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Old 04-18-2020, 01:57 PM   #21
Russ/40
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Default Re: 24 stud crankshaft in a 21 stud engine

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Originally Posted by flatheadmurre View Post
Have you testfitted them in the block yet ?
Hopefully you donīt even need to machine the full 1/16...
Any shop with a CNC Lathe around ?...then you can machine the pistons...maybe easier then machining the heads.
First: I am currently half way through another build, so although I have the block and rotating assembly for the build ( less the rods ) we are discussing, I havent even cleaned the block yet. I am in the theoretical stage as to the pop-up motor.

I know cutting the pistons would be the easy way to go, but i'm thinking there could be some performance gain with the pop-up approach although it probably be minimal. I can sculpt the combustion chamber for good squish, and maybe better flow. Certainly this has been done before. I think Navarro had heads for this. Which tells me there had to be a reason for doing it.

Gary, the pistons I have that are .125" over are Egge's EP-1091 which spec the CH as 1.625" thus the interest in a pop-up piston configuration. Also, the deck has never been cut, and I usually don't unless absolutely necessary. I have 21A rods, but they all need to be worked, and I know their lengths are all over the place as they are.

Thinking out loud, as this is the one engine I have wanted to do, have never attempted before, and will probably never do again, if I do it. Clearly i need the benefit of experienced advice. I appreciate all the help thus far.
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Old 04-18-2020, 02:50 PM   #22
GOSFAST
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Default Re: 24 stud crankshaft in a 21 stud engine

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Originally Posted by Russ/40 View Post
Gary, the pistons I have that are .125" over are Egge's EP-1091 which spec the CH as 1.625" thus the interest in a pop-up piston configuration. Also, the deck has never been cut, and I usually don't unless absolutely necessary. I have 21A rods, but they all need to be worked, and I know their lengths are all over the place as they are.
Hi Russ, never had a Flathead piston with that C.H. You need to call Egge and ask if they are including the dome in that dimension?? Sounds like they are but it shouldn't be configured that way!

We use Ross pistons regulary, they consider the C.H. from the center of the wrist pin hole the very corner of the pistons, this is the "deck". Their domes add .188" to the total hgt.

In short, we use 1.436" C.H. pistons not counting the dome hgts. This is for an OEM 4.000" stroke.

Thanks, Gary in N.Y.

P.S. Am going to keep my posts a bit short, still in the middle of some major health issues, half-way out of the woods so far!
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Old 04-18-2020, 03:27 PM   #23
JSeery
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Default Re: 24 stud crankshaft in a 21 stud engine

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Am going to keep my posts a bit short, still in the middle of some major health issues, half-way out of the woods so far!
Hope your health progress stays that way!
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