Suzuki GT250 -73 racing: the crankshaft

[Erkki]

Hey everyone,

Are there any Suzuki 2-stroke experts around?
I'm building a GT250 for classic racing, and I’ve read on the Suzuki2strokes forum that the stock crank isn’t great for racing. I have read that some people modify the crank by going from 3 bearings to 4.
I’ve tried registering on Suzuki2strokes but can't get in, and the forum’s pretty much inactive now, so I can't get in touch with anyone who knows about this mod.
Has anyone here raced with the stock crank? How did it go? And does anyone have info on the 4-bearing mod, like what parts are needed, etc.?

Many many thanks in advance!

[husaberg]

Hey everyone,

Are there any Suzuki 2-stroke experts around?
I'm building a GT250 for classic racing, and IÂ’ve read on the Suzuki2strokes forum that the stock crank isnÂ’t great for racing. I have read that some people modify the crank by going from 3 bearings to 4.
IÂ’ve tried registering on Suzuki2strokes but can't get in, and the forumÂ’s pretty much inactive now, so I can't get in touch with anyone who knows about this mod.
Has anyone here raced with the stock crank? How did it go? And does anyone have info on the 4-bearing mod, like what parts are needed, etc.?

Many many thanks in advance!

No idea but the earlier T20 ones they used to use TZ or RD cranks but that required line boring the cases.
What I do know is the Gt250 A and k/M had different conrods with different little end sizes
I have the drawings for one model here somewhere
https://www.oldjapanesebikes.com/mra...lletins/page-2


Conrod differences
https://www.mitaka.co.uk/rod.php?id=175
https://www.mitaka.co.uk/rod.php?id=176

first google

Lifted from the Suzuki history site

Just a quick bit of info on the differences that between the Ram Air J-K-L-M (1973—1975) models and the later A-B-C (1976—1978) variants: Ram Air motors are quite closely related to earlier T250's and share the three main bearing crankshaft arrangement, and the carbs are bolted directly to the barrels. A-B-C variants have a revised crankshaft using four main bearings and different lubrication arrangements, also the barrels are different in construction using different stud spacing, and rubber inlet stubs which mount the carbs


Don't think any GT250 had electronic ignition except the later X7 which isn't really a GT250.


So in short No!!

youre welcome.

[Erkki]

Thanks!

Yes, the GT250K engine is still mostly the same as the earlier versions, at least in terms of crank bearings.
I'm planning to use 14mm small-end rods and RG250 pistons.
Machining the cases to fit a Yamaha crank is out of scope for now, but maybe in the future—we’ll see.

Do you know if the Yamaha gear wheel fits the Suzuki flywheel directly? (with cases machined)

[husaberg]

Thanks!

Yes, the GT250K engine is still mostly the same as the earlier versions, at least in terms of crank bearings.
I'm planning to use 14mm small-end rods and RG250 pistons.
Machining the cases to fit a Yamaha crank is out of scope for now, but maybe in the future—we’ll see.

Do you know if the Yamaha gear wheel fits the Suzuki flywheel directly? (with cases machined)

I was looking for the mag that had it as I thought I had posted it before, it was a late 80s Classic racer mag about Mac macdermaids post classic TT 250 winner about 1986.

Actually here it is here.
Different bike...

http://t20suzuki.com/racing.htm

For racing I would be looking for single ring mx pistons.

[Erkki]

I was looking for the mag that had it as I thought I had posted it before, it was a late 80s Classic racer mag about Mac macdermaids post classic TT 250 winner about 1986.

Actually here it is here.
Different bike...

http://t20suzuki.com/racing.htm

For racing I would be looking for single ring mx pistons.

Thanks! Lots of great info!

I wonder which center seal was used and if he utilized two stock center bearings.

I've also been considering CR125 single-ring pistons, but that depends on whether I trust the crank enough for using high RPM.

[F5 Dave]

Typical suzuki you won't have any problems with stock parts in as new conditions for many road miles (or even kilometres if your dials allow).

But kick that shit to the curb if you are racing it. Spend your money into the crank first. I've only raced singles but took me a while to learn that. New parts, and ideally Larger big end bearing to suit yamaha or any sort of modern mx rods. But will depend on crank webs and a decent engineer to bore webs.

[jato]

Crankshafts ... i recently built up an early kawasaki triple crank (they use 2 main bearings per cylinder which is essential for high rpm) using wossner rod kits from RM 125, with the crankpins machined to give a race-worthy interference fit into the crank webs, with a small shoulder to press against. it has clocked up many hours of 10000+rpm use so far and is still true and apparently happy.

[Erkki]

Ok, I’ve almost figured out my "Stage 1" setup.
I’m going with GT250A conrods and RG125 pistons.
After disassembling the engine, I found that I can use a set of four outer bearings to achieve a more stable four-bearing crank.

However, I’m unsure about the middle seal setup. Is there any reason a standard shaft seal wouldn’t work in this case?

Does anyone know if 25x65 labyrinth seals are readily available anywhere?

Also, are there any guidelines for DIY labyrinth seal design and machining?

[F5 Dave]

I understand that people use two outside seals back to back where not available for centre. Not to dissuade labyrinth mod.

[husaberg]

Ok, I’ve almost figured out my "Stage 1" setup.
I’m going with GT250A conrods and RG125 pistons.
After disassembling the engine, I found that I can use a set of four outer bearings to achieve a more stable four-bearing crank.

However, I’m unsure about the middle seal setup. Is there any reason a standard shaft seal wouldn’t work in this case?

Does anyone know if 25x65 labyrinth seals are readily available anywhere?

Also, are there any guidelines for DIY labyrinth seal design and machining?

From what I understand if you go the lab seal route you have to make sure the cases have been line bored at the factory or do it yourself.
The only reason I am saying this is I have seen stuff where they tried to put lab seal in Kawsaki tripple and it ended in tears and this was the reason given.
not sure what others had them std but RD125/200/250/350/400 and TZ did.
You can just look in the parts fiches for the main being the part number on most says is the bearing size say 6202 then you just look at the bearing size.


I think earlier Suzuki cases ones were factory line bored not sure later stuff.

RD350 is 62mm od

well aren't you lucky a JS450 jetski has lab seals and a 6305 bearing which is 64mmx25mm not sure width though.
https://www.mitaka.co.uk/JET_BEARINGS.php
I have a big online catlelogue somewhere that has a lot of snowmobile and jet ski stuff linked.

[Erkki]

Great info, thanks! I hadn’t noticed that Mitaka has parts for jet skis. I think many snowmobiles use labyrinth seals too.

Yes, the cases are factory line-bored, and since there’s no space for two back-to-back Suzuki seals, I first tried going the labyrinth seal route.

I found this Kawasaki seal number: 59096-502. I ordered it from Promo-Jetski to check the width, but they refunded me since it's no longer manufactured.

I think the Yamaha seal has a 30mm inner diameter, so it would require some kind of collars inside and outside.

So, I went back to the back-to-back seal route. The space for the center seal is 12mm, so I machined a 65x45x12 collar and will use two 25x45x5 seals. I’ll drill lubrication holes from the transfer ports, and with the DIY collar, it’s easier to route the channels all the way to the bearings.

If this sealing setup fails, I’ll either try making a labyrinth seal myself or find one that fits. So husaberg, if you find that catalog, it would be really helpful!

[husaberg]

Great info, thanks! I hadn’t noticed that Mitaka has parts for jet skis. I think many snowmobiles use labyrinth seals too.

Yes, the cases are factory line-bored, and since there’s no space for two back-to-back Suzuki seals, I first tried going the labyrinth seal route.

I found this Kawasaki seal number: 59096-502. I ordered it from Promo-Jetski to check the width, but they refunded me since it's no longer manufactured.

I think the Yamaha seal has a 30mm inner diameter, so it would require some kind of collars inside and outside.

So, I went back to the back-to-back seal route. The space for the center seal is 12mm, so I machined a 65x45x12 collar and will use two 25x45x5 seals. I’ll drill lubrication holes from the transfer ports, and with the DIY collar, it’s easier to route the channels all the way to the bearings.

If this sealing setup fails, I’ll either try making a labyrinth seal myself or find one that fits. So husaberg, if you find that catalog, it would be really helpful!

Not 100% but i think BDK were making lab seals pretty sure they were doing one for H1r i think they were the same od nor sure the id but someone like Flettner or Sketchy could make them.
i used to have a link to a massive snowmobile and jet ski site that had al the sizes and manifolds etc but it appears to be dead.
I tried the wayback machine but that was dead as well.
I might have the data saved on my old PC or lappy.

but I would go the Yamaha route and then you can use generic TZ stuff.
Prox list all the dimensions for pistons and rods and bearing
I would use MX pistons if I could as they have better thinner rings and are just better than road stuff
Honda tended to have long windowless skirts.
Even you you are doing it use decent silver/ alloy cadges for big ends and small ends.
if you are big 20mm and 14mm gudgeon pins look at KT100s stuff for the bearings.

bottom right
tech
https://www.pro-x.com/media/catalogue/

[jato]

As above regarding bearings ,thin-ring pistons etc... a labyrinth seal will be easy to make if you are fairly crafty on a lathe, you could also build a split one with a pair of m5 cap screws holding it together . what type of bearings are adjacent? C3?. i would expect your main bearing bores to be around 0.03 smaller than nominal maybe even a whisker more being aircooled. once measured up you might settle on the seal OD being 0.02 or so interference and depending on the bearings maybe 0.03 or 4 radial clearance, with 4 lands and 3 grooves if you've only got 12mm width available. i have an old TZ seal here which has 6 + 5 but it is 16mm wide. an appealing possibility would be to make a split one out of acetal and close the running clearance to almost that of the bearings.

[Erkki]

Oh crap, I just realized I've misled myself regarding fitting the Yamaha crankshaft. I previously read on the T20Racing site that Yamaha's bearings are larger than Suzuki's, so I assumed they must be really big then.
But now I’ve realized that the T20 actually has smaller bearings than the T250/GT250.

If Yamaha's bearing inner diameter is 25mm, I might be able to use Suzuki's 25x65x17 bearings with the Yamaha crankshaft. That would instantly open up a much wider range of piston options with 15mm and 16mm wrist pins.

But for now, I’m going to put this Suzuki setup together for at least next summer to see how it holds up. I’m still deciding on the center seal solution, either a back-to-back oil seal apparatus or a POM-C labyrinth seal, since I’ve got a 70mm bar of it sitting on the shelf. Time is running out since I plan to do the crank next Sunday

I actually would have already made the labyrinth seal from aluminum, but my blank had a 31mm hole in the center, so I ended up making the back-to-back seal adapter sleeve for 25x45x5 oil seals instead.

The RG125 pistons have 1.2mm rings, would it be safer to run just the top ring to avoid ring flutter?

Thanks for the ProX link! I hadn’t come across that before. I’ve been working with 4-strokes for the past 20 years, so I haven’t really explored these sites until now.

[husaberg]

Oh crap, I just realized I've misled myself regarding fitting the Yamaha crankshaft. I previously read on the T20Racing site that Yamaha's bearings are larger than Suzuki's, so I assumed they must be really big then.
But now I’ve realized that the T20 actually has smaller bearings than the T250/GT250.

If Yamaha's bearing inner diameter is 25mm, I might be able to use Suzuki's 25x65x17 bearings with the Yamaha crankshaft. That would instantly open up a much wider range of piston options with 15mm and 16mm wrist pins.

But for now, I’m going to put this Suzuki setup together for at least next summer to see how it holds up. I’m still deciding on the center seal solution, either a back-to-back oil seal apparatus or a POM-C labyrinth seal, since I’ve got a 70mm bar of it sitting on the shelf. Time is running out since I plan to do the crank next Sunday

I actually would have already made the labyrinth seal from aluminum, but my blank had a 31mm hole in the center, so I ended up making the back-to-back seal adapter sleeve for 25x45x5 oil seals instead.

The RG125 pistons have 1.2mm rings, would it be safer to run just the top ring to avoid ring flutter?

Thanks for the ProX link! I hadn’t come across that before. I’ve been working with 4-strokes for the past 20 years, so I haven’t really explored these sites until now.

One model gt has the 22 big end. 16m. Pins.
Isn't rd centers 30mm inner bearing pretty sure it's one of the reasons to use rd crank in preference to real tz. Or was it the other way around? I think it might be .
Look on the tz site or rank to the wiltons. He used to be on Kb ...malcy? Or something like that.
I have drawings for both somewhere.
The suzuki ones are on ozbook in service bulletins.
The yame ones were on another site. They would have been in service bulletins as well for rebuilding cranks.