318 Pistons: baffled

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swing69

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New thread, just to clean up the topic.

Engine: 1968 318. thought to be original, turns out, its a replacement 318 with no vin stamping on the pad. From 1968 Dodge.

Now that I've cleaned up the parts abit; quick caliper measurement....not micrometer.
Crank MB journals measures: 2.495 crank bearings, good, no oversize markings.
ConRod measures: 2.123 Spec. 2.125. no oversize marks on bearings. bearings down to copper.

PISTONS: 2 "look" different....slightly different machining finish, lighter in color. no number on piston tops. Other 6 marked: 2899446 OS Thats the one on the right....

Quick check on the bore looks to be 3.930 (.020 over) gotta measure......

here are picts of the pistons removed.

Also shown is the 285 NP purchased piston, in std. size. (now that I know I'm over.....its gotta go back anyway. :( what the hell! no valve reliefs! may be a truck piston in the wrong box! more research needed.

Anyone have a clue what could have happened? Previous owners no help.....

Still trying to decide what to do........
 

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3.930 is .020 over.

Make sure when you get the next set to get the tallest compression height. There are several.
 
If there are no identifiers on the block, usually a stamp somewhere that identifies the bore being oversize from the factory, the engine has been rebuilt at some point in its life. There have been several after market suppliers over the years. My best guess on the piston differences are that either they're different runs from same supplier or a mix from different vendors.
 
Someone else just got that same flat top piston in another thread. If it is a truck piston, the compression height will be in the 1.650" range, versus the 1.720" range for the 285NP. OH, that is YOU LOL

BTW, that cross hatch pattern on the side.... I have seen that on Silvolite /6 pistons before.
 
The piston on the right in the second picture looks exactly like the pistons I found in this 30 over 318 I'm in the middle of.
Right down to the tapered edge, valve reliefs, and notch.
 
6 of one brand, six of another....not a great sign! lol. yup: my piston from the other thread. I figured and new post was in order, since I was straying from the OP.

This thread is about the existin pistons, and mismatch of parts! I need to get my good micrometer and measure everything as accurately as possible.

Question: the 2899446 OS marking; anyone see that before? Usually the aftermarket oversized are stamped with the size .020, or whatever.
 
A caliper is not very effective making really accurate measurements on pistons. It's real easy to get them on crooked. The 318 standard bore is 3.910. Pistons always measure smaller than the bore size in order to make the clearance necessary. So a standard piston would probably measure around 3.907 or so.
 
RustyRatRod said:
A caliper is not very effective making really accurate measurements on pistons. It's real easy to get them on crooked. The 318 standard bore is 3.910. Pistons always measure smaller than the bore size in order to make the clearance necessary. So a standard piston would probably measure around 3.907 or so.
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Thanks RRR. I'll have my mic's in on Friday.....if I remember....and get some good measurements of the pistons.
 
swing69 said:
Thanks RRR. I'll have my mic's in on Friday.....if I remember....and get some good measurements of the pistons.
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You want to measure them 90* from the pin up high on the skirts.
 
forget the past and the drama of the old pistons. measure bores and hone and move on. get the tallest pistons and what size cam determines need for notches. if u do not need notches the better, no notches = more compression
 
BTW, OP, I see in the SilvoLite catalog that they make some flat top 318 pistons with a higher 1.741 compression height than the flat top truck piston. Again, the crosshatch pattern says Silvolite to me. They may have sourced some to FM or they got in the wrong boxes. Were these the $7 ones?

Go to this link and look at page 23:

www.uempistons.com/catalogs/silvolite_catalog.pdf

BTW, the Sterling pistons are an older company that FM absorbed in the 60's; see Wikipedia.
 
There are also some 1.745 compression height offered. Splittin hairs really, but that's the tallest I have seen.
 
stock 8.5:1 car pistons had 4 valve reliefs and offset wrist pins and 1.759" comp distance, same as L2329F speed pro
 
Marcohotrod said:
floating rods are better and easier for do it yourselfers, admit it Rusty-you missed it
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No I saw them.......I would swap for pressed rods so quick it would make your head spin.

I'm not in this for a contest. I am trying to help.

Are you going to tell us where to find those pistons you recommended, or is that a moot point because they are obsolete?

Cause here's what I get.

http://www.summitracing.com/parts/slp-l2329f
 
would you swap for the heavier later pressed rods, cuz the 1964-71 1618699 lighter 273-318 rods only came floating. and I do not have any L2329f+20 pistons, but summit is not the only piston source in the world
 
and if u saw the pistons u suggested need different rods and maybe a rebalance, and are really helpful, u would have told the OP that your suggested pistons would not hook right up to his rods
 
Marcohotrod said:
would you swap for the heavier later pressed rods, cuz the 1964-71 1618699 lighter 273-318 rods only came floating. and I do not have any L2329f+20 pistons, but summit is not the only piston source in the world
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I sure would. In a flat second. Because it opens up more choices for pistons.
 
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