GT250b pistons

[rattly250b]

Hello all,
I'm new to this forum so please bear with me.
I have a 78 GT 250b which has a persistently rattly top end. Over the last couple of years I have the crank rebuilt with new bearings, seals , rods etc and also both barrels rebored with new pistons from Wemoto. This rebore failed to cure the top end rattle that prompted it in the first place.
I also have a spare pair of barrels which I have recently had bored to the original Suzuki specs which can be found on the Ozebook website. The result is the same top end rattle.
Both sets of pistons that I have are tapered by approx. 0.35mm (.015") from skirt to crown.
My question is; has anyone else seen problems like this with after market pistons? Better still, can anyone recommend a supplier of pistons to avoid this problem.
I have read that Mitaka are a big name in replacement pistons these days but having contacted Grampian, who distribute them, they state that the taper on these pistons is 0.38mm which is almost identical to the items I have.
I should stress that the bike runs fine and goes very well, it's just the top end rattle I would like to lose.

Hopefully someone out there can give me some advice.

Regards

Paul

[itsnotateapot]

Have you tried ear plugs

[teazer]

Pistons are machined to be smaller at the top to allow for the fact that when running, the top is hottest and expands more. So they are tapered bottom to top. They are also typically cam ground which means they are smaller side to side that they are front to back. Pistons are not supposed to be absolutely round and parallel. Your measurements and what Grampian said, confirms that they are probably OK

Which brings us to your noises. What is important is piston to bore clearance and it is possible that the bores are oversized i.e. excessive clearance. Unlike old British twins taht seemed to like a whole lot of clearance, Japanese bikes generally run best with very tight clearances - typically in the .0015 to .0025" range.

And air cooled two strokes do tend to make a fair amount of engine noise compare to say a new modern water cooled 4 stroke.

[barney01]

It definately sounds like you may have piston slap rattle caused by too much clearance normally by cylinder wear.
Is the rattle present all the time? Even at startup or only at high revs? Or only when the engine is warm. Is it consistent or does it get better or worse at any time.

[malky]

All two strokes rattle - I have never had one that doesn't - I bought a brand new GT185A back in 1976 and ran it in as to the book but after a few thousand miles it rattled - got one now and a recently rebuilt GT250K engine work done by Grampian and they both rattle - I think it's just a trait of the two stroke engine they are never going to be as smooth and quiet as a four stroke
Mal

[senicoli]

All two strokes rattle - I have never had one that doesn't - I bought a brand new GT185A back in 1976 and ran it in as to the book but after a few thousand miles it rattled - got one now and a recently rebuilt GT250K engine work done by Grampian and they both rattle - I think it's just a trait of the two stroke engine they are never going to be as smooth and quiet as a four stroke
Mal

+1 the 550K has just been rebored new pistons and rings it rattled before and it rattles now its a two stroke colin

[barney01]

It is difficult with these engines if your used to a sewing machine running four stroke. However i have uploaded a video of my 550 J with piston slap right cylinder. You will see the difference i think.
https://youtu.be/WAYLOfNntcA
That offending cylinder was fairly worn when stripped down and has now been rebored.

[yeadon_m]

My GT380B was swishy quiet after an SEP rebuild from ground up in 2006. Its now done 5 thou miles or so and only when cold does the right pot rattle and only then at tickover. If I hold the revs above 2500rpm it doesn't rattle and, as it warms up just a bit, the rattle goes. Once hot, swishy quiet. Not bad for an air cooled triple.
https://www.youtube.com/watch?v=ew6C1Wo ... e=youtu.be
My GT550B has a slight left side ticky tick when hot but I've decided its fine. That bike, like your 250, rattled like that when it first came to me and has done so through two full top end rebuilds. My GT750A just rarely ticks on right pot when cold and running slow but is fine when even slightly warm.
I don't think they should rattle when running faster than idle and are hot. Cold clearance is designed, as Richard has said, to give it room to expand. I would guess that clearance is somewhat of an art. Too little and it will initially be quiet but there's a seizure risk as it gets run in. Too much and it will always rattle.
Cheers,
Mike

[Alan H]

+1 the 550K has just been rebored new pistons and rings it rattled before and it rattles now its a two stroke colin

I told you it was OK and 'they sounded like that'!!!!!!
Hopefully it'll be sunny soon and we can get out on them to compare rattles.

[senicoli]

+1 the 550K has just been rebored new pistons and rings it rattled before and it rattles now its a two stroke colin

I told you it was OK and 'they sounded like that'!!!!!!
Hopefully it'll be sunny soon and we can get out on them to compare rattles.

cant wait al been putting a few miles on the 550K to bed her in a bit and build some mileage range confidence up all going well till it pi$$ed engine oil out of the back end drive sprocket oil seal or neutral sender unit should get time to have go at it this weekend incidentally anyone know what size socket I need for the drive sprocket nut I can borrow one from my mates breakers yard if I know the size thanks colin