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Thread: Difficulty starting a rebuilt 48 Panhead FL - Part 2

  1. #1

    Default Difficulty starting a rebuilt 48 Panhead FL - Part 2

    Hi Guys

    A few weeks ago I posted the following problem

    Prior to chroming and painting this 48 needs to run perfectly and I'm having problems starting her. She splutters for a few seconds but doesn't catch on.

    I've never had the bike running and this is my first Harley so I need to check a few things;

    Timing is spot on, it has the later auto advance unit, points gap is correct.
    Spark Plugs are new Champion J12YC, gap correct, sparks ok
    Timing gear marks all line up
    It has solid pushrods, adjusted properly
    Inlet manifold has been removed cleaned and new brass packing bushings fitted, greased - not pressure tested
    Inlet nipple holes sealed - not pressure tested
    Heads removed, valve seats look very good, but not lapped in by me.
    Heads are from 1950, not 48.

    I have two concerns - compression is only 60 psi on front and back cylinders, after about ten kicks, with oil in cylinders - this seems low?
    The carb is an M-74, not the original, this has been cleaned, and set up as per the service manual, float level checked, needles set up per manual, no leaks - is the larger carb a problem?

    Following advice I have now; lapped in the valves and pressure tested the manifold. The manifold was leaking and this has been fixed.

    As a result, there has been no change in the compression test, but the manifold is sealed under pressure - the bike still does not start.

    The compression is still low, so I decided to remove the cylinders

    Each piston has 'D 4' on top which I assume is 40 thou over, pistons/bores show wear but not excessive. In rear cylinder, compression ring clearance in bore is 50 thou (both rings at different heights), and in front cylinder piston to bore clearance is around 20 thou all around.

    Whilst I haven't fully checked the front pot, my suggestion would be;

    Hone both cylinders and fit new set of rings to each piston (40 thou over), reassemble, new gaskets etc, check compression has improved - reassemble and test?
    On the other hand, if these compression numbers (60 psi) are OK, I should look elsewhere - carburettor?


    Thanks Greg

    Last edited by harleygreg; 05-11-2019 at 10:48 AM.

  2. #2
    Join Date
    Jun 2007


    How are you measuring rings, end gap ? feeler gage ? Are you checking compression with carb WOT?

  3. #3
    Join Date
    Nov 2018
    Reno NV


    It has been a long time since I built a panhead, but the 20 thou piston to bore clearance seems way to much to me. This motor would be very noisy.
    I think the clearance should be in the 0.001/0.002" range, measured at the bottom of the skirt, front to back.
    You might need to look at a rebore.

  4. #4


    Hi Guys,

    I have now checked the front cylinder and compression ring clearance in bore is 50 thou, both rings were checked in their cylinders at two different heights. The 20 thou clearance between piston and cylinder was incorrect as this was measured at the top of the piston. When measured at the skirt, at front or back it measures <4 thou which I believe is about right? I believe the correct compression should be around 100 lb PSI? All compression tests were done with WOT. All ring clearances measured with feeler gauges.



  5. #5
    Join Date
    Nov 2018
    Reno NV


    That piston to bore clearance is close. The last picture shows a little bit of scuffing on the front thrust face.
    I think your original idea is the best way to go, run a ball hone through the bore to deglaze it, and fit new rings.
    The compression ring gap should be .01-0.02".

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