Engine Stud Problems

Discussion in '2-Stroke Engines' started by Timbone, Apr 17, 2016.

  1. Timbone

    Timbone Active Member

    Since putting in my most recent engine, I've broken two engine studs on the rear of the engine. Strange. I thought my transition plate method was rather foolproof.

    The way to remove the broken thread at the engine block is to use the dremel and cut a slit in the end of the stud and then back it out with a flat head screwdriver. It's never fun to remove the rear tire (dirty job!) but it's the only way to get there. Cannot understand why these engines do not come with 8mm studs...

    I removed the stock studs and placed longer ones, locking the back of the studs onto the seat tube, hoping to add stability. There was some kind of instability there. I can tell you that the engine is absolutely rock solid. Zero deflection. My hope is that the motor is as much support member any part of the system.

    Here is my pic:

  2. Frankfort MB's

    Frankfort MB's Well-Known Member

    What I did awhile back on one of my bikes I broke a stud and took a small drill bit and drilled the stud out then I rethreaded to a standard size which is a lot easier to find a grade 8 bolt or stud.... Bolts seem to work well for me but if you have an odd frame then it might be a challenge
    Timbone likes this.
  3. crassius

    crassius Well-Known Member

    all the force is at the back mount, that one should have studs perpendicular to seat tube and a bit of PVC between motor & frame

    motor must sit down solid in frame before tightening mounts - no amount of tightening will stretch it to fit
  4. Timbone

    Timbone Active Member

    Good advice, as usual!

    I took a long look at my rear motor mount. It is perpendicular to the seat tube and the engine is where it wants to be. You can't see it, but I added a stainless steel spacer for a perfect sit. I've never considered PVC, though.

    One of the things I didn't mention: the rear transition plate I made out of 1/4" mild steel was being grazed by the motor chain. Instead of grinding it down for clearance, I just allowed the chain to eat down into it. I added some grease and the result was pretty cool: it was serving as a kind of second chain tensioner. Very little resistance rolling over it.

    But with the rear studs obviously under too much stress, I ground the plate down so there is no longer any contact. My hypothesis is that the knocking of the chain onto the plate was causing a tick tick tick shearing for to the studs.

    It's all about R&D.
  5. sbest

    sbest Active Member

    All good advice here, especially Crassius about keeping bolts perpendicular to frame.

    Why 6mm instead of 8mm? Bigger is not always better.
    Part of what holds a nut tight is the tension created by the stretch of the stud.
    If the stud is too big, the stud doesn't stretch, the other parts have to compress, and often the force involved is more than they can bear.

    As for tapping it out for 1/4" studs?
    Typically mixing metric and SAE fasteners is bad practice, but it is your bike and it will work.