Announcement

Collapse
No announcement yet.

Quick engine build question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Quick engine build question

    I am down to a bare block with my S52 and will be doing aftermarket pistons/rods/rings. I have decided to go with Molnar rods and JE pistons for the rotating assembly. I talked to the machine shop today and told them I was going to be ordering custom pistons and that once they arrived I would bring everything in for them to do the work. The engine ran just fine before teardown and the cylinder walls had some very faint vertical scores but nothing major at all. Factory crosshatch is still intact (everywhere in the bore I believe but it has been weeks since the block saw the light of day). So here's my question/dilemma:

    The machine shop told me to bring the block in and they would clean it up and do the bore/hone and then tell me what size pistons to custom order to fit the bore. This seems a little odd to me as I was under the impression that each bore should be made for the corresponding piston and that the piston should be right in front of the machinist when the bores are done to get everything precise. The shop's logic is that they don't know how much they will have to bore out the block to get the cylinders perfectly round so they recommended not buying a set of pistons that could potentially be undersized by the time they get the bores where they need to be. Like I said, the bores LOOKED to be in pretty good shape overall and I was planning to go .1mm over at 86.5mm. He was saying that .1mm is .0039" so that's not much material to remove to get the bores straight and that he may have to take more off.

    I have to place a custom order either way since JE does not have off-the-shelf 10.5:1 pistons and I also want to get a few upgrades they offer when they custom make a set for you. What do you guys think? Would you just get .2mm over to be safe and have them machine the block to each specific piston or would you let him bore out the block and hope JE get the dimensions EXACT on my custom order? FWIW this is a reputable shop in the area and was recommended to me by the local BMW race shop.

    #2
    Bore first. This is standard. And when boring, there's a minimum cut they want to take to get a nice.
    even bore.

    In the racing universe, it's 'as big as the rules will let me' or 'as big as I can go before finding water'
    depending, of course, on the rules...

    A really high- buck job will bore, then wait to get the pistons, then hone-
    but let's face it, the piston maker specifies the hole size to the tenth, and makes the pistons to a tenth, so really,
    what's not to just do? A good machinist can get the holes to +-1 tenth, and you're good to go.

    Always listen to your machinist.

    t
    now, sometimes I just mess with people. It's more entertaining that way. george graves

    Comment


      #3
      OP, almost every reputable machine shop i've gone to asks for the pistons before they bore any block... i would be slightly wary if they told you boring the block would be fine with boring the cylinders without using the pistons.

      also, there's a good reason why domestic pistons come in increments of .01" - anything under that and the shop won't be able to do a good cut on the cylinder. so basically, at a minimum, you'd need to go .254mm over the standard size

      but honestly OP, i wouldn't bore the block unless there was something wrong with the cylinder walls, and yours sound great. who cares about what JE rates their piston "compression" at? that certainly wouldn't be a good reason to order custom pistons.

      get one of their higher compression pistons, say a 11.5 or 12:1, play with an online compression calculator like this and figure out how thick your head gasket needs to be (based on what's available) to get you to your ideal compression ratio, then order your head gasket. all off-the-shelf, no custom bullshit, call it a day

      Comment


        #4
        bump

        Comment


          #5
          Ended up bringing everything 5.5 hours away to VAC where I know the job will be completed properly. Thanks to those who replied. I will make a build thread for this once I get my parts back.

          Comment

          Working...
          X