Opinions sought on pistons (GT550)

[yeadon_m]

http://www.kettleclinic.co.uk/kcforum/v ... f=2&t=6419" onclick="window.open(this.href);return false;

Folks,

Still in mid-nightmare with my SEP rebuilt motor. See link for todays fun and games. Any advice from m'learned colleagues here?

Still smiling, but through gritted teeth.

Cheers,
Mike

[Vintageman]

I think the ring gap location is a problem. It will snag. If suppose you can put a larger than normal taper on port edge as it will help ring ends run in and out of port. Yes small windows

Are both piston types from Suz?

I would not worry about getting piston weights matched since crank not too long and has multiple bearings separating each cyl and its an inline cylinder topology engine

Yes the oil hole are to lube piston as it runs a long intake bridge. More important if bridge was on exhaust side since it would be thin and get hot.

[Coyote]

Mike, if you are not building a racing motor, I wouldn't be too concerned with the piston weight differences. I would however be VERY concerned about the piston ring pin locations. You would be opening the door for all kinds of future headaches you don't need. Those pistons you got must really be old or they were intended for a different motor. Possibly a twin. I think the t-350 used the same diameter pistons.
The first thing I would do is compare them really closely to your center piston. I would make sure all the ports, windows and cutouts match the known good piston. If you confident they are the same, I would have machine shop relocate the pins. Don't try to do that by hand. One little slip and you will damage the ring groove edge.
I would have responded to the other post but I didn't know how. If it makes you feel any better, the 3 pistons I just put the jugs on this weekend have 7 grams variation in weight. What does SEP mean?

[yeadon_m]

Guys,

The pistons are genuine Suzuki 12110-34101. I will confess to much confusion about how the ring retaining pins are in a different place. I know that the non-US late model cylinders (like mine) are plated and have 7 ports rather than 5, but I thought the extra ports are simply in a vertical line, not further rotated round the cylinder. But if thats true, wouldn't that mean on the US (iron liner) 550s, these pistons are carrying the ring end gaps across some of the intake ports? maybe the intake ports on the 5 port cylinders are sufficiently low down that the piston rings don't cross then at BDC. Who knows.

I will be exploring finding some other pistons, where ring retaining pins are in the right place. I'm deeply irked by all this. SEP is the engineering firm that did the 2013 rebuild. That these recent pistons appear incorrect however isn't down to them.

Cheers,
Mike

[Craig380]

Posted this on the Kettleclinic too:

By the sound of it, the new pistons are for the US-spec iron-liner barrels, as the inlet port on those will be below the piston rings, and those barrels don't have the extra transfer / boost ports which the ring gap now overlaps in the plated cylinders.

Trying to move the ring pegs is probably your best bet at this stage. I don't know if the peg is cast as part of the piston, or if it is added by drilling the piston AFTER the piston is cast.

Here's a thread from the RGV250 forum that may help: http://www.rgv250.co.uk/forums/index.ph ... ring-pins/" onclick="window.open(this.href);return false;

I don't know what the 550 rings are like, but on the 380, the chrome rings have cut-outs so the gap fits OVER the peg and (in theory) stops it from ever popping out.

[Vintageman]

When Suz added extra transfers to GT250 and GT550 they made new piston for it and changed ring locating pins. The new piston works in the old engine not the other way around.

[tz375]

Mike,
The 750 has ring pins in the same place ie mid point of a port, and that is OK. What is not ok is the pin at the edge of a port allowing a long end of a ring to spring into the port.

With the 750, the rings and the ends are not normally exposed to the inlet, but when we fit a lift plate, the lower ring can be see. On drag race motors revving to 9000 the peg position is not a problem as long as the intake ports are correctly chamfered.

On your motor, at BDC the rings are exposed to the extra pair of transfer ports and are pretty much dead on the middle of the ports, so it should not be a problem, but if you would feel more secure, have SEP move the pegs to a spot between ports on solid metal.

[yeadon_m]

Richard,

Thats good info - thanks.

I've just spent a half hour mucking about with one of the new pistons and a nice new set of rings installed the correct way up. Having oiled a bore and the piston / rings and carefully placed it in the bore, its clear that the pins do ensure the ring end gap is precisely at the centre of a very narrow (no more than (15mm wide) intake port and for the life of me I could not see how it could snag. I can see that the port edges are nicely chamfered.

Are you saying that the stock 750 pins, while lined up to run across a port, do not do so unless the barrel is lifted? or that they run across a port anyway? (I think the former, and your additional info is on lifted barrel motors, which is pretty reassuring to me).

I'll have to weigh up all the choices before making the decision. Leave as is, reposition pins, buy expensive correct pistons (at risk the sizes won't be spot on for my nice new plated cylinders). Or I could push the whole plot into a lake

Cheers,
Mike

[yeadon_m]

VM / Craig,

I'm now sure you're right and I have iron-liner suited pistons. What a pain. All sent to try us

Cheers,
Mike

[Craig380]

Might be worth asking what moving the ring pegs might cost as well as drilling the lubrication holes - at least you have reference pistons for the machinist to work from.

By coincidence, in a recent issue of CMM, Stan Stephens' article described how he had to modify some Suzuki pistons and Yamaha rings to work in a Yam TZR250 V-twin race bike, as Yamaha piston kits were NLA ...

[tz375]

Rings might be an issue too. Usually rings for Nicasil are different to chrome plated rings for iron liners.

Yes, the rings are only exposed on a 750 with a lift plate or someone raised the port roof too far.

Speak to Pete O'Dell about the peg position and see if he can move them if he thinks they need to move.

[Coyote]

I'm now sure you're right and I have iron-liner suited pistons. What a pain. All sent to try us

I'm not so sure. Maybe Europe motors are different. The pistons I just installed have the pins at the 4 and 7 o'clock position The old ones in the motor are the same The 550 cafe I built a few years ago were 4 and 7 also. Now it sounds like the center piston is wrong for a plated motor and the extra ports.

[yeadon_m]

Coyote,

The way I read the bulletins, the later pistons (with their further-apart piston ring retaining pins) are fine in the earlier motors too. Its the earlier pistons (with their closer-together pins) that sweep the ring end gaps centrally across the narrow intake ports. However, at least 3 separate sources have confirmed that provided the port edges are carefully chamfered, this should not be a problem.
So, if you have an iron-liner 550, you're fine whatever Suzuki piston type you fitted.

Cheers,
Mike

[Coyote]

One thing you can be sure of. There were no plated liner motors distributed on this side of the pond.. None. Another neat thing you guys got and we didn't was the elimination of the cross over pipes on the later 550's. In the USA, the cross over pipes remained on all production years

[Craig380]

It's strange to think that the factories went to such lengths to produce such variations for different markets, when the expected sales volumes couldn't have been THAT high.

Like the special 384cc (55mm bore) versions of the GT380 for the Italian market ...