Piston numbers for a GT550

[tennesseetwostroke]

Does anyone happen to know why Suzuki changed the pistons for the GT550? My 74 original motor has 12110-34000 pistons in it. They are about 159 grams and the pins that hold the rings in place are a little less than an inch apart. The piston that superceeded this is 12110-34101, weighs in at 172 grams and has the ring pins about an inch and a half apart. It also has three drilled holes in the rear skirt. I have ended up with two of the number 34000 and one of the number 34101. It looks as though the placement of the pins won't matter as they both travel across the intake ports but the weight difference is quite a bit. It looks like it would be easy to get the weight the same. I was just wondering if anyone has used a mixed set of these and what the reason for the changes were.

[yeadon_m]

What a coincidence! I have a GT550B (nikasil cylinders) with two heavy (mid 170s grams) and one light piston. I don't know if they had a reason to change them, but they evidently did.
The light one (high 150s grams) as you say has piston ring retaining pins that means that the ring ends run across the intake ports (the nikasil cylinders have 7 ports and the US iron liner cylinders, 5).
On advice that as the ring end gap is central to the port and that the port is a narrow one, I decided to fit it despite it being 'wrong'.
I have run that 'incorrect' piston for hundreds of miles. When running its the quietest of the three and, on dismantling, there appear to be no problems with ring ends and port edges. Piston and bore are unmarked.
I made the weights balance on that lighter piston by installing a heavier (thicker walled) piston pin of identical external dimensions, from a Vespa Malossi/Polini 210cc kit where the piston pin is 16mm OD, 50mm long, 9.5mm ID and weights just under 10g more than a Suzuki stock pin. I then made slight, biconic weight reductions on the other two, stock Suzuki pins and on assembly, each piston/pin assy was within 4g. Thats a lesser variation than I had seen for example with a set of 3 Suzuki GT750 pistons right out of their boxes, so judged it would be fine. So far, limiting revs to 4K rpm, its nice and smooth.
I would have much preferred to have chosen 3 identical pistons, but its a long story and they are hellish expensive.
Good luck!
Mike

[tennesseetwostroke]

Thanks Mike. It looks like there is room that they could have placed the ring pins where the gaps would not cross the open ports so I guess that may not matter. Where did you get your heavier pin from? That might be easier than lightening the piston if I can't find another matching one. I tried to order a set of Wiseco pistons from Paul Miller and after waiting weeks for them to ship they said they lost them. Really disappointed on that note. Thanks again.

[Craig380]

Do the rings actually cross the intake port at the bottom of the stroke? On my old 380 the rings were never visible in the inlet port, otherwise the inlet charge could conceivably go straight into the cylinder at BDC ... I thought the roof of the inlet port was usually just below the rings?

[yeadon_m]

Craig,

When using the 'wrong' pistons on a 550 (ie pistons spec'd for an iron liner, 5 port cylinder in a nikasil plated, 7 port cylinder) the ring end gaps do cross the narrow intake ports (one ring end gap per port) but because no more than 5mm of 'loose' ring projects into free air, it cannot spring out and into the port.

TN two stroke: I found the piston pin online from here I think. https://www.beedspeed.com/piston-16mm-5 ... s7acs8l7j2" onclick="window.open(this.href);return false;

Cheers,
Mike

[tennesseetwostroke]

Yes Craig is right. Wow it has been so long since I worked on a two stroke. Either type piston in these iron cylinders do not go low enough for the rings to cross the main intake port. They do cross the transfer ports but the ring ends are not in the open space with either one. Thanks for pointing that out Craig. One less thing to worry about, and every little bit helps! I know I can get more of the heavier pistons if I want them to match. Maybe I will run across another older lighter one before I get ready to put this back together. Or the heavier pins. One way or another. Thanks for the link Mike.