Suzuki GT250 -73 racing: the crankshaft

[Erkki]

Ok pulled the crank apart and found out that the inner wheels had aprox. 6mm shoulders which I didn't see before. So no room for 2 bearings and seal.

Actually with those shoulders the crank started to look more solid than I thought. Also I had to apply hefty amount of pressure to get the pins move, so interference fits felt quite tight.
Didn't want to weaken the crank by machining those shoulders off so I just assembled the crank with new 20/14 rod kits, bearings and seals in stock configuration. future will show that will it hold together.

I'm trying to find cheap Yamaha crank somewhere to see the dimensions but no luck. If anyone can help with the Yamaha specs it would be very helpful!

One model gt has the 22 big end. 16m. Pins.

Interesting!
Only 22 big end I can find is GT550.
Is there some model with 54mm stoke I don't know about?

[husaberg]

Well looks like i misspoke sorry
The version of the eGT has only bigger little ends 16mm vs 14mm gudgeon pin
20mm big ends will be well strong enough they were good enough for nearly 50hp on the RSW125. Although they were that tight a fit used occasionally the webs used to crack.

this lists both

all below
https://www.mitaka.co.uk/con_rod_kits_all

I will find the basic dimensions. for Suzi here.
https://www.oldjapanesebikes.com/mra...lletins/page-2

from Memory the Barton 3 used to bore the gt380 suzi crank to TZ dimensions but not 100% they run the similar set up but narrower pins and so forth.
but just do the Yamaha conversion.



well you go...
click three times
below is Vincent crabtrees account of how in theory to make a big rgv

[Erkki]

Yeah 20mm should be ok, I don't believe I get anything huge out of the Suzuki cyls anyway.
But I would like to have that 15/16mm small end option. current 14mm really limits the piston selection.

About fitting the air cooled RD250 crank.
From service manual I found that Yamaha bearings are 2606 inner and 6305 outer, so shaft diameters are 25mm and 30mm.

I think the Yamaha crank could slip in with stock GT250 outer bearings and for inners GT750 09262-30042 30x65x16 + DIY lab seal
Would still need the axial dimensions and drive gear specs to verify

[Erkki]

So I was wrong about the center inner dimensions, it's 25mm so GT250 Suzuki bearings there too and the 25x65 lab seal I have already made from acetal.

Thank you so much husaberg!!

[husaberg]

https://www.tz350.net/tztips.htm#crank
years of experience and lots of kiwi stuff from the wiltons
more stuffre rd cranks

http://erlenbachracing.co/RD_TZ_LC_R...rences.htmAlso

ii) The crankshaftThe first problem is getting hold of a good crank. The TZ 250 and 350 used the same crank so either will do. However, TZ cranks are getting very rare and expensive. The centre flywheels are also prone to cracking, between the crankpin opening and the centre shaft. Lea recently took apart three TZ cranks for a customer and two were cracked and the other one was too loose to use! There is a better alternative though and that is to use an air-cooled RD 250 crankshaft. These are the same dimensions as the TZ crank other than the centre shaft which is 2mm narrower - ie 30mm instead of 32mm. This 2mm difference gives the flywheel more strength between the crankpin opening and the centre and so prevents cracking. Lea now uses RD cranks almost exclusively when he needs to find or replace a TZ crank. You can use the TZ outer flywheels, with the two inner RD ones if you want.

There is though another slight problem - the ignition rotor fitting is different on the TZ/T20 to the later RD ones. The old points-ignition RD crank has the same rotor taper as the TZ one and the T20 rotor fits this too (with a little modification). So the early RD cranks - models A,B,C,D,DX - or an original TZ crank are the ones you want. The electronic ignition 'E' model has a very different end as you can see in the picture

Remember rd350s are still being made.heaps of aftermarket as well

New ones not sure the quality ORG Yamaha ones were quality kit
https://hvccycle.net/yamaha-rd250-rd...haft-assembly/

[husaberg]

oh yeah the guy that is doing my fibreglass for the 851 also has tr250 molds. but judging by the number thrt camr out of wangas I guess there are plenty up north too.

Grumph before he moved on mentioned a way to do the Ignition pretty cheaply.

petronix