Packard Forum Index » V-8 (1955-56)

I have a 1955 Packard 320 V8, which needs a rebuild. I also have a near perfect set of 352 standard pistons. What problems will I run into if I try to bore the 320 out to 4" and convert the 320 to a 352 and use the 352 pistons?

Shouldn't be any problems at all. Packard only cast one V-8 block, from which they bored the 320, 352 and 374 displacement engines. Are your 352 pistons originals?

IMHO the 352 pistons are original. They were removed from a low mileage 1955 Patrician engine which I had to bore out to 20 over and get new pistons.

Have someone blueprint the engine to make sure. Although Packard had only one size block externally they may have used different cores when casting a 320 vs a 352. And don't forget the cranks are different and there is a reason for that. 3/16ths of an inch is a hell of a lot to bore. Blueprinting will verify whether that is feasible.

Randy Berger, Thanks for the reply. How is the crank different? The throw is the same if I remember the specs correctly (my manual is with the car about 40 miles from here).

I don't know the specifics, but they carry a different part number from the 352-374 crank.

Greetings, Mr. Pushbutton,

You are the acknowledged expert on the pushbutton Ultramatic, but I cannot agree with your Packard V8 block info. The 320", 352" and 374" blocks are each completely different blocks, with different casting numbers. A 320" can potentially have serious problems if bored to 352".

I have sonic-tested many Packard V8 blocks and found cylinder walls as thin as .200" in some spots. Boring a 3-13/16" 320" to the 4" 352" bore requires taking out a minimum of .093" of this, leaving only .107". The engine will start, run and maybe last, but there are dangers of overheating, cylinder wall flex and even failures, should there be any corrosion weakening the cylinder walls from inside the water jacket. The 352" blocks are still very common and no reason to take a risk on boring a 320".

The August 1955 Hot Rod Magazine article by Racer Brown goes into great detail on this and recommends a maximum overbore of 1/8" (.125")on any Packard V8.

Yes, back in the bad old days, some racers bored the 352" a full .250" oversize, but that was before we knew any better. Current experience shows the better ring seal and power output from strong cylinder walls is preferable to the few more cubic inches gained from an overbore which would weaken the cylinder walls.

Finally, the 320" crank has much smaller counterweights and cannot be successfully used with the heavier 352" pistons. Get a late 1956 352" and start from there. It is a much better engine, anyway, with many improved oiling system parts and valve train parts.

thnx, jack vines

Excellent advice Jack. Excellent. The 1/8 inch overbore limit is not just for Packard V8's but is a good general rule-of-thumb to be applied to any engine including air cooled.

Dear Jack Vines, Many thanks for your expertise. Needless to say I have given up on the conversion idea. I could always use the 320 block as a boat anchor (if I had a boat).

Jack--thanks for that correction, I was not aware of that. You have certainly spent time in the trenches enough to know.