Could I have my zr 800 cylinders bored to 900 specs and use 900 pistons and ecu? Is there more to it than this?? Seems cheaper than buying 900 cylinders...

I'm not much of a motor guys so let me know if I'm missing something here..

Another thought is; would the zr 800 power valves still work?

Wiseco sells an 880 big bore for that sled i think. You just recut your valves to the new bore. Many options they can go as far as a 925. As far as i know the 900 pistons will work,need to recut the heads. Get the porting done also.

The ports are different, the most cost effective switch is if you can sell your 800 cylinders and buy 900 cylinders. You could have your 800 cylinders bored to a 900 but would not produce the same power as a 900. So get some 900 cylinders and have them ported. Or if you have the cash do Bikemans 800 to 900 kit that does porting and all that you need for like 1700 bucks. The 800 valves would have to be ground for the extra clearance that is needed. Which is included in the bikeman package as well as matching your heads to 900 specs.... Some have used their 800 ecu and not have problems, but essentially the 900 ecu would be a good idea. It would be interesting to know the actual fuel delivery difference. I bet its not much different.

900 cylinders with you're 800 heads. You'll LOVE it

These guys are right. Paying someone who knows what they are doing to bore and then port your 800 to 862 (900) specs with the required head and powervalve work will definately cost more than just buying a used 900 topend. Essentially the same cost as a BMP bigbore kit. If your carbed, your ecu, carbs and pipe will all work great. (I'd look for a cheap 900 pipe though).

If you're carbed? I'm EFI and i'm running 900 cylinders on my 800, with my stock program

I'm efi as well - you just bolted on 900 cylinders with 800 heads and it worked well??

Best mod i've ever done to my sled! My sled ran pig rich before the 900 cylinders and now my plugs are minty brown. All you gotta do is lathe out the 800 heads to the 900 bore and change you're powervalves. THE 800 heads are sweet cause it gave my sled alot more compression and gives you the sleeper look. If your sled runs dirty rich like my 800 did then you're good to go. If not you may want to run a boondocker or TFI control box so you don't blow up

I may be able to hook you up with the necessary parts. Should also have a set of these pipes built.

Built? Why wouldn't you just but a set of 900 pipes?

Because these work really well.