big big bore

robbier6 · Post by **robbier6** » Wed Oct 05, 2011 1:16 pm

Hello again,

just a quick question, does any body know of a company that would do a big bore i mean 400cc plus conversion for the kr1s, i know the 300cc is available but would prefer alot more.

Thanks

scooble · Post by **scooble** » Wed Oct 05, 2011 1:36 pm

300cc is about all your going to get with modified cylinders and standard crank - and even that is a bit unreliable.
I worked with stan stephens some years ago on a 350version, but it didn't stay together long enough to make it viable.

I believe falconman is experimenting with a KX125 cylinder configuration though which may be the way to go (if he gets it finished and working)

ScottaKR · Post by **ScottaKR** » Wed Oct 05, 2011 1:45 pm

Hi Robbie.
In all honesty, even the 300 conversion leaves the bores a little weak between the kips and exhaust ports, and I really don't think theres any way the stock barrels could cope with such an overbore. I think the only way you could do it is if you used barrels of a bigger bike ( KX___ ones perhaps), but there's a heck of a lot of work involved in that. If you did the 300 barrels and a stroked crank you might get a bit closer to what you want, but then your talking massive amounts of work (have a look at falconman's project to see what I mean).

robbier6 · Post by **robbier6** » Wed Oct 05, 2011 3:22 pm

best i forget it it then, and start saving for the TSS 500 conversion for the rs250

falconman · Post by **falconman** » Thu Oct 06, 2011 1:27 am

The main problem I am having is that since the bike was never sold here the people that make cranks don't want to bother with it as they can't sell enough to make it worth while. Cost would be around 350 to 400 quid for a new crank (with 54mm stroke) if they could sell about 50 of them. So I'm stuck with having to modify something to fit. Currently I live 400 miles from my shop so progress is slow lately.

robbier6 · Post by **robbier6** » Fri Oct 14, 2011 12:17 pm

It's frustrating, would like to find a big bore conversion rather than dispose of the bike, guess i shall keep it as a 250 if the 300 is unreliable too.

Luders · Post by **Luders** » Fri Oct 14, 2011 12:55 pm

The 300 conversion Stan Stephens did was not that good anyway and power gain from it was disapointing to say the least.

You're much better off tuning and developing what you have as a 250, you'll get more enjoyment and satisfaction out of doing it that way round.

dave32 · Post by **dave32** » Fri Oct 14, 2011 1:55 pm

That might have had more to do with stan than the actual idea,i had a 570 big bore for my rg5 the cylinder head was left the same size as the 500 along with the head gasket,result major detonation(the gasket overhung the bore by 1.5mm on all sides, when i questioned him on this he said it was fine?!.
In my mind it was poor workmanship and just a half arse job.
I got a local engineer to mod the heads and gasket to suit and bore it out(after the resulting 570 let go due to the head) to 585,since then its been fine for countless trackday and about 10,000 road miles.
Many years ago i got to ride a friends kr1 that was taken out to 400cc by Taffspeed,that was far from dissapointing.
Unfortunately ive no idea what was involved in this,but it would be good if it could be reproduced.
Even the stan rgv300's are dissapointing so it says something.
I agree with luders though,your much better developing a 250 as it is,50cc is neither here nor there.