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Compatible IO-540 variants

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  • Compatible IO-540 variants

    This has probably been discussed before but I was not able to find it.

    I’m at the point where I need to find an engine and I’m looking for an IO-540. According to the plans the bearhawk 4-place is limited to the 260hp variants. I’ve assumed this is due to weight? What engine models are acceptable or have been used by other builders?

    I am talking with a seller that has a IO-540-K1H5 available, this is a 300hp variant narrow deck engine.

    What variants do you recommend and which ones should be avoided?

  • #2
    The parallel valve engines seem to be on the high end for weight on the 4-Place. Ours has around 250hp. Some parallel valve engines are able to make 300. Honestly I think 250 is more than we need.


    • #3
      That is an angle valve engine, about 50 pounds heavier than a parallel valve engine due to heavier cylinders and crankshaft. When shopping for an engine ask if it is parallel or angle valve. Narrow deck or wide deck refers to the way the cylinders and case are bolted together. Narrow deck engines are the older style, not sure when Lycoming swapped over. Most if not all O-540s are parallel valve and can be easily if not cheaply converted to fuel injection. IO-540s up to 260HP like the IO-540 C and D series are parallel valve.


      • #4
        Thanks Rodsmith! That’s extremely helpful.


        • #5
          Its all about the weight.

          Mike Araldi's 4-place had 330hp from a parallel valve engine (the lighter weight one) built by Barrett, and that plane has been around for about 16 years if I recall correctly. So it's a very workable combination.

          My point is you can ignore the horsepower to a greater extent.

          It's the engine model "number", more correctly called the engine model designation, which you need to look at. This will tell you about the engine configuration and hence the weight. There's a few resources available online which tell you how to interpret the Lycoming engine model numbers, which will help you determine ones are parallel and which are angle valve.

          Here is one such resource:


          • rodsmith
            rodsmith commented
            Editing a comment
            I have an old paper copy of that, didn't know it was available online, great!

        • #6
          Aren't the angle valve engines a bit wider and might not fit in the factory cowl?

          Other than that, weight is just a matter of do you want the extra empty weight and if the balance works out. If you're a guy that flies around with 1 or 2 people up front and a fishing pole in the back, the balance will be off. OTOH if you load these things to the gills and fly around at gross most of the time, you'll love 50lbs on the nose.


          • Battson
            Battson commented
            Editing a comment
            I thought there were factors other than CG of the aircraft. I can't recall exactly, but I have a feeling Bob once said the engine mount would need to be beefed up to take the extra weight safely. Remembering there's no "fat" in a Bob aircraft design, everything is lean and mean - only as strong as required plus safety margin - no more. So if you go past the recommended weight, you're eating into the safety margin.