750 sei question on piston rings help please!!
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Thread: 750 sei question on piston rings help please!!

  1. #1
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    750 sei question on piston rings help please!!

    Hi

    A bit of background to my question!

    Last year I bought a "restored" 750 for quiet a tidy sum and it has never run well ,lucky if I get 4 cylinders some days!
    Plugs sooted up so strip carbs ,really grubby so ultrasonic clean ,new float valves and set them up.
    Air filter was solid with crud ,looked like it was original!
    Still no good so check compression --low as 99 psi on one pot! about 110 on others chucked some oil in and hey presto 135 -140!

    The motor was re built and re bored with new pistons and rings according to seller.

    Getting really cheesed off now so head off to see new pistons and bores newly done with honing marks so it was as he said,,relief to some extent!

    Barrel off and check rings and the top two on every piston are the same i.e. top rings plain rings, not a stepped 2nd ring in sight!

    Well I am no 750 sei expert in fact I know nothing except the general facts as I have only had it a short time but unless this is some mod or update I can't find any mention of it .But itseems to be the answer to the lack of compression!

    Also there are no o ring seals around base of sleeves (bores)

    should also say the bike smokes from breather when warm,not from exhausts though!

    As you can imagine I am quiet miserable after all this so advice would be much appreciated!

    I have given up with the restorer.

    pondman

  2. #2
    Senior Member black nails's Avatar
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    John
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    That's interesting, on the 500 ls, which is basically the same engine with a couple of cylinders missing, there are no 'o'rings fitted on the cylinder bases.
    Regarding the piston rings, have you removed them to check for markings, I wonder if they are slightly tapered in section and fitted upside down on the bad cylinders.
    Just a thought.

  3. #3
    Senior Member Engenia's Avatar
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    Errol Kowald
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    Sounds like a dodgy rebuild.
    I have no experience with the Sei, but do have some with your symptoms.
    The issue is with the piston/ring/bore seal, judging by the increasing pressure with oil added, but 135 psi is still not brilliant, so while the head is off you should lap the valves to get them sealing properly. Use kero to confirm that they are sealing. You'd be surprised how badly freshly ground valves/seats seal.

    You need to check a couple of things:-

    1. Have you measured the bores? (They might look good but measurements will confirm)
    2. Were the ring gaps lined up? (They should be set at 180 degrees in line with the crank.)
    3. Are ring gaps correct?


    Whatever you do, you should put new rings in it and scour the bores with scotch-brite to give them a chance to bed in. Then run it in properly to make the rings bed in. That means - maximum cylinder pressure to push the rings into the bore - and you get that by:-
    1. cycling the engine wide open throttle (WOT) to 75% max revs, letting it drop back to 25% then repeat, or
    2. find a big hill, select a high gear and labour it WOT, or
    3. find a dyno and run it WOT under load at ~ 50% revs.


    It's not a two stroke so you don't need to seal the bottom of the barrels. A gasket will do to stop oil leaks.
    hooroo, Errol www.engenia.com.au [136,351 km - and counting .....]

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  5. #4
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    thanks

    Quote Originally Posted by Engenia View Post
    Sounds like a dodgy rebuild.
    I have no experience with the Sei, but do have some with your symptoms.
    The issue is with the piston/ring/bore seal, judging by the increasing pressure with oil added, but 135 psi is still not brilliant, so while the head is off you should lap the valves to get them sealing properly. Use kero to confirm that they are sealing. You'd be surprised how badly freshly ground valves/seats seal.

    You need to check a couple of things:-

    1. Have you measured the bores? (They might look good but measurements will confirm)
    2. Were the ring gaps lined up? (They should be set at 180 degrees in line with the crank.)
    3. Are ring gaps correct?


    Whatever you do, you should put new rings in it and scour the bores with scotch-brite to give them a chance to bed in. Then run it in properly to make the rings bed in. That means - maximum cylinder pressure to push the rings into the bore - and you get that by:-
    1. cycling the engine wide open throttle (WOT) to 75% max revs, letting it drop back to 25% then repeat, or
    2. find a big hill, select a high gear and labour it WOT, or
    3. find a dyno and run it WOT under load at ~ 50% revs.


    It's not a two stroke so you don't need to seal the bottom of the barrels. A gasket will do to stop oil leaks.



    Thanks for reply!!!!


    Well things are progressing

    It seems they are 500 Honda pistons and looking pics of new sets it seems like it has 2 top rings installed and not what should be there!
    I have ordered a set From David Silver Spares to check ,so I can't really say this is problem yet.
    It has new pistons and re bore and valves are good so ?

    Cant see how a mistake like that can happen but we will sei!

  6. #5
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    Well I have got the new rings and as I suspected the Honda 4 rings in Benelli are 2 top rings and oil ring. no middle ring,its quiet different apart from being black!

    Ordered rest of ring sets but need quiet a bit of time to re build so will wait till Winter when I am less busy!

    hope this is last of probs!!

    pondman.

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