Piston ID

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4spdragtop

4spdragtop

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Bought these, measuring 4.04, so std bore(no good for me)
I thought they would be marked "std"? Can't find markings anywhere other than 265-99 on the underside.
Weigh out at 721ish grams, pins measure .984 and weigh 154ish.
I've done some searching and look like Mopar Performance ??
Compression??
Thanks
Steve

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Either been polished off or they are 360 .040 over pistons.
 
They look like the old 340 TRW 12.5 pistons I used to run back in the day. I believe you are measuring in the wrong spot. The ring lands are smaller than the skirt diameter, so they may be oversize
 
Compression height is the distance from the middle of the wrist pin to the top of the flat part of the piston.

340 from 68-71 stock compression height with a 3.31 stock crank is about 1.81”
360 stock compression height with its 3.58 stroke crank is about 1.67”.

Measure the flat part of the piston to the top of the wrist pin bore and add 1/2 of the wrist pin diameter or (0.984/2 = 0.492”) to the above measurement.

This should give you an indication which stoke crankshaft the piston is for.
 
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Thanks guys, got pretty close to 4.070 with my verniers, and approx 1.785" comp height, so with my redneck capabilities call it the 1.81.
So .030 over 340. You would think they would put something besides just the 265-99 on it, but what do I know lol.
Thanks for the lesson,
Cheers
Steve
qkcuda said:
They look like the old 340 TRW 12.5 pistons I used to run back in the day. I believe you are measuring in the wrong spot. The ring lands are smaller than the skirt diameter, so they may be oversize
Click to expand...

Rocket said:
Compression height is the distance from the middle of the wrist pin to the top of the flat part of the piston.

340 stock compression height with a 3.31 stock crank is about 1.81”
360 stock compression height with its 3.58 stroke crank is about 1.67”.

Measure the flat part of the piston to the top of the wrist pin bore and add 1/2 of the wrist pin diameter or (0.984/2 = 0.492”) to the above measurement.

This should give you an indication which stoke crankshaft the piston is for.
Click to expand...

20241012_213116.jpg
 
Thanks Al, ideally 273 Commando lol. I think I'm good with the ones in the 340 now(I should calculate my compression lol). From what I've read the compression will be very high(too high for me)
I'll probably SALE these...isn't that's how it's said here? :lol:
RAT ROD AL said:
What pistons are you wanting ?
Click to expand...
 
I’m guessing that most of the markings were milled off some time ago .
But the diameter and compression distance will tell you most of what you need to know .
Those markings inside came from the forging die,,,,it tells you what forging blanks they were .
Judging from the hole for the fire pocket in the dome,,,they were fairly high compression originally .

Tommy
 
I picked up a set of 340 I think in my last parts pick. Haven't even opened the box to look at em yet. I'll have to figure out what they are the same as you I guess. Then maybe put em up for sail !!! :poke:
 
I would not use digital calipers if you want accuracy down to a few thousandths [ of an inch ].
Think about what is actually doing the measuring......
Just for reference, copy paper is about 0.004" thick.
 
I agree that they look like the old TRW 12.5:1 pistons. Comparing yours to Mr. Sinister's, though, it's pretty obvious yours had some custom work done. Other than the obvious mill marks, compare the plug pocket. Yours seems to travel further into the "valve" face of the dome, which is what I'd expect from material being removed there.

You could measure the depth of the valve pockets and compare them to what Sinister has to get a rough idea of how much was removed. Calculating actual compression? I don't know if I'd want to do the math involved finding the area of that area of the dome.
 
Yeah, I think I'll stick with the TRW pistons I got installed, even tho the ones I posted here would increase compression. I think I'm better off looking at a different converter(currently stock) and/or different rear gears(3.23).
Engine runs great and has been pretty reliable since figuring out distributor issue.
Carb/timing tweaking would benefit also.
 
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