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Engine Rebuild Questions

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  • Engine Rebuild Questions

    I have a car that has not been run in 30 years. I have removed a cracked cylinder head and have a 5.5 Snyder to replace it. A couple questions. The pistons come up to 0.030 above the planed engine surface. I think I have read somewhere that this requires a special gasket to make up this combination. Also I want to clean off the carbon (very slight build up) on the pistons and valve tops, but I am panicked about getting grit down into the rings/cylinder wall area and the valve guides. Any suggestions for what gasket I need and how I should clean up the carbon areas? The valve clearances are within 0.001 and all look quite good on the seat to valve interface, and the rod journals are at about 0.0017 max by plastigage, so I would like to just get her all buttoned up and started.

  • #2
    agree with Purdy. The .030 above the deck should not be a problem.
    Use a shop vac directed at your scraping and go very slowly.
    You can rotate the crank and select the valves that are closed to scrape them


    • #3
      Thanks for the advice. I will start scraping off the carbon with the shop vac real close. I have a B head gasket but it is one with a sealant as part of it. May buy the "Best" gasket instead.


      • #4
        I'm still learning even at the age well over the start of Medicare. Does this rise above the block deck require chamfered pistons or is .030 insignificant.?


        • #5
          For those wondering about the math for a new engine off the assembly line, it goes like this:
          Rod length (7.500") + 1/2 stroke (4.250"/2 = 2.125") + original piston compression height (1.906") = 11.531". If the deck height is 11.500 the piston will protrude 0.031". Original blocks usually came off the line on the high side of the 11.500 - 11.505" range, so pistons were at least 0.026 and not more than 0.031" on a new engine.

          Every time the block gets a haircut the piston protrusion increases, but most of the replacement pistons the vendors sell are somewhat short of the original 1.906" compression height. Reground cranks are also notorious for having each throw 'off' from the original stroke spec. a different amount. Replacement rods do not necessarily measure 7.500", either. All I can say is measure everything twice, and keep a build sheet.

          At operating temperature and 3000 rpm you will get about 0.012" 'stretch' from a combination of several things. Add this to your specific rise above deck height then subtract that total from your compressed gasket thickness plus head flycut depth. You should have a positive number. This is the actual running clearance.

          You need at least 0.020 and not more than 0.045 for an effective 'squish' in a flathead. Too little and you risk induced detonation in the squish area, too much and you will have a sluggish flame front with incomplete combustion and resulting carbon. Squish was not well understood in the A era and the stock engines all had too much clearance, resulting in less than stellar cylinder flame performance and a lot of carbon deposit.
          Mechanical engineering 101: If you put an adjustment knob, screw, bolt, or tolerance specs on something, some people will immediately fiddle with it. If you mark it DO NOT TOUCH everyone will mess with it.


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