Blowing head gaskets

Discussion in 'Mechanic and Repair' started by dfor, Nov 15, 2001.

  1. dfor

    dfor LawnSite Senior Member
    Posts: 815

    I have an 8 hp Briggs I/C on a Little Wonder blower. About 2 weeks ago I cranked up the rpm's just by adjusting where the cable mounts at the engine. A week later I blew a head gasket. Now about 4 days later I blow another. All 7 or 8 bolts seemed to loosen up some from the vibration (may not have torqued them down enough). Unit is 4 yrs old.
    Question: would the rpm's possibly being too high blow a head gasket? Or would the second one have blown due to not enough torque on the bolts? Thanks for any help.
     
  2. justractors

    justractors LawnSite Member
    Posts: 39

    Many blown head gaskets on these little engines are caused by overheating and/or improper cool down prior to shutting down.
    Make sure the cooling fins and cooling fan are clean of debris. Make sure you Idle the engine for a couple of minutes after running it hard.
    It is very possible the second one blew from the bolts being too loose.
    Beg, borrow, or steal a torque wrench
    Bill
     
  3. General Grounds

    General Grounds LawnSite Senior Member
    Posts: 902

    :blob3: you may want to put a small dab of thread lock on those bolts to help with the vib., cool down is very important.T
     
  4. Richard Martin

    Richard Martin LawnSite Fanatic
    Posts: 14,700

    Never use any sort of thread locker on any bolt that needs to be torqued! The thread locker will throw off the torque readings on the bolt being torqued. Additionally when you go to retorque the bolt a few hours later the thread locker will make the retorquing useless as it will definatetly show incorrect readings. A lack of retorqueing could very well be the reason the second head gasket blew.

    The only time you should ever use anything on a torqued bolt is when instructed to do so by the service manual.

    What I recommend is obtaining a new head gasket, tap the head gasket bolt holes and die the bolts to clean up the threads and then torque and retorque the head gasket bolts according to the factory recommendations. Also check the head to see if it warped.

    If the problem persists I would turn the rpms back down to the factory maximum.
     
  5. captdevo

    captdevo LawnSite Senior Member
    Posts: 932

    if you over-revved it you probably caused the blown head gasket.

    bring the rpm's back to spec, clean head bolt threads and holes, deck the head and block ( use a large file and make sure the surfaces are square), install a new head gasket, make sure all cooling fins are clean...

    never use lock-tite on head bolts unless it's aluminum flange type (gasketless), then you would gel-seal gasket maker.
     
  6. sdwally

    sdwally LawnSite Senior Member
    Posts: 385

    As state above don't use a thread lock unless the repair manual says to. B&S head bolt do not use thread lock, also do not use a lubricant on torqued bolt as you will over torque the bolt(unless required as per a manual). Antiseeze compounds are about the only thing you would want to use on head bolts especially around the exhaust manifold.
    If the head is warped this would be the reason you would be losing head gaskets also. Heads can be resurfaced on a thick piece of glass(min 3/8" thick) using a sheet of 220 grit silicon carbide sand paper. Normally I spray the sheet of sand paper with a lubricant like WD40 to help hold the sheet in place. If it is warped bad you can use a coarse grit dry to remove more material. With no lubricant, when the sheet is loaded with particles you can shake it off and reuse the sheet.
    Sand the head one direction for so many strokes, then rotate 90 degrees and use the same number of strokes. Once you can see full contact of the gasket area with the sand paper use a figure 8 stroke to finish up with.
    Use a torque wrench in a criss cross patern and recheck torque a second time in the same criss cross patern. You will normally find that the first 2 to 3 bolts will tighten down some more.
    Engine high rpm should be set at or just below 3600rpm with no load. Since this is on a blower you will need to unload the engine. This is done by plugging the air intake to the blower. You will not feel any air flow when you have unloaded the engine. Once you set the high rpm in this manner, you not have any more problems with you head gasket. Note that at full throttle with the blower loading the engine it will not run at what you set the rpms at. If you adjust it to do so you will have engine failure in the near future.
     
  7. Robert Doubrava

    Robert Doubrava LawnSite Senior Member
    Posts: 342

    you most likely ran it too fast. my rider's 7 horse will die if you run it too fast.
     
  8. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    They run 5 hp briggs 6000 rpms when racing with a stock head gasket all day. As stated above your problem is in the torque and retourque of the head bolts. Check to see if the head and block are flat and clean. Also check to see if the cooling fins are clear. I'm mostly repeating most comments above. You can overheat the engine is you run too low rpm's while lugging it. Usually you should use engine oil on the threads and a little onder the bolt head. Most of the time I use anti-seize compound there. Good luck.
     
  9. FIREMAN

    FIREMAN LawnSite Senior Member
    from n.j.
    Posts: 318

    first you should actually torque the bolts to spec.....if they fail to torque properly you should replace the head bolts as they can stretch.......good luck
     
  10. big james

    big james Banned
    from texas
    Posts: 152

    THAT IS AREAL GOOD WAY TO BUST OUT THE THREAD BOSSES ON CERTAIN AREAS ,.SILICON WILL DO THIS WORSE THAN ANYTHING , iSAW AN OLD HARLEY ONE TIME THAT A GUY HAD FORCED SILICON DOWN INTO THE THREAD BOSSES ON HIS PRIMARY COVER AND BUSTED EVERY ONE OF THEM :(
     

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