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Jahns (domed) pistons for 283 SBC...how to CC dome?

Discussion in 'The Hokey Ass Message Board' started by Bass, Mar 1, 2008.

  1. Bass
    Joined:
    Jul 9, 2001
    Posts:
    3,287
    Location:
    Dallas, TX

    Bass Member

    I recently scored a set of NOS Jahns domed pistons for my "period correct" 265 that I am piecing together to go in my Model A Roadster. They're actually 283 STD pistons, but my 265 has already been bored over .125 to bring it to 283 ci.

    I'm trying to get a rough estimate of the compression these pistons will yield. Here's some numbers for the combination I'm using:

    bore= 3.875
    stroke= 3.00
    piston/deck clearance= .025
    head gasket thickness= .051
    head gasket bore= 4.00
    combustion chamber= 60cc ('57 Fuelie/ Power Pack)
    top of piston to top ring groove= .325
    piston dome volume= ??


    The pistons have a pretty substantial dome on them...the guy I bought them from said they are supposed to be 11:1 compression. So by plugging that into a compr. ratio calculator, then the dome would have to be somewhere around 17cc...which I have no idea if the dome actually is.

    So the real question is, what is the proper way to CC the dome of a piston? Is it similar to the process used to cc a cylinder head's combustion chamber?

    The point of all this is that I what the compression ratio to fall somewhere in the 10.5:1 to 10.75:1 range. Should be fine in a light car with a cam that has a tight lobe center,a 4 spd trans, and running 93 octane. Right?

    I guess I could adjust compr. slightly with a head gasket change, but I don't want the gasket getting too thick and making the quench area any taller than it already is. The domes on the pistons are solid, so I think I could have them chucked up in a lathe to take off a bit if necessary...but first I need to know what the dome volume is to nail down the compression ratio.

    Anybody want to take a guess at the cc of the dome from looking at the pictures below?

    Thanks...

    Attached Files:

  2. Tman
    Joined:
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    Tman Member

    Well, the dome is smaller than on my MT 283 slugs. We figured mine were in the 13:1 range...no help on your actual question tho.
  3. FWilliams
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    near seattle wa.

    FWilliams Member

  4. R Pope
    Joined:
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    Saskatchewan

    R Pope Member

    I'd put a ring compresser on a piston, exactly !" above the top. Fill the comp. level full with sand, salt, whatever. Then do the same with a flat top piston, measure the difference in volumes, and there ya are!
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  5. lostn51
    Joined:
    Jan 24, 2008
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    lostn51 Member

    those are close to 11:1 just by looking at them compared to the 12:1 that i have
  6. aka brandons grandpa
    Joined:
    Dec 31, 2006
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    Location:
    atwood baby.....northern ky

    aka brandons grandpa Member

    i have used clay from a hobby shop to make molds for checking cc,s.jahns were not top of the line piston. but they had neat decals.do you have rings yet?jim
  7. CNC-Dude
    Joined:
    Nov 23, 2007
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    Ringgold,Ga.

    CNC-Dude Member

    The best and correct way to measure the effective dome volume of your piston is very simple! First start with the tools of the trade. You will need a burette of adequate volume to hold enough solvent or your fluid of choice,probably at least 100cc's. Then put the top ring on the piston. Then get some medium density grease and lightly coat the inside of the cylinder you choose to insert the piston into for checking. Don't overdo the amount of grease you apply to the cylinder bore, just apply enough to allow a good seal around the edge of the piston. If you have a depth mic or dial caliper, make sure that the piston is square in the bore(be as precise as possible to the .001 if you can) by a predetermined depth(.500 or .750,etc..). This of course will be determined by the actual height of the dome. Make sure that it is only slightly lower than the top of the deck, but equal at 90 degree spacing. Now, take a Q-Tip or other slim dowel with a kleenex wrapped around the tip to now remove the excess grease from around the piston edge and cylinder bore.Any remaining grease will count toward piston volume,so be thorough and remove as much as possible. Double check that your piston is still square and hasn't been moved by your grease removal. Now you will need a piece of plexiglass or lexan that will cover the entire bore. This plate should have 2 holes drilled into it, one for the tip of your burette to fit into, the second will be for a vent to help eliminate air pockets as the cylinder fills. Lightly run a bead of grease around the top of the cylinder with your fingertip to seal the plexiglass plate. Make sure grease doesn't ooze into the top of cylinder, this again will add to the piston volume that you measure and give you an inaccurate reading. Now fill your burette with your fluid(I use varsol or safety kleen), and zero the reading with the petcock. You may have to practice a few times. The fluid in the burette will appear to be curved on the bottom edge of the level, this is normal(its called a miniscus),that your reading at the bottom edge of it. Now slowly fill the cylinder with your burette. As the cylinder becomes full, and your close to topping it off,allow any air bubbles to find their way to the vent hole and escape. Then, top the level off just to the bottom of your plate and not up the holes in the plate. Also check for leaks, you will know by now if you have any! Take the reading in cc's from your burette and write it down. You need to know the exact bore size of the cylinder down to the .001th. Use this formula to calculate the volume the bore should be without the dome present: .7854 x bore x bore x depth of piston in bore(your measured depth with depth mic). Now subtract your cc'ed volume from the calculated volume and this is your piston dome volume(Finally)! Now you can plug all those #'s into your compression formula and see what you have! PM me if you have additional questions, but this is fairly simple once you start getting into it....
  8. Bass
    Joined:
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    Bass Member

    Thanks guys...that makes a lot of sense to check it actually in the cylinder bore. I don't know why that didn't occur to me.

    Fred, I tried to use the search before I asked this question, but I guess I didn't enter the correct terms. That old post was pretty much exactly what I was looking for, thanks.


    OK, next related question. How much side clearance should I run with these heavy old cast pistons? I've read that there needs to be more clearance, but how much? I measured the bottom of the skirt at 3.870.
  9. Roothawg
    Joined:
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    Slackerville, OK

    Roothawg Member

    Tell us what you find out. I'm sure everyone here will be surprised.
  10. Tman
    Joined:
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    Tman Member

    BTTT, good tech here, bumped for others to see. Thanks guys!

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