All right, we've been through this before, but I guess I need to do it again.

Key to understanding early hemi and poly engines it that Chrysler decided to copy what was the GM practice at the time, in which each division developed its own engine line. So four separate engine families were developed, in which virtually nothing interchanged from one family to the other.

First Chrysler OHV V-8 came out in 1951.

Then De Soto in 1952.

Then Dodge in 1953.

And finally Plymouth in 1956.

They each have their own bore center-to-center distances and so cranks, cams, heads and blocks do not and cannot interchange one from the other, as they are not and cannot be the same length.

Chrysler, De Soto and Dodge engines were developed as Hemis. Later, poly versions of the Dodge and Chrysler engines were developed. There was never a De Soto poly.

Chrysler and Dodge poly engines can be converted to hemis by swapping heads and related items, but only Dodge to Dodge or Chrysler to Chrysler.

The Plymouth V-8 was developed as a poly engine. There was never a Plymouth-specific hemi engine (the later 426 hemi was used by both Plymouth and Dodge). Plymouth poly engines cannot be converted to hemis.

However: Because its own V-8 wasn't quite ready for the 1955 model year, all 1955 and some 1956 Plymouth V-8's were actually Dodge poly engines, and these can be converted to hemis by using Dodge hemi heads.

Also: The lowest-priced 1957 De Soto models used Dodge poly engines, and these too can be converted to hemis, using Dodge hemi heads.

And finally: There was a very limited run of HP 57 Dodge D-501's that had 56 Chrysler 300 engines; i.e., dual-quad 354 hemis.

(Clearly, by the mid-50's, Chrysler Corp. was already moving away from a strict each-division-gets-its-own-engine mentality.)

The original Plymouth poly of 1956 had 277 cubic inches. Fury models introduced late in the model year used Canadian 303-inch Plymouth poly engines, which were essentially a 277 Plymouth poly with a .060 larger bore and a longer 3.31 stroke. For 1957, Plymouth poly engine choices were 277, 301 and 318. The 318 was a larger-bore version of the previous year's 303 and used the same crank. The 1957 301 used a 318 bore size with a 277 crank.

Externally, 277's, 301's, 303's and 318's look identical.

(BTW, there was a 301 Chrysler poly in 1955, but it is a completely different engine than the 301 Plymouth poly.)

In 1959, Dodge phased out its old hemi/poly and began using Plymouth poly engines with a .040 larger bore to make it a 326 incher (the Dodge was a mid-priced car and needed a bigger engine than the low-priced Plymouth). The 326 was a one-year-only version of the Plymouth-designed poly motor. Next year the base V-8 for both Dodges and Plymouths was the 318 poly, with 361 and 383 big blocks optional.

So the Plymouth poly evolved into the corporate standard V-8, and was also the basis for the later small-block 273, 318, 340 and 360 wedge V-8's, all of which share the same bore center-to-center distance and other critical dimensions with the old Plymouth poly.

Here's a picture of the 277 poly in my 56 Plymouth Belvedere:



That clear things up??