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  1. #1
    urugol's Avatar
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    Bore Size issue Pro 785

    I decided to measure the bores on the 785 cylinders today. Now that I have the engine out I can get started on my project but I was a little surprised to find the diameter of the bore seems to be below the factory spec as per the picture. The manual quotes 69.7mm but all 3 cylinders return 69.0mm. If I measure - excluding the sleeve I get approximately 71.0mm. Interestingly, there is a fair degree of piston slap, at least it feels that way - approximately 1.0mm between piston and sleeve.

    As I'm trying to order some piston kits, I'm a little confused as to what size to get. Can these sleeves be bored? Do I need new sleeves? Do I need sleeves?
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  2. #2
    2&4strokepolaristech's Avatar
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    you can't get a accurate measurement using a vernier caliper. If you want to get a better rough measurement use the very ends of the caliper. You have them down to far. Also move them around side to side and back and forth till you get your highest reading. In either case looks like you have a standard size cylinder. Now looking at the picture you have it appears that you have some piston material on that cylinder. You should fully inspect that cylinder. The chrome like to come off at the top where the ring switches direction(very high wear spot). You only have one option as far as using the factory bore/nikasil and that is Polaris pistons. Polaris factory pistons with rings/bearings/clips and pins are about 525.00.....

    We have a ductile iron sleeve that will allow you to run a standard 750 piston all the way up to a 1.00mm oversized 780 piston. This way you can use a cast piston such as Polaris or WSM. You could also run a Wiseco forged pistons as well. this sleeve is one that we came up with that is thick enough to allow the cylinder to be bored out this far.It keeps your cost down on the pistons as well as the down time if you have a engine failure. If you oversize your cylinder more than .010" from standard you will need to have your head re-chambered. Also if going larger that .040" you will need to have your port timing checked and or corrected. If we bore you cylinders(I know a long way from Australia) we check port timing for you each time the cylinder is bored and correct if need at no charge. Head machining is an additional fee. If you are running a aftermarket head we can re-chamber your head right off the bat for 1.0mm of oversize. we can also make sure you are safe to use 93 octane fuel.

    I hope this helps.....

    Jay

  3. #3

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    Heres what i would do get a set of Polaris 750 pistons rings and wristpins with new bearings and then have the cylinders checked and if they need to replated just have them redone
    750 pistons should be easier for you to get over there

  4. #4
    ujustlost's Avatar
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    Quote Originally Posted by 785 lugs View Post
    Heres what i would do get a set of Polaris 750 pistons rings and wristpins with new bearings and then have the cylinders checked and if they need to replated just have them redone
    750 pistons should be easier for you to get over there
    and easier on the bottom end. +1

  5. #5

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    I think you had one cylinder with a Wiseco piston in it correct ???
    if they did it right they replated the cylinder for the right clearance of the Wiseco piston so it might have one cylinder 3-4 thousenths bigger then the other 2

  6. #6
    urugol's Avatar
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    Yes, there is 1 Wiseco however I am still getting 69.0. If I pull the Vernier to the very top of the chamber I still get about 69.3mm stretching them right out. So the plating cannot be bored, needs to be removed and replated is that correct? I am looking for a good workshop locally to do this, as shipping to USA and back would kill me.

    I will go for 750 pistons for sure, but why not forged pistons over stock cast ones? Given the stronger properties, and the fact they are cheaper would I not be better off with Wisecos? In any case, I will probably buy them from USA it will save me some.

    So given the bores, should I get stock 69.72mm or should I go oversize. My understanding is oversize=greater displacement=more power.

    I should also mention I will be running a Hot Seat 92 Octane head.

    I put in some pics of the chambers, they look fairly smooth and clean.
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  7. #7

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    The 785 engine was not setup to put bigger pistons in it , if you open up the cylinders you throw the port timing off by cutting back the ports .The 785 in it's stock bore is all you need if you know someone who knows how to port a 785 motor correctly it will build a TON of power but theres only a handful of guys with this knowledge . Leave it setup with the 750 pistons and have the cylinders setup for them if any of your cylinders are ok you can just hone them and reuse them
    The nicasal coating is only 3-4 thousenths thick so you cant hone them much without cutting through the coating

  8. #8
    urugol's Avatar
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    As tempting as a ton of power sounds, I think I want to go with whatever is best and most reliable at this level, so I won't pursue oversize!

    Ok, I found out about replating and apparently there are only 2 places in Australia that do this, none of which is in my state. Replating costs more than I care to tell anyone, it's really really expensive to do.

    So I spoke to a Polaris guy over here and I wanted to verify something. He told me that the nicasil shouldn't need to be replated and I would be ok with standard size pistons - need some clarification on this one. The nicasil still looks pretty good in my opinion, more pics attached.

    I'm getting my hands on an inside micrometer hopefully this weekend so I can get exact measurements of the bores and pistons. I have a Wiseco dealer here with a last set of 750 pistons (69.72mm) ready to go as well. The reason I lean toward Wiseco is I suspect with a higher compression head forged pistons will handle detonation a lot better (please correct me here if I'm wrong). I haev read about cold piston seizure with forged pistons, no probs I always warm up my ski before riding in any case.

    I noticed a bit of scuffing on some of the pistons (pics), and also measured the Wiseco albeit with the vernier however I cannot get greater than a 69.3mm diameter.
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  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    You'd better do a little more research before you buy the Wiseco 750 pistons.

    Pretty sure the rings will not be compatible with the Nicasil plating.

  10. #10

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    Quote Originally Posted by urugol View Post
    As tempting as a ton of power sounds, I think I want to go with whatever is best and most reliable at this level, so I won't pursue oversize!

    Ok, I found out about replating and apparently there are only 2 places in Australia that do this, none of which is in my state. Replating costs more than I care to tell anyone, it's really really expensive to do.

    So I spoke to a Polaris guy over here and I wanted to verify something. He told me that the nicasil shouldn't need to be replated and I would be ok with standard size pistons - need some clarification on this one. The nicasil still looks pretty good in my opinion, more pics attached.

    I'm getting my hands on an inside micrometer hopefully this weekend so I can get exact measurements of the bores and pistons. I have a Wiseco dealer here with a last set of 750 pistons (69.72mm) ready to go as well. The reason I lean toward Wiseco is I suspect with a higher compression head forged pistons will handle detonation a lot better (please correct me here if I'm wrong). I haev read about cold piston seizure with forged pistons, no probs I always warm up my ski before riding in any case.

    I noticed a bit of scuffing on some of the pistons (pics), and also measured the Wiseco albeit with the vernier however I cannot get greater than a 69.3mm diameter.
    you need to measure the bottom of the piston the top is smaller

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