Tuesday, September 14, 2010

Project 1977 TY 250 Twinshock Yamaha Bike Modifications

My TY 250 Yamaha trails bike has had many modifications and is a ongoing project.
In this blog I will share all my knowledge and info with you.

The engine has been bored out to 72mm and fitted with a Yamaha DT 250 piston which gave it a bit more CC, approximately 264 CC.  This alteration is commonly known as the 270 conversion. You will have to take 2 or 3 mm from the bottom of the piston skirt as it may hit the top of the big end. The DT piston top ring is flush with the top of the piston, this helps in increasing the compression as the standard TY piston rings are approximately 3mm lower down the piston skirt.  The engine fly wheel has also been lightened slightly by putting it in a lathe and taking a few millimetres off outside of the fly wheel.  The engine has also been fitted with after market Boyson reed valve.  These modifications coupled with the fitting of a new points free electronic system makes the engine feel a lot more lively with plenty of low end torque.  I also have a bog standard TY 250 in fairly good condition and there is a big difference now between the 2 bikes' performances. I would also recommend that you try to obtain the original Hepolite Yamaha DT 250 pistons where possible. I found the best quality pistons at www.classic-motorcycles.co.uk

My exhaust system is a stainless steel front pipe from Sammy Miller and the rest of the silencers are all alloy and also from Sammy Miller.  This exhaust system is much lighter than the original TY. www.sammymillerproducts.co.uk

The original TY 250 front forks have been removed and replaced with the newer TY250 monoshock forks as they are a better fork and give the bike more fork travel and a very slightly longer wheel base.  This makes the bike's handling, in my opinion, better.

To fit these TY mono forks you will have to have the original TY 250 twinshock fork yokes honed out slightly.  I also had my forks re-chromed and fitted with new seals.  These forks do feel good and work very well.

The front wheel is also from the TY monoshock bike and is fitted with a set of rare Z spokes. The front wheel brake plate is from a Yamaha IT 250 and has been machined slightly to fit the wheel and forks.  I prefer the IT brakes because there is more leverage in the brake and this gives me a good keen and light front brake.

I had the rear wheel re-built using stainless steel double butted spokes and nipples.  Using the original TY twinshock rear hub and  new gold anodised alloy rim.  The wheel was re-built by Juan Knight (the brother of the (world enduro champion David Knight).  My front and back brake shoes are water grooved for good braking in the wet.

Rear suspension (rockshocks) I had specially made by John Bull.  They were made to take into account the weight of the bike and rider.  They are also lighter than the standard shocks as they are made from quality aluminium.  They feel good and look great. www.rockshocks.co.uk

My frame is a standard TY250 twinshock but with alterations to make it very close to the more expensive majesty frames.  Basically, you need to alter the position of the rear shocks and foot pegs.  To alter the position of the rear shocks you will need to weld a new suspension mount about one and three quarter inches down from the original top suspension mount.  This puts the shocker in a more laid down position.  The modification for the foot pegs is much easier to do as I bought a kit from Sammy Miller and it was quite straightforward to fit.  This puts the foot pegs slightly further back and lower on the frame. 

The original TY kickstart was quite worn and kept swinging around and denting my exhaust so I replaced it with a modern trials/enduro light alloy kickstart that Juan had lying around his workshop.  We are not sure what bike it is from but the kick start did fit easily onto the splines.  However, there was no  way of securing it to the shaft as there is no bolt to tighten it up so we drilled and tapped through the bottom of the kickstart and then screwed a grub screw into the groove on the kickstart shaft.

My handlbar set up is quite a bit different from standard.  I did this modification as I am quite tall (6'3") and TY trials bikes are small.  Moving the foot pegs did help with this a bit but the bars were where I could make the most difference.  To gain the height I wanted I fitted a set of S3 bar clamp extensions that were designed to take modern tapered bars.  The bars are contour pro-tapered gold anodised.  I bought the highest bar they produce.  I really like these bars and the bike now feels just right for  my height.

Brake a clutch levers are a special design.  To enable you to get the feel you want there are 3 different hole positions on the levers and they are also made from a good quality light alloy.  I bought the levers from TYTrials (www.tytrials.co.uk).

There are now many new re-manufactured parts available for the Yamaha TY250 as there are still plenty of these trials bikes around.  Other bits I have fitted are lightweight alloy wheel spacers, snail cams, rear wheel spindle and nut, steerer head bolt and many other nuts and bolts.  I have not had the bike weighed yet but I think I may have saved over 12lbs in weight over a standard TY.
I found this really cool classic trials t shirt on the web. I love wearing it, It is made from  a heavy cotton and I have washed it a few times and it has no shrunk or fad d.

  I used my TY all last year in the Isle of Man Vintage Motorcycle Club trials and ended up winning the clubman 2010 Trials championship trophy. Here I am been presented with the trophy.


  1. Love your bike! I have a '76 TY175 and have all sorts of modifications I plan on doing. Your handle bar extension idea is interesting (to get the handlebars up higher). I've thought of doing something similar and now I have some reference pictures!


  2. hello Mike

    i'm looking for ty 250 forks (twinshocks) do you still have the ones comming from the ty ? would you sell them ?
    contact me bob.ryckaert(@)hotmail.com

  3. when you mover your upper shock mount how long of shocks did you use?
    Did it change the steering angle? Does the bike turn any better?

    Thanks, you have a great looking bike!

    1. I moved mine and use 14" shocks. I'm not so sure that moving the mounts did so much but using a longer shock helped. If you do add travel to the rear you will also need to lengthen the kickstand.

  4. Hi what modification did you do to the IT250 brake plate to get it to fit the mono wheel?

  5. Beautiful bike.! Great quality work.