273 Commando Piston Dome Volume

Discussion in 'Small Block Mopar Engine' started by Alpine White, Aug 5, 2018.

  1. Alpine White

    Alpine White Member

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    Hey Guys,

    I’ve recently purchased a new set of Rebuilders Choice 4bbl Commando Pistons and need the dome volume in cc’s to calculate my CR. I’ve contacted Rebuilders Choice but they are unable to provide the information but said they made them from factory 273 Commando Pistons.

    I’ve attached a photo of my one of my new pistons and a photo I found on the net of an original Commando piston.

    Any help would be greatly appreciated.

    Cheers guys

    Luke

    52A56A73-D6D1-4894-AA63-BE93331BE67C.jpeg

    430045CF-DC8D-4B16-B463-B072829A7F79.jpeg

    A9015F9C-9CCB-4C59-BBDC-311DD1FC02EB.jpeg

    9D073D55-E905-43FE-92FC-B807E5C81309.jpeg
     
  2. toolmanmike

    toolmanmike FABO Staff Staff Member FABO Gold Member

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    I have no answer to your question but you are lucky. Egge's were the only piston available when I rebuilt my 273 10+ years ago. That info wasn't available for me either. I got a cc kit and measured with a piston installed.
     
  3. Plymouth 65

    Plymouth 65 Floorable Deplorable

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    Stock Commando pistons and heads were 10.5 to 1 compression ratio. 65'
     
  4. Alpine White

    Alpine White Member

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    I’ve been really lucky I think. Parts are very expensive here in Australia. I purchased a full engine rebuild kit out of the States for the price that I would have paid for pistons and rings here in Australia.

    I’ve been reading anywhere from 15-11.5cc’s for the Commando Pistons. I read a guy cc’d a set and they came in at 11.5 but just wanted to be certain.
     
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    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      Yes but the op is looking for the dome volume so he can figure his ratio.
       
    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      I like the looks of these pistons but I would think it might be a good idea to smooth the tops to prevent detonation. Maybe @yellow rose, or @rumblefish360 can chime in with a recommendation. What say you @RustyRatRod, or @krazykuda ?
       
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      • yellow rose

        yellow rose Doctor of Thinkology.

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        The only way I know to know for sure is to do the .500 downfill and then you'll know for sure what you have. I do have an old DC manual around here somewhere. I'll dig it out and see what I can find.
         
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        • toolmanmike

          toolmanmike FABO Staff Staff Member FABO Gold Member

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          That's the procedure I used.
           
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          • krazykuda

            krazykuda FABO Gold Member FABO Gold Member Technical Editor

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            Use this procedure with the .500" down fill to find the true compression of your combo...

            How to Check Compression
             
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            • Alpine White

              Alpine White Member

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              Thanks for the response guys, it’s appreciated. Does anyone know what cc’s the factory Commando pistons were? There is so much literature on the net in regards to practically everything else bar that.
               
            • toolmanmike

              toolmanmike FABO Staff Staff Member FABO Gold Member

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              Hey Karl, please read post #6 then look at the rough cast on the tops of the new pistons.
               
            • toolmanmike

              toolmanmike FABO Staff Staff Member FABO Gold Member

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              I really wish I would have checked before I took mine apart.
               
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              • rumblefish360

                rumblefish360 so close yet so far away

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                That is the weirdest piston top I have ever seen, like a kit, finish to suite the need.
                 
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                • toolmanmike

                  toolmanmike FABO Staff Staff Member FABO Gold Member

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                  It sure looks sand cast. I would smooth them and then cc the chambers.
                   
                • Wyrmrider

                  Wyrmrider Well-Known Member

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                  we can walk you through the .500" downfill method if need be- oh I see a link but have not looked at it
                  closed chamber heads?
                  agree on polishing those piston heads
                  original piston would not have been coated in 65- what you got there?
                  you might want to od hone those pistons or even give them a light knurl on the thrust side, run a bottle brush or brake hone through the pin bores- anything to hold some oil- lightly
                   
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                  • Alpine White

                    Alpine White Member

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                    My mechanic as pretty switched on so can work it out. RC said they made them from factory 273 Commando pistons so was curious as to what the factory specs were. I did read on a thread somewhere that a guy did cc the pistons I have and they came in at 11.5.

                    Yep, closed chamber heads. It’s a 1965 Australian built AP6 4PV factory V8 car. We missed out on the 4bbl Commando version here in Australia so am building it to as close a spec as I can. I have an Edelbrock LD4B, Edelbrock 500cfm 4bbl Thunder Series carby, and have had my cam reground to a similar grind, bit more duration. Components are going away to be balanced this week then a dummy assembly to chase around 10.2-1. I’ll attach a photo of the car and a photo of the cam specs.

                    B0E2B27D-E8D6-484E-82BC-6358F704B597.jpeg

                    445DDC1F-0657-40C1-8190-B1F32DB2FF28.jpeg
                     
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                    • perfacar

                      perfacar Well-Known Member

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                      the dome volume is 16 cc. this is from TRW and ross and other dealers. NHRA specs show the dome volume. it's in engine specs on their website.
                       
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                      • Alpine White

                        Alpine White Member

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                        Really? Wow that’s a lot more than the 11.5cc I read elsewhere. Will have to measure them up as that’s a lot of variance.
                        Cheers mate.
                         
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                        • krazykuda

                          krazykuda FABO Gold Member FABO Gold Member Technical Editor

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                          If you run the book numbers for the compression, they do not work out with the advertised compression... That's why I came up with the method to check your combination vs going by book numbers...
                           
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                          • krazykuda

                            krazykuda FABO Gold Member FABO Gold Member Technical Editor

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                            Doing a clean up cut on the top of the pistons will help a little, but I do not think that it is absolutely critical... I don't see much sharp edges that can become hot spots... That little bit of 'porosity' shouldn't hurt it...
                             
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                            • krazykuda

                              krazykuda FABO Gold Member FABO Gold Member Technical Editor

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                              The Edelbrock LD4B is made for the 66 and later head bolt pattern.... The D4B is the one made for the early 64 & 65 heads with the unique 5/16" bolt pattern...

                              I would recommend using a set of 66 or 67 heads instead to be more compatible with the intake bolt pattern, or you could enlarge the intake bolt holes in the manifold so the bolts will go in properly... But if you want to keep the intake manifold virgin, then I would go with a set of 273/318 heads from 66 or later... 66 & 67 will have a little smaller combustion chambers to help keep the compression up.... The edelbrock intakes are getting harder to find and it may be easier to find compatible heads for the LD4B or you will have to modify the intake bolt holes to use the LD4B on the early 65 heads...
                               
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                              • Alpine White

                                Alpine White Member

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                                Definitely no sharp bits, I’ll post a better photo.

                                EBBC5C40-1322-40D5-9D14-098394719974.png
                                 
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                                • Alpine White

                                  Alpine White Member

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                                  I’ve had 8 heads crack tested (4 x closed chamber & 4 x open chamber) and they’ve all come back cracked, either through the combustion chamber or around the two end bolt holes closest to the exhaust manifolds. I’ve just payed for a NOS set of closed chamber heads, which is what I was wanting anyway to chase the CR of the Commando. I know I’m going to have to alter the bolt angles on my LD4B but I’m ok with that, as you said these Edelbrock manifolds are getting hard to come by and these cracked heads have put a real dint in my budget. I appreciate you taking the time to reply to me, it’s really appreciated mate.
                                   
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                                  • krazykuda

                                    krazykuda FABO Gold Member FABO Gold Member Technical Editor

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                                    That sounds like a good way to go with what you have and modify the intake bolt holes to fit the older style heads...
                                     
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                                    • Alpine White

                                      Alpine White Member

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                                      I’ve had my cam reground with this grind, should hopefully wake her up a little with the 500cfm Edelbrock on top of the LD4B.

                                      84723464-819B-4C37-97CE-946A7300256D.jpeg
                                       
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