GT250 T305 Twins similarities

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Indy650
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Suzuki 2-Strokes: '74 GT250

GT250 T305 Twins similarities

Post by Indy650 »

Hi Im doing some work on my GT250 and wondering if anyone knows if you can use the 59.9mm T305 pistons in the GT250 engine. I've never worked on a 305 so I know the stroke is the same but don't have the other measurements like deck height and wrist pin size. I'm assuming the 305 is just a 250 with bigger combustion chambers and larger sleeves and pistons but would like to confirm before I buy the parts. Also if anyone knows if the entire top end from the 305 will bolt on the 250 as that would save me milling the heads ext...
dollydog
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Re: GT250 T305 Twins similarities

Post by dollydog »

easy swap, or use the 315 top end?
i think [but don't quote me on it] the t305 and 315 pistons use 16mm gudge pins, so you would need a 73 or early 74 crank as the later ones only used 14mm gudge pins. hope this helps? :D
cheers, dd.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
Indy650
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Suzuki 2-Strokes: '74 GT250

Re: GT250 T305 Twins similarities

Post by Indy650 »

Thats helpful thanks. I wish i could find a triple especially a 750 liquid but even these little twins are super rare where i live. They are on craigslist a matter of hours before someone scoops them up.
Vintageman
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Re: GT250 T305 Twins similarities

Post by Vintageman »

I am late to this.

I think 1975 Gt250 start to use the smaller wrist pin. I own a 1974 wicj is L model and M seams to ne where it changes. If you have 1976 -77 whole different stud patter for cyls.


I think Dollydog (or someone) bored an early gt250 quite a ways but may not quite 60mm like T305 std piston.

The t305 an t250 bring larger port specifically exhaust.

The t305 is more of the "rocket" in my opinion.

To do the swap you need

T305/t350 exhuast (Cyl mount thread larger DIA than 250) or use chambers
Oil Pump. The T305 and T350, different (?) but both flow much more oil than 250
The T3xx has stiffer clutch springs. If you don't change the extra torque will cause clutch to slip
The carbs are different and Carb insulated spacer to cylinder is to.
The air boot is different
The airbox is different where the t305 is the most free flowing (and hurst your ears the most).

I think if you could fit gt250 carbs and more strictibe gt250 air box the 250 carbs may work ok on t3xx. What is really nice about the t3xx is those 32mm carbs
You may want to add the rubber handle bar isolator or you hands get the tingles no doubt. Slight fab drill effort to GT250 brace to accept isolator bushing and need chrome t3xx handle bar clamps, stock GT250 bars OK. This works. no more tingles at all. The GT250 did not need this but you will after swap to t3xx even with fresh bore to spec
You need the heads the later T350 with canted fins cools the best of the two style. T305 head little smaller dia and may be problem on t350 if it is not std bore.
I think you need to swap throttle cable also to get full WOT without binding.
You need the t305 piston or T350. The t305 is at 1mm OS fits std T350 cycle and the transfer cut outs wider. But not sure if it adds much, T350 works well in T350. I think if you cound T305 at 1.00 mm os you maybe be able to bore it the 61.5 mm and use 0.5 OS T350 piston... liner gets thin so bore job needs to be aligned well.
the sprocket is 38 tooth rear vs 41. 41 works but cruising you wish little taller. 17 tooth fron is almost as tall or 40 rear not as tall


Hmmm what else.... That's it me thinks
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dollydog
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Re: GT250 T305 Twins similarities

Post by dollydog »

yes it was me :D i put a pair of +2mm ts125 pistons in overbored barrels. it gave me 58mm bore x 54mm stroke = 285cc. i used the 250a barrels on a ramair bottom end [don't ask, but it was a long job], because they had the 4 transfer ports instead of 2. also milled the crankcases out to suit. used ramair heads and covers, so unless you're a complete anorak you'd never know the difference, which is what i wanted. i used 28mm gt250a carbs on it as well. i had a hell of a job getting it running right until i sourced a pair of flat topped [instead of domed] pistons. these had a different skirt shape which made the inlet ports open sooner and close later. they also had 14mm small ends, so i had to change the crank for a 250m. after that it started first kick and ran like a dream :D you can do alsorts with these lovely little engines if you put your mind to it :D
cheers, dd.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
Indy650
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Re: GT250 T305 Twins similarities

Post by Indy650 »

Thanks for the replies guys, lots of good info. I decided just to stick with the 74 250 topend for now I have parts to build a couple extra engines where I may play frankenstein a bit more. I much dislike the single transfers and which they had the bridge for more controlled flow. Also I was surprised to see these cylinders have auxiliary exhaust ports but they are way too low being about even to the transfers rather than farther towards the top of the exhaust and they are very small. Being so low IDK if there is anything I can do with them I may try raising them and widening them a little but I'll decide when I get to them. The brass oil nipple makes for a pain to port the intake anyone ever tried taking them out for porting and then reinstalling? Im thinking if I heat the surrounding aluminum they may pop out. As for the heads I will probably re-cut the chambers and lower the size of the squish but I need to re-assemble the engine to take the initial measurement and I'll also check intake and exhaust duration and decide what I'm going to change them to and start planning my combustion chambers. I don't want the bike to be too peaky but I think most Suzuki's were very tame stock so I'm aiming to bring the redline up to around 9000 rpms so probably an ex duration of 190 or so. I'll try to add to my restoration thread and here as I proceed.
dollydog
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Re: GT250 T305 Twins similarities

Post by dollydog »

hi indy, those little brass pipes definitely come out. i've got a pair of barrels that somebody removed them to mill the inlet ports out - but i don't know how he did it, sorry :( he also - AND DON'T DO THIS - milled out the bridge in the inlet ports. this is important if you want your piston rings to last :D
cheers, dd.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
Indy650
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Suzuki 2-Strokes: '74 GT250

Re: GT250 T305 Twins similarities

Post by Indy650 »

Thank you for that info dollydog. Ya many people make the mistake of narrowing or removing the bridge on ports. However the crown of the piston never goes past the top of the intake so how could it hurt the rings? Is it close enough on these that the lower ring catches without the bridge? The worst is when someone does it to a bridged exhaust port and then the tiny strip left gets super hot and expands. then its not long before the pistons all scored up and rings are snagging the port. I slightly angle the beginning of the bridge for air flow sometimes and clean any casting marks but thats it.
dollydog
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Suzuki 2-Strokes: several gt250 ramairs

Re: GT250 T305 Twins similarities

Post by dollydog »

hi indy, i may have dreamt this, but seem to recall it was something to do with ring flutter. like i said memory not what it was 40 years ago, so i may have dreamt it. must google it one day :D it's definitely a no-no anyway, whatever the reason.
cheers, dd.
ps, vintageman, they changed from 16mm to 14mm little ends in about mid 1974. in engines dated 74 you can get both sizes.
GTS250 road registered. TS250 engine, Ramair frame.
GT250 big bang road registered. Both pistons fire the same time. USD forks.
GT285 road registered. Overbored - 58mm and TS125 +2 pistons fitted.
GT10 road registered. '65 T10 engine, GT250 frame.
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tz375
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Re: GT250 T305 Twins similarities

Post by tz375 »

If the rings stay above the intake port - and they usually do unless you have a lift plate, the rings never enter the port, so the bridge has no effect on rings.

I have three sets of GT750 barrels here with the inlet bridge removed for extra port area.

BUT, if you get later style GT550 pistons, the bridge must stay because those pistons have a line of small holes in line with the bridge to keep the bridge cool and lubed.

Ring flutter is more a function of ring mass and RPMs. Thick rings flutter at lower revs. That's why many bikes now come with 0.8mm thick rings - to reduce flutter.

If the rings are exposed to the port the way they are on say an exhaust port, then the rings bulge into the port as they pass the opening and that's why many really wide exhaust ports are bridged.
Indy650
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Re: GT250 T305 Twins similarities

Post by Indy650 »

ya ive always known ring flutter to usually happen at really high piston speeds. I've always likes dykes style rings but you don't that many of them. My favorite is the keystone ring though with its concave style top.
As for bridged exhaust ports I despise them. I much prefer the 3 port exhaust that has the main port and the auxiliary side ports. The bridged exhaust ports always have issues with the bridge overheating even if you have cooling holes in the piston. That small strip of metal gets too hot and expands and there goes your piston.
As for the auxiliary exhaust ports they still have to be done correctly. Take the GT250 for example some of them have this style exhaust but the auxiliary ports are far too low actually about the height of the transfer ports where they should be higher up about in line with the top of the main port. Especially with the GT250's single transfer windows the flow of the fuel mix isn't controlled as well.
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