Big Bores for 70 degrees

I have kept a lazy eye on big bores for 70 degrees for a few years now, more or less waiting for something bad to happen to my cylinder "forcing" me to make it big bore. That never happened but my friend ****** up his engine royally and in the end my cylinder ended up on his "new" engine. So as I have his ****** cylinder and a motard without working engine I am finally forced to do something. I will go easy and make it a 590.

One trend I noticed over the years is how the big bore kits have become more difficult to find. The athena 590 kit disappeared 1-2 years ago and now I see that thumperracing doesn't offer anything for the 570 anymore. You can find the same kit but for KTM so the parts are still there, luckilly the engines are more or less the same as KTM is big as a brand.

I have found one way to make a 590 and that is to buy the 520cc big bore kit for the KTM 450. That uses a 102mm piston. It seems one need a KTM 530 conrod to make it work in a 570. So I have ordered a piston like that and I will order a 530 conrod. I was thinking of buying a 610 kit and put it on the shelf for the future, but it feels a bit to extravagant to do it.. So the parts are still out there but not as a convenient "click to order for your bike".

Does anyone know where I can find a 102mm top gasket that will fit? Is this what I'm looking for? http://thumperracing.net/index.php?route=product/product&path=49_51&product_id=282

Yes you are right, I will send them an email ask.
I'm thinking of putting some money aside every month until I can afford that lovelly 660 kit...

I have an extra 450 cam I will send to TR to get their profile. The question spinning in my head is if I am to spend money on 590 but instead put that money on bigger TR stuff.... :/

And Alan, I am a good friend because he is a good friend. I wouldnt do that to anyone

If you search this forum for 70 degree cams it seems lika TR is the only true option, but they dont give the numbers. As I have an extra cam it is worth to test and honestly I'm too much of a grasshopper to use the numbers in a proper way. I understand how cam timings work but there are too many variables and I lack the tools and empirical knowledge.

But.. I really need to make a DIY dyno to properly test and tune all things. And I just dont have time for all of this right now.... Maybe tune the ecu from lambda readings, laptop in backpack while doing *** dyno..

berglsmerg said:

I found that with 610+44mm TB that I had a big gap in the mid range and it dropped off up top, the TR cam filled it all in and restored the top end with a slight loss of bottom end response. I tried and failed to measure the cam specs with the motor in the frame but I can tell you there is more lift, duration and a narrower lsa. When I get around to rebuilding the motor I'll take the specs properly this time.

Jon I did the laptop in the bag thing + AFR, and got pretty close, the final tune on the dyno ( a week before it exploded) got me 2 hp from about 7000 onwards and smoothed the bottom to mid transition. Go the 610 Jon, you won't be sorry, just say **** it and do it !

Click to expand...

Well.. The dollar is weak, and so am I :unsure:

After going through some economical soul searching I had a look into one "forgotten" bank account and found the money I need, so I'm going 660cc

I did order a 102mm piston as my humble plan was to save the ruined cylinder and go 590cc. **** humble, that piston is on the shelf for future projects.

The thumperkit is ordered, just waiting for a reply from them. Work is already started; my garage is organized and cleaned, the engine split and some Husaberg stickers applied all over for the Swedish husa-God.

I will use this thread for the build. Probably ask after peoples input now and then and share pictures. I have a few questions already for this build, one about a suter slipper clutch that has been on the shelf for quite some time and another about lambda sensor placement on my doma. I'll get back tomorrow regarding that as it's bedtime here now.

While waiting for a reply from thumperracing I had a look at the suter clutch that I bought with the bike but has been in the box. The former owner bought it used but couldnt get it to work as it was slipping while benching the bike (had closer to 70 hp on the back wheel he told me), so he removed it and ordered a 1500N spring.
I seriously doubt a bike like this need a 1500N spring as they are for R1s and similar strong bikes, but what do I know.. It can be that the clutch is worn out?

I had a look and the bottom plate are for sure worn from the inner friction plate. I checked it with a 0.2mm feeler gauge but luckilly it seems like it is less than 0.2. The pressure plate on top looks used but good. So it should be alright (I hope..). The holes for the ball bearings do look very used, what do you guys think about them?


I managed to find plates that barelly made the stack into spec; 35.75mm. It should be within 35.35mm -0.2/+0.4. They emphasize the importance of that in the manual.


This is the clutch in all its glory! All parts there and all look good. Now it is back in the box until the engine is done.



If anyone with experience can give me a tip what springs to use? I could see that the small 950N and the big 1235N had been used. But I have a few more to choose from:
Small for slipper effect
750N
850N
950N
1500N
Big for clutch pressure
1235N
1310N
1400N
1600N

I guess I have to play around with it later on when the bike is up running (whenever that is).

Yep. But as the former owner had trouble with it slipping and ended up buying the strongest one I found it a bit strange. It is a quick change anyway so I guess I go strongest first and then change it step by step down until it starts to slip and then back one up. Talking about accelerating just to be clear.