Maximum safe size for boring out 903cc. barrels

[Lee McEvoy]

Thinking of putting a few more cc's but want to keep it looking stock 903 on the outside? What s the maximum safe size to go for and is high compression worth it as well? Thanks

[zed1015]

You can go to 1045cc ( 71mm ) but it leaves the liners a little thin the same as pushing the 1015cc barrels to 1105.
1015cc (70mm) is the max I go to and a Wiseco 10.25-1 piston kit will do the job nicely.

[Lee McEvoy]

Out of curiosity, can the thin liners if I took it out to 1045 be managed? Or is it just not worth it. My aim is for an every day rideable bike?

[zpower]

You could also buy a big bore kit with sleeves and pistons.

1170, 1197

[Pigford]

You could also buy a big bore kit with sleeves and pistons.

1170, 1197

The norm for that route is a 1260 - but they're trouble

[zed1015]

Out of curiosity, can the thin liners if I took it out to 1045 be managed? Or is it just not worth it. My aim is for an every day rideable bike?

Wouldn't bother myself, there's no real power advantage to be had for an extra 30cc.
Also there's a good risk of the liner spigot (the bit that protrudes below the barrel casting) cracking as that portion is thinner than the rest of the liner and if the borer goes even slightly off centre it can get wafer thin.

[zpower]

1260 = thin liners allowing the sleeves to warp, crack etc...

1170 or 1197 kits leave more metal.

My 1197 liners are 2.5mm thick and standard 1015 liners are 2.8mm.

[Lee McEvoy]

Sorry for ignorance, but can those big bore and sleeves go into a set of 903 barrels? Am I not then into having to alter cases as well?

[neilbarrett]

id go for a 998cc kit ,the old f1 limit,

[ADRIAN H]

Sorry for ignorance, but can those big bore and sleeves go into a set of 903 barrels? Am I not then into having to alter cases as well?

Big bore kits complete with liners, would mean the crankcases would have to be machined out - to allow for the extra diameter of the liners. Talk to ray at Debben Performance in Ringwood Hants, (as he is relatively local to you) - for more info.

[jimmock]

This is the kinda thread that makes me want to go and thrash myself into a coma!!!

[ADRIAN H]

This is the kinda thread that makes me want to go and thrash myself into a coma!!!

Go for it mate.

[zed1015]

Sorry for ignorance, but can those big bore and sleeves go into a set of 903 barrels? Am I not then into having to alter cases as well?

Why has a simple straight forward re-bore question turned into a full on engine strip ???

Just bore it out to 70mm if you want a nice quick motor with no hassles.
You will feel the difference with a set of 10.25 comp Wiseco's and up a size or three on the main jet.

Anything involving bigger liners (even 1015 liners in some cases) will required the crankcase mouths opening up.

[zpower]

Sorry guys it's my fault this thread has delved into the intricacies of installing a big bore kit

It was just a suggestion

[Lee McEvoy]

Gents thanks to every one for exposing all the possibilities to increase the capacity. Really want to keep it looking as stock as poss so going to look at the costs of the 1015 option with the higher compression pistons. Suppose I had better start saving!