Stihl - Cylinder Base Gasket Mystery?

Discussion in 'Mechanic and Repair' started by Steve Sr, Sep 20, 2012.

  1. Steve Sr

    Steve Sr LawnSite Member
    Posts: 4

    Hello,

    I have been contemplating on whether or not to try to rebuild a Stihl 4137 series (FS-75,80,85) trimmer with a bad piston and cylinder. I was looking on Ebay to see if anyone had a cylinder rebuild kit for a reasonable price. While doing this I found a seller that was selling used saw and trimmer parts. In one of his listings I saw a complete short block in what looked to be *very* clean condition for $55. Having a soft spot for old 2-strokes I decided to take a chance the piston and cylinder were still usable and hit the "Buy" button.

    The short block arrived and it was indeed very clean... like someone had recently worked on it. From what I could see through the ports the engine had been run but only for a short time. The piston still had the horizontal factory machine marks visible on both intake and exhaust sides. The cylinder wall looked like a mirror. The rings were sort of a dull gray color like they hadn't even been worn in yet.

    Now the big mystery. There were not one but *two* cylinder base gaskets between the block and cylinder. So it appears that someone had this engine apart cleaned it up and replaced the the piston and cylinder using two cylinder base gaskets. The engine was run for a short period of time and then the trimmer ended up in the junk pile and was disassembled for parts.

    I suspect it was turned into parts because it wasn't running very well. I know that the extra base gasket definitely lowered the compression ratio but was this enough to make it run bad enough to be scrapped? My only thought is that some tech thought he was trying to fix a leak somewhere. Can you think of anything else? Unless someone gives me a very good reason not to I am planning on removing one of the cylinder base gaskets.

    Speaking of things that can possibly leak. Is it advisable to change out the crankshaft seals at this stage in the rebuild process? Can the old ones be removed without having access to Stihl's special seal puller? If so can you recommend a method or procedure?

    Thanks,

    Steve
     
  2. dboyd351

    dboyd351 LawnSite Bronze Member
    Posts: 1,099

    If you just need to know about the 2 base gaskets you could go ask your Stihl Dealer. If you need much more than that you could ask them to order you the "Spare Parts List" for that machine. The spare parts list is a detailed multipage schematic/parts list that has blown up drawings of every part of your machine with all the parts and the part numbers. If I recall correctly, it cost me about $15. The one for the FS 90 was about 30 pages long.
     
  3. Steve Sr

    Steve Sr LawnSite Member
    Posts: 4

    Dboyd,

    Thanks for the suggestion. I have a couple of years previous (to this particular trimmer) parts list and service manual shows only a single gasket. The actual trimmer itself where this small block is going also had only a single gasket.

    So it would appear that there should only be one cylinder base gasket on this engine. I was just wondering if someone knew more than I did and could offer an explanation why this engine ended up with two cylinder base gaskets.


    Thanks,

    Steve
     
  4. cgaengineer

    cgaengineer LawnSite Fanatic
    Posts: 15,782

    Yes, change crank seals. It's possible that the gasket surface is warped and two gaskets were required to make it seal but I doubt it.
    Posted via Mobile Device
     
  5. Steve Sr

    Steve Sr LawnSite Member
    Posts: 4

    CGA,

    Any suggestions on how to remove the old crank seals without access to Stihl's special puller tool?


    Thanks,

    Steve
     
  6. cgaengineer

    cgaengineer LawnSite Fanatic
    Posts: 15,782

    Can you post a picture of the seals (a shot of the crank in block)

    Generally a seal pulling hook, small screwdriver or a small screw can be used on them.
    Posted via Mobile Device
     
    Last edited: Sep 25, 2012
  7. piston slapper

    piston slapper LawnSite Platinum Member
    Posts: 4,240

    I also doubt that double headgaskets was anything more than an assembly error.
    Even very small differences in headgasket thickness will affect the timing and compression...as the intake and exhaust ports are integrated into the head...
     
  8. dutch1

    dutch1 LawnSite Silver Member
    from Jayhawk
    Posts: 2,231

    I'll second your thoughts, Slapper. I've replaced a number of jugs/pistons/rings in the past and have never seen a unit with more than one jug gasket. The last guy who worked on the unit may not have even seen/removed the original--who knows. Judging from what I have seen some "shade trees" do, anything is possible.
     
  9. BigFish

    BigFish LawnSite Gold Member
    Posts: 3,043

    The OP's unit ain't got no headgasket. Cylinder base gasket,yes, headgasket, no.
    I always take a smooth flat file and swipe the cyl. base to clean up any bumps, etc. And I always use a spray sealer on all the gaskets.
    I'm not big on using dry gaskets on anything, plus it usually makes it easier to take apart down the road.
     
  10. piston slapper

    piston slapper LawnSite Platinum Member
    Posts: 4,240

    Hmmmm...I must paint pictures in peoples minds...
    No matter if I call in an order for a HEADgasket for a BR420. ...or a BR549..the guy at the parts counter ALWAYS gets me the right HEADgasket..
    Posted via Mobile Device
     

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