I guess I gou a bad crankcase

Discussion in '2-Stroke Engines' started by FelipeCobu, Oct 11, 2011.

  1. FelipeCobu

    FelipeCobu Member


    A few weeks ago my 70cc engine seized - by the time it has not more than 30 miles of work-, disassembling it I found massive bearing failure.

    Next to it I had my crank trued and I balanced It, rebuilt the engine w/ quality bearings (INA and SKF) using heat/colt method.

    When bolting the case halves together I discovered that my crankcase haves misalignment because when I tight the bolts the crank can't spin free.

    I give it a few beats and more heat and torqued the bolts some way that the crank spins - not free as I would - but spinning.

    Last night I figured out my engine trying to seize again, I stoped it and removed the clutch assembly, magneto cover and the piston jug and header, noticed the crank was too tight, not seized. Torqued down the bolts as I do before and again the cranks can spin almost free.

    I'm considering leave this engine some time idling as a "break in" and see if this if it gets free. What else can I do? Any suggestions? Do someone had the same issue?


  2. motorpsycho

    motorpsycho Active Member

    are you sure that the bearings have the proper clearance?
  3. HeadSmess

    HeadSmess Well-Known Member


    theres paper thin shims on each side of the crank to stop float... never the same number or thickness from engine to engine. obviously some poor girl has the job of sticking cases together, measuring endfloat and shimming to compensate. remember, mass production has tolerances... tolerances to allow the largest shaft "off the line" to fit in the smallest hole "off the line". this means the smallest shaft is very loose in the largest hole. i doubt they do selective assembly in the factories to minimise this... tools break, wear out, machines heat up, cool down...no two parts will ever be exactly the same!

    it sounds to me like your crank wasnt pressed together tight enough... just a 10th of a mm is all it takes to be too wide! i do believe you had it pulled apart? you should need at least 1 shim on the crank. and there should be some endfloat to allow for when the things running... like 0.1mm

    if it runs freely before you tighten the case up. yep. too wide. and having hit it with a hammer will do the world of good for the trued crank... :jester: youve just untrued it :eek:

    otherwise, it wasnt pressed square, the bearings arent seated properly in the cases, the bearings are too large for the hole...(they feel notchy if so) or the holes are eccentric/eggshaped

    also...open cage with shield on the OUTSIDE only bearings? anything else except full opens will sieze from inadequate lube.

    heat/cold...if a bearing doesnt go in with a few gentle taps with a drift at room temp...its too big. depending on the style of bearing as well. the bigger and heavier, the tighter they can be...
    light up to get the odd one out, sure...but never when installing. bronze bushes that get reamed to size afterwards need the shrinkfit.

    in all my experiences...running a bearing of any type that isnt running freely wont help it "loosen up".

    it either eats :poop: or...eats :poop: :jester:

    its a 70 :rolleyes7: stick with the 50 :p
    Last edited: Oct 11, 2011
  4. bideronit13

    bideronit13 Member

  5. FelipeCobu

    FelipeCobu Member

    Thanks HeadSmess

    A few more information about my case:

    The first time the engine failed and I dismantled it I didn't found any shims besides the crank, what I found was the clutch side bearing burned and w/o three spheres.

    I disassembled the engine again last night and the same bearing was burned again.

    I had my crank trued by a shop, not at home. It was pressed to allow proper clearance between connection rod and the crank.

    How can I know if the crank is to wide? How can I shorten it not killing the clearance to the rod sides?

    The crank could move freely but when close to BDC was hard to move it, that is why I think in misalignment.

    Thanks for helping.

    Last edited: Oct 12, 2011
  6. HeadSmess

    HeadSmess Well-Known Member

    arrrrgh! i dont know!

    this is why i just dont bother and replace the whole engine. something failed? theres a reason for it.

    you could be right. crankcase was dodgy from the word go. you mention it had three balls missing when you first stripped it...where did they go? and why did they break? there may have once been shims and they went the way the balls went, or...it was a tight case/crank combo in the first place, wasnt shimmed, and was actually too tight all along... once again, 0.1mm!

    i will spend money and time on "hotting up" a perfect runner, but once theres been a major trauma...its like keeping dribbly things alive in wheelchairs...whats the point?

    and if you went back all the way to the start, before you got any spanners out...calculated what youve spent... a new motor would probably have been cheap.

    thats really all i can think of :( soz dude....

    maybe use two crankcase gaskets to get a bit more thickness there? if that didnt work, i would seriously just walk away!
  7. FelipeCobu

    FelipeCobu Member

    Thanks again Headsmess

    I'm at Brazil here is not as easy to buy things like in US my engine kit costed me roundly 350 bucks.

    I will try the thicker gasket and see if I can screw the halves and still spin that crank.


  8. HeadSmess

    HeadSmess Well-Known Member

    brazil = australia :D different spelling i think...

    almost. a full kit is 300 depending on where you go down under...

    so not that far off.

    bare plain motor is fairly cheap. cheaper than any workshop after 1.5-2 hours of work, anyways!

    cant you get a relative to send you one as a gift or do they just hit you with duty at customs anyways?

    even id do it if brazil wasnt so freakin far away!
  9. FelipeCobu

    FelipeCobu Member

    I came to a gasket shop and asked for a thicker gasket and some shins will try to assembly the engine caring a lot about clearances, I'm not sure if the failure was due to misalignment or lack of clearance, so I'll try once more. As plan "b" I,ve found a dealer that have a spare bottom end to sell.

  10. FelipeCobu

    FelipeCobu Member

    Apparently solved!!!

    Finished assembling last night, firstly even with my 1,5 mm gasket I had the crank locking, then i figured the machine shop leave it with a 0.40mm gap between rod and crank, gone back to the shop to get it pressed to 0.20.

    Now the cranks goes freely even w/ all the bolts pressed. After all this issues I understood the reason the bolts came loose from the "factory" at China; How poor the things are done there!!!

    First test ride was last night and wow it bangs! Accelerates much better now!

    Let's see if I'm going to have some happy times now...

    Thank for all the useful info I'm getting here!