Me personally I would have it bore to the smallest overbore size. Or next available size piston. Since you are not gain much in performance I would want to take the least amount out of the cylinder wall as I could in order to preserve the cylinder as long as possible. With ATV’s parts become unobtainium too quickly as it is. Better to have some bore left available instead of possibly needing to find a cylinder that could very easily become hard to find.

 

Thank you for the advice. Do you know or does anyone else know whether Rev6 make good quality parts?

 

Hello. First post on Polaris ATV Forum.

I have recently picked up a 2009 850 XP. It is in pretty good shape but has a cracked piston. Sorry for the terrible picture. View attachment 193087

So I am looking for advice on rebuild kit. I could get the Re6 Refresh kit (+.5 over bore). There are also kits from JE that allow a 6mm over bore and one that is even larger. Namura makes 1.5 mm over bore piston kit.

Couple of questions I hope someone can help with. My 850 has the iron/cast sleeve so it can be bored safely. Quetion - how much can/should I over bore to keep a reliable engine. I am quite sure I will require boring but I won't have then engine out until tomorrow.

Also, anyone have any experience with Rev6? I am a long time motorcycle guy but new to ATVs. I have no experience with Rev6. I like the idea of a refresh kit so all seals, bearings etc also replaced.

Any thoughts? So thankful for any help!

Welcome to the forum

 

Thank you. Much appreciated.

 

J&E pistons like Wiseco are forged - forged pistons are stronger and will handle higher rpms, but require more clearance and longer warm-ups than cast pistons like Polaris and Namura. Thus the engine makes a bit more noise during warm up, but will handle higher rpms than a cast piston. J&E (like Wiseco) use thinner rings for less friction and faster acceleration, but wear out faster and need more frequent replacement (it's a cost of racing) - Namura pistons are cast (same as Polaris pistons), run tighter tolerances and the rings last longer. In my experience it makes no difference how much you bore the engine for renewal - with the biggest bore available, you will gain a bit of low end torque, bit will lose a few hundred rpm at the top of the rpm range especially if you bump up the compression. Bore it to whatever oversize you desire or can afford. The engine will run the same on a .010" overbore as it will with a .050" bore. Part of the decision is what is available at the time you want it. Go overboard and bore more than 2mm oversize and you will get into needing a special head gasket - any size under 1mm larger than stock is considered stock.

Personally I have never heard of REV6, but there are a lot of companies I have never heard of who have been in business for years.

 

Thank you. That was super helpful. I am not keen to tear this apart again so perhaps it would be better to go with Namura. I'll have to see what the cylinders look like first. I'll update once I know.

 

I wonder, does anyone know the best place to find a service manual for my 09850 xp?

 

Check your email

 

Well, I was able to remove the engine and get it completely disassembled. What a mess.

Somehow, I am guessing the engine timing changed and the rear piston appears to have hit the spark plug, split into pieces and fell into the rotating lower end. The result is a piston in a million pieces. The con rod for the affected piston rotated into the bottom of the cylinder wall and is now bent. Otherwise the rest of the engine looks fine. I doubt it is but it looks okay.

The crank seems unaffected. The bearings etc seem unaffected. The balancing shafts also seem okay. The previous owner reported that he didn't try to run the engine or even turn it over once he heard the bang. The lower case is okay with no damage but obviously the upper case/cylinders are toast.

I'll post pictures later once I get them from my phone to my pc. It is an epic mess in there.

 

I suggest buying a an nFLOW remanufactured engine - I doubt you engine would even be considered a 'core'.

 

I have already sourced a cylinder head/upper case. I would rather buy a rebuild kit and build myself. I love mechanical work and i have lots of time.

The reman engines are really expensive. I can buy a complete kit for much less.

 

Good luck with it - the cases are sold as matched sets - if you put the upper half of one on the lower half of another check the bearing bores for perfect alignment - only .001" is acceptable mis-alignment - any more than that and the crank will bind and not turn - if it turns, but has resistance, it will seize the bearings in a short time and all the time and money will have been wasted.

 

Great. Nothing is a simple as it seems. I didn't know they were sold as matched sets.

Perhaps I will have to have a local shop line bore for me. Would that solve this issue?