Suzuki GT285

[dollydog]

Hi, my first post since I joined. Found the forum [again] after Googling for GT250 big bore - just to see what others were up to. As you can see from the header, I built my own big bore from a Ramair engine with GT250A barrels and milled out crankcase transfer ports to match. First problem was 250A barrels don't fit, so I reamed the back 2 stud holes perfectly round, made some ali slugs on a lathe, Loctited them in and redrilled them at 65mm centre to centre. I'd already bought a pair of domed TS125 pistons on +2, making them 58mm. Took the barrels to my local machine shop, with the pistons and had them bored out [cost me 3 rebores for a 4mm overbore] and had the nockpins on the pistons moved to where GT250 nockpins would be. Rebuilt the engine and it ran ok, with 28mm flatslides from an RG250. Ok wasn't good enough, so i stripped it again and found some flat topped TS125 pistons, same size. Moved nockpins again and rebuilt the engine, this time I used standard GT250 Ramair carbs. Oh yes, I had to change the crank, flat tops had 16mm gudgeon pins, not 14mm. This time it ran well, using my own electronic ignition, adapted from a Fiat 500. Alspeed spannys and starts first kick and runs like a dream - dead crisp. I did loads more - but don't want to bore you all to death on my first post. With the Ramair covers on it looks at a quick glance as though it's just a standard GT250 Ramair. Love it
cheers, dd.

[Vintageman]

I am impressed you got those later cylinders to fit the earlier case. I think I would have looked for A cases since I am lazy (unskilled) like that.

Next, I am surprised there was 4mm of cast you could bore out...I hear the GT380 is only good for 2mm any more you brake through liner. I was going to use ts125 pistons as well, but if I can only go std size there are 2mm after market piston out there on ebay for 250A that would fit gt380. GT380 will take either ring pin locators.


The only thing, maybe better on a 250 is T305/t350 cylinders. Here you are 60mm to 63 mm (if you use 2.0mm Wiseco piston getting rare). so 5mm more than what you did max

A 58mm GT250 cylinder must make a difference and unique. The porting on the A makes it a hot tuned bike. How high is the compression. The later A/B head added 10mm to head hight to lower compression on stock bike... Be careful of too much compression.

Did you widen exhaust port/intake port to take advantage of wider cylinder bore? I don't think port gets wider as you bore it wider.. so you could have made port 1 - 1.5 mm wider each side for example... In take as well. I would raise and 250A ... its already 29.5mm.

Are you running expansion chambers?

[dollydog]

hi mate, just to answer a few questions:
i wanted to use the A type barrels simply because they had 4 transfer ports, not 2.
unfortunately, i don't like the A engine shape, but love the ramair. can't stand the later x7 reed valve engine.
the liner on the 250 is at least 8mm thick, so boring 4mm out of it was no problem.
compression - i've no idea, but it's now using the flat top pistons, which i presume are less than the domed.
heads were milled out to 59mm and the squish just followed on from normal 250 squish - it works anyway
handmade copper head gaskets - 0.3mm copper sheet, annealed and fitted. the ali ones blew time and again.
timing? i set it at standard and then advanced/retarded til it sounded right - all against the rules, but works for me.
downpipes are standard gt250, cut and shut, spannys are an old set of allspeeds.
this is what we call 'shed built'. no $5000 paint job or custom made spannys, no cnc in sight, just the tools in the shed
and a budget of about 50c including the t350 frame, new tyres, forks and all the rest of it, it came in at well under £1000,
i love it and it has a very unique sound, starts first kick and everything works.
cheers, dd.

[Vintageman]

8mm thick. Then, Why not use T350/T305 pistons that would have been 6 or 7mm os using std size.

I have both early and later GT250 cylinders and engines. It would be nice to take a GT250A and see if it would swallow a T350/T305. Clearly need to to widen ports to take advantage of that extra volume....simply use T350/T305 OEM specs for how wide to go or split the difference as in your case

You are very creative and thanks for FYI on how much extra meat there is on a 250 cylinder.

[Craig380]

@Dollydog, welcome and that is very impressive - you make all that machining work sound easy, when it is anything but!

@Vintageman, as you probably know you can go 2mm OS on 380 barrels, Suzi sold a factory 384cc bike in Italy (55mm bore) and offered 1mm OS pistons for it to make 403cc. Triplerocky (Rocco Garzone) had a 380 with this full-fat motor.

[dollydog]

hi craig, thanks for the comments
i seem to remember that t305 and t315 pistons have 18mm gudgeon pins - same as the 550? might be wrong.
i never touched the porting - i've never ported anything in my life after seeing a lot of bikes get worse.
the flat top pistons have very tight rings on them and after trying to fit one side about 15 times, including breaking the top ring, i decided to change my MO. you can't just put the ring gap near the nockpin and hope the barrel will slide on, because it won't. so what i did in the end was this: put circlip [1] in piston while on my knee. put barrel upside down on my knee. feed piston in so the rings are just inside.this took a few goes before i got both rings right. put barrel over studs and put a couple of chocks to hold it at the right height. turn crank over until conrod at the right height and gudgeon pin will slide in. put circlip [2] in. take chocks out and lower barrel down. repeat for the other barrel. what a ball ache
the real good thing about the flat tops was the shape of the skirts. they opened the inlet ports earlier and obviously closed them later than the domed tops. i suppose that was as good as a mild tune? oh, one more thing to really miff me. you can't get the rings for flat tops here and i had to buy a complete piston kit
cheers, dd.

[Craig380]

Are the rings for the TS125 pistons a keystone (wedge profile)? It's unusual that they are a tight fit in the ring grooves.

On the standard 380 pistons (keystone rings top and bottom) they move really freely and are a piece of cake to feed into the barrels with a squeeze from your fingers (providing the ring gaps line up with the pegs on the piston)

[dollydog]

hi mate, with the flat tops the rings are normal, but only 1mm thick and they are very, very springy. the ends of the rings can easily ride up over the nockpins and that's how they get broken. you must put the nockpin dead in the middle of the gap and squeeze both sides of the ring equally to exactly seat them. as you can imagine, it's pretty damned difficult. the bloke i had them off had his 'questions' turned on on his web page and several people had messaged him asking if he was sure the piston kits fitted, because they couldn't do it and they were only trying to fit one, i had two to do.
normal gt250 pistons are so easy to fit [and 380, 550, 750], just put the gap somewhere near the nockpin and slide the barrel [well oiled] on. these? they would make the pope swear
another thing i haven't done is the carbs - apart from fitting 112.5 main jets, which everybody with a gt250 seems to do. i've refurbed about 100 pairs of the mikuni carbs and probably 70 of them had 112.5 jets in. pilot jet stays the same.
if i could get bluetooth to work on this steam driven laptop i could post some photo's.
cheers, dd.

[Vintageman]

i never touched the porting - i've never ported anything in my life after seeing a lot of bikes get worse.

I agree, I have gone too far on EX height... no more street bike. But width should not been a problem. Keep to OEM specs for long ring life for example. Compare a T305/350 width versus gt250 and you'll know how far to go. I would keep same shape on sides as GT250A. This is a significant performance gain that simply matches what you did for bore size increase. But, I can understand your reservation.

The T305/T350 is the same wrist pin dia as the early GT250. The pin is longer though of course. Maybe same as GT550, but need to check

[dollydog]

if i could whip the heads and barrels off, do the porting and get them all back together in a reasonable time then i might be tempted. but i'm not going through all that feeding pistons into barrels malarky again i'll keep this one as it is and might start experimenting with my big bang gt250
cheers, dd.

[dollydog]

vintageman, the t350 [actually a 315cc] standard piston is 61mm, which would be 7mm oversize and only leave about 1mm of liner. don't think the engine would last very long i'll stick with my GT285, as far as i know it's unique
cheers, dd.

[dollydog]

i measured a spare gt380 barrel liner last night and this is what i found:
standard liners are [near as dammit] 5.3mm thick, which doesn't give you a lot to play with.
on the plus side, the bottom of the liners aren't chamfered like the 250 liners. when overbored to 58mm, i had to flatten them off slightly as they were like razor blades, very thin, sharp and could have snapped off easily.
i just discovered the rg150 - standard pistons are 59mm, which would give me a 295cc maybe when it needs a rebore?
cheers, dd.

[Vintageman]

The T305 is 60mm.

The GT380 liner at the top I think is only a little over 2mm thick if I recall

To honest I am very surprised you got that much out of 250. I fear if If you tried T305 piston (6mm OS), you would break through somewhere. What about the bridge thickness between the two intake port windows. I did an engine (forget) "big Bore" and had to shifted the bore off center and a bit towards the EX port (not windowed) to preserve bridge on intake. The last "big bore I did was a gt185 I think it was 4mm. Got kit off ebay out of UK. That is where I widened the ports. I didn't do that at first so I know what difference it makes. both torque and top end.

If you like GT250 and get another keep your eyes out for either T350/T305 cylinders. I think the factory spec shows the T3xx at 4Krpm is equal to the GT250 at its peak. If you now slip your clutch I think the T350 springs are stiffer than 250 and what I did.

My approach these days (trade time for money). For example if you had a kawasaki S1 (250cc) put a s2 top end on it (350cc if you can find it) . Or if you have a RD/Ds7 Yam 250, put a 350 top end. But for example if had a yamaha DS7 250 ccs I hear a yam ds6 piston 250 cc is widernand will make it few mm big bore like you did. There are many other examples of both ways.

If you go 350 top end still not easy for you need carbs, spacers, airbox, Y boot, oil pump primary gears/clutch/sprockets, etc parts to do it right. And in case of suzuki the stock exhaust needs to be changed if that is the way you want to go.... way too long reply


Again you have a sweet bike as is!

[dollydog]

hi vintageman again, in case you hadn't noticed, i like doing things the hard way - or differently
"don't bother, it will never work" is the worse thing anybody can say to me, i'll try anything - until it works
to be honest, i don't just want to put a standard big bore kit, that thousands of other people have done, so a
305 or 315 is out of the question, it's too easy and too common
i bought a rg250 gamma engine in a job lot, stripped it down and found it had been completely rebuilt inside -
everything was new, but i still rebuilt it. total cost? £100 dragged a ramair frame in from outside, ground most
of the lugs off it and installed the rg engine. i went from there. anybody can fit a gt250 engine in the same frame
and anybody can install a rg250 engine in a gamma frame - too easy try it t'other way

next project? i've got one in mind. has anybody ever built an inline 4 cylinder gt500? the cranks will press up pretty easy.
i've got plenty of spare cranks, crankcases, gearboxes, barrels, piston kits, etc etc, plus over 20 complete engines, the only question is: will i live long enough to complete it? any thoughts? i don't mean about my demise, i mean the 500? perhaps a new thread for this?
cheers, dd.

[Alan H]

GT550 pistons are the same size as the T305, so what about a pair of outer GT550 pots on a 250 engine?
That should be a straight install, Shirley?