Im looking into buying a spare engine from a XT660Z and doing a rebuild of the engine and while the engine is apart upgrade it for some extra hp.

Ive seen that some people say that the stock barrel can be bored to 103mm but not more, but at the same time there are 104mm pistons on the market. Is it possible to bore and replate a stock cylinder to 104mm?

If you would like to do a proper build on a XT 660 engine, what would the modifications be?

Im thinking of bore and replating to 104mm piston, ported cylinder head, OTR cam, exhaust with better flow and improved airbox. What else is there that gives a bit more punch?

If i would do the things listed above, how would it affect the durability of the engine? How would the engine run compared to a 690? Would it just be a waste of money since im better off with a 690 from the begining?

Been there done that, any engine performance modification will affect the long term reliability of a motor. The cylinder is to thin at 104mm & you may brake the cylinder skirt off with a 104mm.

Bore the casing & go as big as you want right up to 108mm.

A 105.5 bore with +2mm valves, 5050 cam stage 3 air filter will see around 76 rear wheel HP.

You can never compare the XT to a 690 KTM they are two different bikes, I am yet to see an XT anywhere the same weight of the KTM, we know we have the capability's of getting these motors up 125 rear HP as done on the 700 Raptors .

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Mods that I have done to my XTX's

My 04 XTX
Stage 1 & 2 DNA filters / Kev fuel mod / snorkel mod / Drilled air box mod / Engine breather mod / Fork Spacer mod / 15w fork oil /
[
My 07 XTX
Raptor 700 cylinder 102mm / Modified Crank Case to take a 105mm Raptor Cylinder / 11:1 102mm JE Raptor piston / Stage 1 Raptor Hot Cam / Ported head / Colder Spark Plug / +2mm Throttle body / DNA stage 1 & 2 filters / Modified Air Box / Snorkel removed / Worry Brothers stage 2 filter cover / 6mm & 8mm Bolts Replaced with Titanium Bolts / Recovered seat in Black / O2 Eliminator /

My 09 XTX
59HP at the rear wheel / Stage 2 Raptor Hot Cam / DNA stage 3 Air Box / Carbon Can Exhausts / Modified Exhaust Link Pipes / PCIII With Custom Fuel Map / Wideband Commander O2 Data logger / LCD100 Dyno Jet display & fuel adjuster / 2500 OHM HT lead instead of a 10K OHM XT lead / Extra Coolant Cooler / Protaper Fat Bars / Tail Tidy With LED Tail Light / OKE Protection Knobs / LED indicators / AIS blocked / Modified rear sprocket rubbers / Rear Foot Pegs removed / 09 ECU With 02 Sensor Changed To A 06 ECU / Complete wiring harnes Changed from 09 to 04 / Home made LED resistor flasher / Hole drilled in the fuel tank filler neck to allow quicker filling / Modified Bar End Weights / Progressive front fork springs, 15W fork oil, forks lowered 25mm, used XTR rear links lowering the rear 20mm, rear spring one click stiffer / Changed front & rear guards from blue to black / 47T rear sprocket / Speedo Healer V4.0 / Kev throttle cam mod / Throttle grip mod / The new 2010 O2 sensor mod is out, works a treat, The new 2012 O2 Controller is out, PM me for details

Now ride a 2018 MT09SP ABS + TCS

My KTM 990 SMR Mod book. http://www.ktmsmt.com/viewtopic.php?f=13&t=4946

Well that it would affect long term reliability i fully understand. But my question would be more if i do those mods will i have to do a full rebuild every 10h or would i still be able to go for longer runs as the XT was built for.

Hum okey i guess im better off just buying a 690 then..

How long is a piece of string, both questions very hard to answer, I have seen a big bore XT's only do 1000kms before destroying the plating on the bore due to poor plating, I have seen big bore seize the small end pins at 3000kms, I have seen big bores do 7000kms then get piston slap, I have seen a big bore destroy its clutch within 200kms, I have seen big bores do over 10000kms with no problems, it all depends on what parts you use & the conditions the bike is ridden under, cheap parts don't last, longer piston skirts last longer, as you can see very hard to say how long yours would last.

All the above cases I have documented over time.

__________________
Click here to join the XT Supporter's scheme

Mods that I have done to my XTX's

My 04 XTX
Stage 1 & 2 DNA filters / Kev fuel mod / snorkel mod / Drilled air box mod / Engine breather mod / Fork Spacer mod / 15w fork oil /
[
My 07 XTX
Raptor 700 cylinder 102mm / Modified Crank Case to take a 105mm Raptor Cylinder / 11:1 102mm JE Raptor piston / Stage 1 Raptor Hot Cam / Ported head / Colder Spark Plug / +2mm Throttle body / DNA stage 1 & 2 filters / Modified Air Box / Snorkel removed / Worry Brothers stage 2 filter cover / 6mm & 8mm Bolts Replaced with Titanium Bolts / Recovered seat in Black / O2 Eliminator /

My 09 XTX
59HP at the rear wheel / Stage 2 Raptor Hot Cam / DNA stage 3 Air Box / Carbon Can Exhausts / Modified Exhaust Link Pipes / PCIII With Custom Fuel Map / Wideband Commander O2 Data logger / LCD100 Dyno Jet display & fuel adjuster / 2500 OHM HT lead instead of a 10K OHM XT lead / Extra Coolant Cooler / Protaper Fat Bars / Tail Tidy With LED Tail Light / OKE Protection Knobs / LED indicators / AIS blocked / Modified rear sprocket rubbers / Rear Foot Pegs removed / 09 ECU With 02 Sensor Changed To A 06 ECU / Complete wiring harnes Changed from 09 to 04 / Home made LED resistor flasher / Hole drilled in the fuel tank filler neck to allow quicker filling / Modified Bar End Weights / Progressive front fork springs, 15W fork oil, forks lowered 25mm, used XTR rear links lowering the rear 20mm, rear spring one click stiffer / Changed front & rear guards from blue to black / 47T rear sprocket / Speedo Healer V4.0 / Kev throttle cam mod / Throttle grip mod / The new 2010 O2 sensor mod is out, works a treat, The new 2012 O2 Controller is out, PM me for details

Now ride a 2018 MT09SP ABS + TCS

My KTM 990 SMR Mod book. http://www.ktmsmt.com/viewtopic.php?f=13&t=4946

Well I've agonised over buying a 690SMCR,more power and much lighter,the pro's and con's go on forever,I've got no problem with price and there's a steady stream of secondhand ones with all the bells and whistles.

Parts ain't cheap,secondhand spares are virtually nil so it's the expensive route at a dealer,assuming you have one local,but the deal breaker for me was the seating position,I could suppress every other doubt I had and that was quite a few,but the seating position!!!! I just couldn't get on with,if you do then it's worth a try,but I found them just impossible,which is a shame because they look the great.

Ive had a 690 Enduro before and loved the engine.

Actually here in Sweden i can get a hold of KTM parts for about half the price of Yamaha spareparts. So for me it would be cheaper owning a KTM in the long run if i want to buy some Yamaha/KTM spare parts.

Hello, I was looking at Boring my engine out in a few months as I have acquired a house with a garage. you say the Cylinder is too thing at 104mm, but then talk about a 105.5mm bore ? is 104mm their safe "limit" ?

As I would love to go around 105mm

Kind regards - Brutank