranky most of the time. Your other options are AlphaSports.com and Partzilla.com I dealt lot's with the later and it's a good outfit. Last time I was at Partzilla their fiches were down, but it has been a while. If they are still missing the fiches, you can get the part numbers off AlphaSports and then go to Partzilla to buy. Partzilla is waaaay cheaper.
Without the proper measuring equipment, it's hard to tell if it needs rebored. Remove the rings and put the cylinder back on.. Use a .002 feeler gauge. If you can slip that in anywhere, you can bet the cylinder is worn oversize. The best check is with inside micrometers or a tri-mic bore gauge. Dial bore gauge works too if you know how to set one up.
p.s. Carbs don't look bad at all and should clean up nicely. You can brighten all the brass parts with a vinegar or ammonia soak. Not for the alloy. Turns it black.

[yeadon_m]

Those main jets, mounted over what looks like a nut or extension piece, don't look right to me. My 550B's Mikuni VM28 carbs have the mains screwed down flat into the carb body onto a brass washer.
While you have the carbs open, please remove and check the numbers on the jets. Pilots should be #25 and mains might be 95 to 100 depending on model year. Nice, well-fitting screwdriver on the pilots to avoid knurling them, but I'm sure you know that!
You can get the aftermarket piston sets from gtreiner1@aol.com in Germany. He's very responsive.
Cheers,
Mike

[Coyote]

I noticed those 'extended' jets too. I figured it had something to do with the early carbs. Never been in one if those before, so I thought that could be normal?? Prompted me to look at the fiche just now and they do look wrong. I have a Suzuki 72 GT550 shop manual. Looked in there and they are wrong. What's with those jets guys?

[tz375]

I had the same extenders/adapters in my 72. The needle jet is tapped for a hex jet but they chose to use round main jets and hence those hex to round adapters. Strange but true.

I would speculate that they wanted to use round jets but Mikuni supplied them with a needle jet/nozzle/emulsion tube tapped for hex type threads, so they needed to use the adapters to allow the use of those NJs and Round mains.

[Desert_75]

Nice looking 550 Good luck sourcing some pistons.

[crazyeights]

Could the adapters be a contributing factor to the failure? Also, what are Nykasil cylinders and how can I tell if my cylinders are Nykasil or not? Thanks in advance

[tz375]

Not likely.

Most probably a combination of partially blocked jets and leaking seals plus possibly poor ignition timing.

[Vintageman]

That looks just like mine when I bought it. I have also seen two others (early models72/73) like this and I knew of one that was running fine after repair with touching crank seals. I did not touch crank seals and have since put thousands of miles.

Here is what I suspect. The person drove the bike cruising along running ~ 1/8 - 1/4 throttle and it was too lean. Those needle jets are probably pitted inside and pilot jet may be partly plugged. The needle jet pitted will restrict flow. In my opinion it’s a little too lean when all is well 1/8 to ¼ throttle.
You will have to pop out needle jet and hold the main jet end into a bright light and with good eyes and/reading glass look down the first 1/4" and see if it looks like the surface of the moon and if so it’s no good. It should be shinny smooth and if you can’t have shinny than at least smooth,

Of course check everything else for jets, float height, jet needle condition and clip position.

Of course it could be something else, but not my experience on these so far... but I have only seen a couple failed like this and they made thousands.