Can you identify these pistons?

Projectile Dart · Nov 11, 2021

I got these pistons but I am not sure what they are for. I can't find much online
The box says they are 0.030" oversize and I measured them at 3.910"
Based on what I saw online, that would be the standard size for an LA 318 but I haven't found anything that would have been 0.030" smaller than that stock size
Also, the small dish plus the weird valve placement makes me think they might be for a poly engine? I am not sure on that...
What do you think?

RustyRatRod · Nov 11, 2021

Those are polysphere engine pistons. Closest I can find is the 313 poly engine from the mid to late 50s. It's virgin bore was 3.875. .030 over would put it 3.905, just .005" shy of 3.910, so that has to be your boy.

Looks like the 313 was primarily for Canadian export.

Locomotion · Nov 11, 2021

Are you measuring at the ring lands or skirts?

PRH · Nov 11, 2021

The area above the ring lands is usually way smaller than the skirts/bore.

Projectile Dart · Nov 11, 2021

Locomotion said:
Are you measuring at the ring lands or skirts?

This is the first time I had ever measured a set of pistons and was unaware that they were smaller at the top
I re-measured towards the bottom of the piston and I was getting 3.935"

RustyRatRod · Nov 11, 2021

Projectile Dart said:
This is the first time I had ever measured a set of pistons and was unaware that they were smaller at the top
I re-measured towards the bottom of the piston and I was getting 3.935"

Ok, then that's .030 over from the stock 318 3.910 bore. Still for the poly engine, though. Look at the valve reliefs.

RustyRatRod · Nov 11, 2021

Locomotion said:
Are you measuring at the ring lands or skirts?

Good eye, Myron! I missed where he was measuring from!

Dubob · Nov 11, 2021

Here’s what I’ve got from my old books. Nylen 952P interchanges to 2268P sealed power. The hand written P260 (safeguard brand)uses the same rings as the 2268P. 1963-1968 premium engine and 1963 standard with hydraulic tappet 318. All I’ve got.

Dubob · Nov 11, 2021

2268P Dodge truck.

Bewy · Nov 12, 2021

The OPs pistons look like they are cast; the 2268 is forged.

RustyRatRod · Nov 12, 2021

Bewy said:
The OPs pistons look like they are cast; the 2268 is forged.

Correct. The pistons in the OP are indeed cast.

Dubob · Nov 12, 2021

Correct his are cast. That’s the issue with the interchanges from one brand to another. They were forged from Dodge but once you started getting into after market different companies produced parts that would “work” but were cheaper, to make them appealing to peoples wallets, that’s how places like JC Whitney were able to sell cheaper parts. They fit but the durability wasn’t there, much like stuff today. In our old auto parts store we always kept the higher end parts.

Projectile Dart · Nov 12, 2021

Thank you to everyone who helped! Mystery solved, much appreciated

RustyRatRod · Nov 13, 2021

Projectile Dart said:
Thank you to everyone who helped! Mystery solved, much appreciated

What are you going to do with them?

Projectile Dart · Nov 14, 2021

I have a '58 dodge in my possession that has a poly engine that I believe is a 318cu. I will confirm if it is and if so then these will be shelved for that motor. If it isn't then I will have no use for them.
Just an idea, any idea of what value they might have in case someone was looking? I wouldn't even know how to begin to price them. They have rings and wrist pins (with snap rings) already installed

Swinger 340 · Nov 18, 2021

Projectile Dart said:
I have a '58 dodge in my possession that has a poly engine that I believe is a 318cu. I will confirm if it is and if so then these will be shelved for that motor. If it isn't then I will have no use for them.
Just an idea, any idea of what value they might have in case someone was looking? I wouldn't even know how to begin to price them. They have rings and wrist pins (with snap rings) already installed

I call them floating pins, as in the 340's. Had a factory 340 once throw a snap or maybe it was not put on at the factory, ring off a floating pin and left a nice track in the cyl. wall. Never did find the snap ring.

65 Cuda 340 · Nov 21, 2021

Projectile Dart said:
I have a '58 dodge in my possession that has a poly engine that I believe is a 318cu. I will confirm if it is and if so then these will be shelved for that motor. If it isn't then I will have no use for them.
Just an idea, any idea of what value they might have in case someone was looking? I wouldn't even know how to begin to price them. They have rings and wrist pins (with snap rings) already installed

58 Dodge would have a Dodge poly motor, 325 cubes (last year for the Dodge poly, by the way). Completely different motor than the 318 poly, which initially was exclusively a Plymouth motor, then became the corporate small block by 1960.

Pretty easy to tell the difference. Dodge poly intake does not function as the engine top cover, Plymouth poly intake does. Also, the Dodge poly valve covers are evenly (symmetrically) scalloped; Plymouth poly valve covers are kinda forward/backward leaning (depending on the side). Also, the Dodge poly water pump housing is a separate casting from the timing cover, the Plymouth poly water pump housing is part of the timing cover casting.

The 325 Dodge poly is a long stroke motor, bore and stroke is 3.69 x 3.80. Plymouth 318 poly is 3.91 bore, 3.31 stroke. Pistons pretty obviously won't interchange.

Dodge poly:

Plymouth poly:

Bewy · Nov 22, 2021

58 Dodge Custom Royals were sold here by Chrysler dealers & came with a 350 BB.
59's had the 361.

65 Cuda 340 · Nov 22, 2021

I should clarify that IF a 58 Dodge had a poly motor (and the OP says his 58 Dodge motor is a poly), it would have been a 325 Dodge poly. Came in Coronets only, which at that time was the low line model. US Royals and Custom Royals all had 350 inch big blocks standard, while D500's (including Coronets) had 361's..