Piston question

JFromm
JFromm Senior Contributor
in HUDSON
While reading an article about an engine rebuild they mentioned that they reworked the pistons. What does that mean, and is that a good idea? Why not just replace the pistons?
Thanks

Comments

  • Glowplug
    Glowplug Expert Adviser
    Knurling pistons is a common practice for engine overhauls.  This brings the piston back to a pre-ware size.  Piston ting grove cleaning is part of the piston reuse.  Our cost to use professional engine rebuilders or machinists make the practice expensive.  But the folks who rehab their own engines save money.  
  • JFromm
    JFromm Senior Contributor
    Thanks 
  • Courtesy Man
    Courtesy Man Expert Adviser
    As I understand it pistons are not exactly round but ever so slightly cam ground or oval or egg shaped. So just knurling or cutting down an oversized piston on a metal lathe may not be good enough. If you measure accurately the piston diameter where the piston pins are located and then 90 degrees from that I have been told there is a dimensional difference - I should try it sometime.
  • Courtesy Man
    Courtesy Man Expert Adviser
    I just measured a NOS 3 inch Hudson piston and find a difference of about .007  between the two locations. Not a lot but engineered on purpose to help seal as temps increase from start-up. I doubt very early cars had that, just a lathe cut leaving it same diameter.
    Look up the early difficulty they had in producing an even round bore for steam cylinders and military cannon.
  • JFromm
    JFromm Senior Contributor
    Thanks for the info. Does knurling make the pistons subject to fail? Does it weaken them?
  • Geoff
    Geoff Senior Contributor
    Knurling, if done properly, is only done on the non-thrust side, that is the one with the split.  It does not weaken the piston.   All Hudson pistons were cam ground from 1934 onwards, which enabled them to be fitted with only 1-1/2 thousands of an inch clearance.  This ensured quiet running and minimal oil consumption.  All modern  engines use cam ground pistons.    The narrower sides expanded to the bore size when hot.  A problem developed with the 3" bore sixes though, when the bores 2 and 5 would distort because of the  siamesed situation,  causing blow-by and oil burning.   To overcome this the rings were pinned in position so that the rings were in the same relative position to cylinders all the time.   Modern ring technology has rendered this  unnecessary.  

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