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  1. #1

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    Questions about balancing 4tec engine internals (crank,balance shaft,pistons,etc...)

    I'm working on building an "fully built" 4tec motor. I will be using crower rods, most likely CP pistons of some kind. possibly 4tec racing, anyway My questions is with balancing the the bottom end.

    I have a crank that I have had rebalanced, blueprinted, knife edged, and the crank journals micro polished. My question is about the balance shaft. What do I need to do to it to keep everything in sync and "balanced". I'm unsure of exactly what the function of the balance shaft is, and need some assistance. Since I've changed the weight of just about every bottom end component, I was hoping someone with some experience in this coudl give me some advice, tips.

    What can I safely Rev a motor like this to? is 10k out of the question? 9.5k RPM?? Of course this is presuming I have all the proper head components (lightened valves,retainers,cam,cam chain sprocket, RR springs and spacers etc..)

    If anyone has any advice or tips on this engine build I would HIGHLY appreciate it. Thanks as always!


  2. #2
    GTXX's Avatar
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    I have no experience with rebalancing this engine but wondering how was your crank rebalanced without factoring in the balance shaft?

  3. #3
    EZ Dock of Long Island Shibby1485's Avatar
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    ... rxpdoo is good person to get input from he's toyed with alot of this i believe building a radical built bottom end all lightened and such... can also trying sending a PM to malte (mrracing)

    i know of throw the CB shaft in a lathe and take the diameter down to like 26mm between the bearings to take some material off.. know a few people that have done that

  4. #4
    Quote Originally Posted by boostaholic View Post
    I'm working on building an "fully built" 4tec motor. I will be using crower rods, most likely CP pistons of some kind. possibly 4tec racing, anyway My questions is with balancing the the bottom end.

    I have a crank that I have had rebalanced, blueprinted, knife edged, and the crank journals micro polished. My question is about the balance shaft. What do I need to do to it to keep everything in sync and "balanced". I'm unsure of exactly what the function of the balance shaft is, and need some assistance. Since I've changed the weight of just about every bottom end component, I was hoping someone with some experience in this coudl give me some advice, tips.

    What can I safely Rev a motor like this to? is 10k out of the question? 9.5k RPM?? Of course this is presuming I have all the proper head components (lightened valves,retainers,cam,cam chain sprocket, RR springs and spacers etc..)

    If anyone has any advice or tips on this engine build I would HIGHLY appreciate it. Thanks as always!
    you have your crank balanced but you have not decided which pistons to use? you have to balance everything together....piston assy's, rods, ring gear, flywheel......etc. this is the only way to do it because you have to know the exact weight of the piston/rod assy.
    you should also lighten the counterbalancer. we remove 355 grams from it.
    the balancer shaft is balanced on its own after it is lightened.
    if you want to rev this engine to 9500+ better invest in some titanium valves.

  5. #5

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    Not all inline motors even have counter-balance shafts. They do nothing to reduce rod or bearing loading. They are there purely for NVH/rider comfort.

    Theoretically, you could remove the counter-balance shaft and plug the oil holes, then balance the rotating assembly just like you normally would and it would be ok.

    Not counting the valvetrain, based on published piston speed limitations, it appears as through 9,500 rpm would be a tad conservative, whereas 10,500 is possible.

  6. #6
    EZ Dock of Long Island Shibby1485's Avatar
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    since 4-tec motors are externally balanced and rotating parts inside the PTO cover (gears etc) are all balanced independently of the crank and rest of reciprocasting assembly... is a counterbalance shaft considered to be independent also in an externally balanced application? or anything within the short block is all integral to be balanced as one assembly?

  7. #7
    King_Of_Fun's Avatar
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    FWIW, the 2.0 liter 4-clyinder engine in my turbo Saab has the balance shaft chain removed (to eliminate a failure that will take out the cam chain) and the the 2 balance shafts no longer spin. True, the engine only spins to < 7000 rpms, but vibration barely increased and it was only at idle... Ron

  8. #8

    Thumbs down crank knife edge

    we tried to knife edge the crank,not a good idea impossible to get it balanced properly, our machine shop in house knife edges 6cld ,4cly and v6,v8 no problems, but with the 4tech 3 cylinder impossible to get a balance thats lower than 70 grams .....
    hope this helps..

  9. #9
    ROTAX RACING performance
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    you need to look for a transmission . the pump efficiency is close to "zero" at 9500 RPM +

    on the race boats, you can use a 9800 RPM REV Limiter , but not the prop speed

    titanium valve and retainer is a minimum at + 9100 RPM Limiter

    what ROTAX RACING valve springs do you have ???, there are no ROTAX RACING valve springs available

  10. #10
    Seadookilla
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    Hi Guys I was wanting to start skydiving from a jet has anybody here done it before and if so how hard is it?

    I have seen people on TV sky diving from a plane I have seen a jet on tv so is it possible? I have a jet but its not real, its plastic.

    oh I do have my own hand made parachute if that helps

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