Tomas Karlsson's Yamaha Z608 replica

Published Date

Tomas Karlsson from Sweden became so interested in the stories about the early factory Z608 350 cc racers that he decided to build one himself.

During last Winter he accumulated all the required parts and started building his Z608 "Replica".

I have asked Tomas to come up with some more details. These are the first 3 pictures he has sent.


A lot of detailed work has gone into this project, i am very curious about the details.


The Yamaha YR series engines were 9are) very strong. Interesting to know what Tomas did to the motor (and the rest of course!)


Hello again Mr Beumer !!

Hopefully I will race my Z608 replica next month in Linköping ,Sweden, at our greatest classic race.

I am in the process of  applying for a machine-registration as a applicable classic racer and getting it through inspection.

The exhaust pipes in the first pictures are not the real job, just borrowed from my TD1c to get some pictures for the application.

Here is some more information for the "" readers:


I´m working like mad now to get the bike ready, I have just made the proper exhausts and i am ready now tois up rebuild the crankshaft.

I have lots of pictures and I can send more complete description later when first race is done of course.

tomas karlsson yz608-1

This is a "start picture" of all the parts I began with...........!


tomas karlsson yz608 frame

Among other things a YR1 frame from -1967 from which I cut off the top half and put a new steering head on, a little lower and  further back.


tomas karlsson yz608 new frame

Then new top loop tubes and a rear frame, again a little lower and narrower!.

The swing arm tubes comes from a Husqvarna motocrossbike.


tomas karlsson yz608 frame from above

The tubes were welded in place with the old "inside" of the tubes turned to the outside, That way the kink in the tubes comes just when the arm needs to be wide,

by the rear wheel sprocket and brake!




More to come !

Yours Tomas Karlsson

Tomas continues telling us more about the engineering of his very special bike:

Hello again!

Some more about building my Yamaha Z608 replica.


bridge bulkhead

The swing arm centre piece, the bridge between the legs was made of a large diameter thin wall tube with a "bulkhead” welded in place.


Then the ends of this tube were pressed to the same height as the swing arm tubes, 40mm. I hope this will give great torsional stiffness to the swing arm.


front engine mountings

I heard from racers that rode these first Yamaha 350cc racers that they vibrated badly and that they gained some revs when the engine bolts was not absolutely tightened, I decided to suspend the engine in rubber bushings and used RD400 bushings at the front and some BMW auto parts at the rear.

rear engine box w rubber elements


rear box w folded top mount

The rear had to be built like a box and suspended around the swing arm shaft to get a clean chain line.

The frame tubes surrounding the engine was rather tight placed so in order to get the engine out and in with cylinder and heads on, the rear top mount had to be foldable, Tricky to fabricate but good for maintenance!

All for now and soon comes more about building the fuel tank!

Hello again folks!

This last weekend (first weekend of August) I had my first race on the new bike. It was a very close call to get there at all because I only had time to run the bike a few days before we had to leave for the meeting. Fortunately it ran pretty well and I had the chance to make some adjustments before the first practice and not waste track time on basic things as bars, pedals and control positions.

From the first few bends in practice for the Linköping Classic race I could feel that this will be a good "roadholder". It felt firm and steady as a wooden plank, not at all jumping and  bouncing with the front wheel as my old TD1C.

Practice went well and I was able to get the gearing and jetting OK with pole position by 2/10 of a second on the last lap.

There were two categories of bikes in my heat, classic 350cc and forgotten era 250cc( mainly TZ250’s up to 1979) so that I was more than pleased with!


tomas karlsson's yamaha racer

In the first race on Saturday I got away first but braked too early in to first bend and tangled with a TZ and got out in fourth position. The couple of TZ’s in front went away and there was nothing I could do about them, but I got back in font of a Morbidelli 125 at midrace and that was how it ended. Me in third overall and 9 seconds ahead of the next 350cc.


Sunday race was on part damp track and I changed the gearing the only way I could, talller gearing to use one gear lower at each corner because I was out of sprockets to make overall gear ratio lower for the damp track.

tomas karlsson 's yamaha racer

Not so good start this time and I kept missing gears out of corners and a bunch of 350cc came by. I climbed back to fourth in 350cc before the end and was breathing down the neck of the third placed Honda.

Next race will be an international classic race in Denmark on 18-19 September and until then I have to look into getting more damping in the forks and some bigger rear sprockets!

All for now!

Yours Tomas Karlsson


Oct. 14, 2010

Hello Ludy!

Do not think I have given up on delivering material, for the website It´s just that I have difficult to export pictures from my mobilephone to the computor. And the next thing i wanted to tell about was the tank and seat but I miss certain pictures!

It´s coming , I have ordered a new transfer cable.


I did the race in Denmark 18-19 September as i told you guys before but I could as well have stayed home.......

I crashed on the warm-up lap for the first race by a stupid mistake . I repaired the damages but on the way out to the second race the gear pedal broke and that was it!

(It is a pity, but with Tomas' engineering skills i am quite sure that the bike will be back to its glory within a few weeks, "shit happens", we all do know that in this hobby!)

Good luck Tomas-San!

yamaha yz608 replica

The weekend after I at least got some racing done. Together with two other friends I raced in a Scandinavian championship 8-hour endurance race.

The bike was a Yamaha R1 but there was no training, so it was into the race directly on a bike I have never ridden before!

A bit scary but fun. We ended as 22 of 28 finishing tems ( 36 teams started).

Back soon!



Yes I have some pictures of our Endurance race:

per larsson

The one with me on it can be used by Classicyams as long as the adress in the lower rigth corner is visible  (professional photographer).

The other picture shows Rider/ owner Robert Eliasson and my buddy Matthew Winstanley iin leathers shaking hand. THird person is our valuable mechanic Filip Åxman.

yamaha r1 2007

The race was in Anderstorp as I mentioned before and it was the last round in a Scandinavian series called ERF, "Endurance For Fun".

Robert Eliasson has done three rounds earlier this year with his ordinary race partner.

The bike is a Yamaha R1 2007 model,  basically stock with even standard exhaust.s The bike ran well execept having a slipping cluch for the three last hours.

Bye, Tomas

Christmas 2010

Tomas Karlsson continues refining his Yamaha YZ608 replica racer.

I have had a planning period in the workshop since the last race. For 2011 I will make a new front wheel with a bigger front brake. The front end I have been using 2010 is actually borrowed from my Yamaha TD1C replica.

tomaskarlsson yz608 front wheel drawing

That brake started life as a Suzuki GT750 4LS brake but I was having problem with front end chatter earlier seasons on the TD1 so I decided to reduce unsprung weight up front. The drum got machined in a lathe and on the mill. Basically I machined the entire rough surface from casting on the drum plus removed the weather seal lip on the edge of the drum. That way nearly 1/2kilo came off!


Then it was a decision to make, whether to modify the anchor plates or to make new ones! I chose to make new ones in a different design! Starting off with drawings and two billets of 7075 T5 alloy I machined the centrepiece of each anchor plate to house all the essential details. By choosing an unusual angle I could get away with using the anchor post as mount for the brake stay. This way the cables could enter the mechanism from above and thus be quite short!


Cover plates was made of 2mm plates attached by 5mm screws to the centrepiece and to titanium rods sticking out front and rear of each centrepiece, Secondary task for these rods was to support the brake shoes sideways.

Finally I made brake stays in 3.6 mm titanium plate and were able to combine the attachment to the fork leg with holder for the cable.  Brake Mechanism was retained from original plates but revised to suit the new angles.


The new brake will use a 260mm TR /TZ drum and more or less get the same treatment as the Suzuki drum. I will however try to make air vents that reminds of the ones used on the "real" Z608.

April 2011

I carry on with the story about building my Z608 replica.


Hopefully I’m a bit more organized now and has named the pictures in the order that I tell about them!


I have not told this earlier but my aim besides building a beautiful race bike was to use very little money on the project.

By choosing and designing to use second-hand parts, which are available at low cost I can stay away from all the expensive TZ parts and Italian replica fabrications.

In terms of cash for parts to get the bike up and running I estimate I have spent 2500 Euro. But then a lot of the small stuff like control-wires, nuts & bolts and wiring + instruments I already had since before.

And another big contribution to keep the cost down is that I build loads of the parts myself.

Like Discovery Channel´s AMERICAN CHOPPER but in magnesium, chrome-molybden steel and titanium!

z608 tank 1

1. Since the frame was modified with two top tubes there as no chance any other tank would fit, so I better make my own. I very much think that a neat tank with deep knee cut-outs can make wonders for the look of the bike. First step was to take the dimension from my drawings of the bike and make a wooden tank dummy or a wooden mould so I could form the sheet metal. I choose 1.5 mm 6061 aluminium alloy. I set out to make the whole of the topside in one piece, but with cuts and welding in it of course.

z608 tank2

2  Then it was time for some serious panel beating. The trick is to control and strap down the parts you have already formed to stop them lifting from the wooden mould. And almost all the forming and beating is done with wooden hammers.

z608 tank3

3 Once the sides are done the rear can be folded in. Each cut is taken bit by bit not to "run out of material" anywhere. I dared to tack- weld with the wood in place but if you try it, turn off the smoke detectors in your workshop!

z608 tank4


z608 tank5

4 , 5   A tank bottom is made with strengthening webs rolled in the material and two tunnels for the top tubes that hugs the steering head. The fuel tap is placed in a "catch tank" that fits between the tubes that extend towards the seat area. The tap is a second-hand for 8 euro from a early RD350 and the rubber dampers on the frame comes from the Swedish army, it used to be rifle muzzle protectors, cost zero!

z608 tank6

6 Marriage between top and bottom, now it shows if you were meticulous with dimensions and cutting.

z608 tank7

7  The front of the tank is cut and folded in and meets with the bottom part. The corners is cut in a sun feather pattern and welded. It gets to be a lot of welding! And grinding! During the whole build the angle grinder has been the absolute greatest asset.

z608 tank8

8 Next up is the filler which I bought on Ebay for 8 USdollars + 5 dollars in shipping. It´s a aftermarket bolt-on filler for Suzuki sv650 from Asia somewhere. I extended the hole to full size and added two more fasteners on the collar.

z608 tank9

9  I made a tool to press a dimple so the filler could be flush with the tank top and that was it!

z608 tank10

10  On the inside there had to be a ring of aluminium for the bolt-threads holding the filler and a fitting for the vent of the tank. I extended it with a small diameter aluminium tube to the upper rear of the tank so the fuel doesn’t get washed out when braking.

Now it’s only to weld the top and bottom panels with a steady hand and add attachment points front and rear.