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Piston Sizing for 850

Discussion in 'Norton Commando Motorcycles (Classic)' started by thunderbolt, Jan 4, 2019.

  1. thunderbolt

    thunderbolt

    Joined:
    May 16, 2016
    I have purchased +.020" Hepolite pistons from RGM for my motor and had a local machinist bore the barrels to suit. Having had work done by him previously to a good standard I did not check anything before I fitted the pistons and rings to the barrels, etc. As I was having what I thought was pushrod trouble and had to remove the head on the bench I decided to slip a feeler gauge down the side of the piston in the bore at TDC just to see what the clearance was. The measurement that I got surprised me and now has me worried.

    My question is: Is the piston the same diameter at the bottom of the skirt as it is at the top above where the piston rings are fitted?

    Thanks Don
     
  2. kommando

    kommando

    Joined:
    May 7, 2005
    No, there is a relief area where the rings are, I witnessed a JP piston where this relief area machining was missed and the bike did not get off the driveway before it seized, once the relief was machined and new rings fitted it ran well. You measure the clearance at the front or back of the piston close to the bottom of the skirt as the piston is not machined round, if the engine is assembled then its too late to check unless you remove the barrels.
     
  3. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    The skirt is the point to measure at, at right angles to the piston pin.
     
  4. thunderbolt

    thunderbolt

    Joined:
    May 16, 2016
    Thanks Timewarp, but is the skirt larger in dia. than the top of the piston??

    Thanks
     
  5. kommando

    kommando

    Joined:
    May 7, 2005
    Yes the bottom is larger in diameter so smallest clearance, measuring clearance at the ring area does not give you useful info as it is not referenced in piston dimensions but will be a larger clearance, you need to measure at the skirt bottom front or rear.
     
    Last edited: Jan 4, 2019
  6. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    Was it a big or small number ?

    I just reread your post and Steve A's was the tip off.
    If the pistons are still in the assembled cylinder/ barrel attached to the engine case and you put a feeler gauge into the space above the piston rings (piston to bore) you would get a mad number (big) so perhaps all is good and as suggested ask the machinist what clearance was used which will be at the bottom of the piston skirt as posted above.
     
    Last edited: Jan 4, 2019
  7. 850commando

    850commando

    Joined:
    Nov 1, 2014
    its really not proper to use feeler gauges when checking the bore fit of a piston to bore. the information you get may not be accurate. I'd suggest you get back in touch with your machinist and have him provide you with the piston and bore dia's...
     
    MexicoMike likes this.
  8. SteveA

    SteveA VIP MEMBER

    Joined:
    Dec 20, 2011
    Sounds like you didn't check the ring gap either. Which could be an issue.

    Sensible way forward if you have the head off already, is to go the extra mile and lift the barrels and check what you now know you should have checked in the first place, there is no other way to have peace of mind.

    But you knew this when you posted and are just putting the job off! o_O
     
  9. SteveA

    SteveA VIP MEMBER

    Joined:
    Dec 20, 2011
    Most people don't have other equipment for the job and it has been sufficient for most applications for decades.

    Of course if you have the tools use them.

    Providing the reborer with the actual pistons, confirming required clearance if you have specific needs, and a few minutes dicussion on collection usually suffices for everything bar the ring gap, which rarely needs any work but you should check it ...... with a feeler guage..... because nothing else works for that job!
     
  10. MexicoMike

    MexicoMike

    Joined:
    Jan 31, 2010
    As noted, the way to check this is to mic the piston at the appropriate location, specified by the piston maker - NOT ALWAYS in the same spot for every brand of piston - and mic the ID of the cylinder wall. The difference is the clearance.

    I agree that if the machinist has the pistons, one would EXPECT the clearance to be correct when the boring/honing was complete.

    Any measurement you are getting with a flat feeler gauge measuring a clearance between two round surfaces is not accurate or useful to determine whether the piston/cyl wall clearance is within spec..
     
    Last edited: Jan 4, 2019
  11. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    Sticking feelers up the side of the skirt is a bit dodgy as a measurement technique.

    Sticking feelers down from the top, past the rings, is completely useless.
     
  12. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Can’t your machinist tell you what clearence he machined it too?

    Sometimes, machinists more familiar with modern stuff can make things too tight rather than too loose.
     
  13. YukoNorton

    YukoNorton

    Joined:
    Sep 4, 2017
    Piston manufacturers have instructions for proper clearances; did your machinist get the specifications for manufacturers clearances before he started the job? Do you know what Hepolite states is the specified piston to cylinder clearance and where the piston is to be measured...along with ring end clearances and ring orientation upon assembly?
     
  14. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    Skirts are larger at the bottom as this is where the most wear takes place, if you take a pair of USED piston you will find differences as to the amount of wear on each one.
     
  15. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    The diameter is larger down on the skirt because the upper parts of the piston run hotter so there’s more expansion higher up on the piston.
     
    kommando likes this.
  16. thunderbolt

    thunderbolt

    Joined:
    May 16, 2016
    It was a BIG number - .012"
     
  17. thunderbolt

    thunderbolt

    Joined:
    May 16, 2016
    What makes you think I didn't check the ring gap??
     
  18. SteveA

    SteveA VIP MEMBER

    Joined:
    Dec 20, 2011
    What you wrote....or perhaps I didn't understand you clearly?
     
  19. SteveA

    SteveA VIP MEMBER

    Joined:
    Dec 20, 2011
    See, there you go.....I have some external Micrometers, I don't have any internal micrometers or telescopic internal guages, I suspect more like me don't rather than like you , do!

    Guess I better get something!
     
  20. concours

    concours VIP MEMBER

    Joined:
    Dec 29, 2011
    Well, maybe a bit dodgy, but if you can slide a .005” feeler gage between the skirt & bore, and a .006” won’t go, it’s better than the guesses going on.
    A course test, but one I do if precision measuring tools are not available.
     

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