[blackadder]

I cannot seem to find an answer to this question anywhere, at least where I have looked. In a Pontiac engine are the wrist pins pressed or floating or is it based on bore or manufacturing date? Any help or leads would help.

[PAUL K]

Pontiac used pressed pins in just about everything. It is the connecting rod that determines if the pin is pressed of floated. However the piston most have a way to retain the pin if a "floated" rod is used.

[AG]

All stock rods use pressed pins.

[blackadder]

Thanks. I noticed that Eagle had the option of either pressed or floating and was curious. I am planning a 421 build and I havnt tore down the engine yet but I expect there is not grooves for C clips in the pistons. Also another question if I may. The bore of a 421 and 389 are the same at 4.063. What are the difference in pistons or are they interchangable? Is skirt length or crown design different enough to make them non interchangable? I know that 400 and 428 pistons although the same bore have different crowns with 428 being dished. Also thoughts on Eagle rods as options here are quite limited.

[PAUL K]

Originally Posted by blackadder

Thanks. I noticed that Eagle had the option of either pressed or floating and was curious. I am planning a 421 build and I havnt tore down the engine yet but I expect there is not grooves for C clips in the pistons. Also another question if I may. The bore of a 421 and 389 are the same at 4.063. What are the difference in pistons or are they interchangable? Is skirt length or crown design different enough to make them non interchangable? I know that 400 and 428 pistons although the same bore have different crowns with 428 being dished. Also thoughts on Eagle rods as options here are quite limited.

Eagle rods are usually a good choice. 389 and 421 pistons have different compression heights (placement of the wrist pin relative to the piston top) they won't interchange.

[firechicken]

389 and 400 have 3.75 stroke.
421 and 428 have 4.00 srroke.
Rods are all the same length so it's the piston that is changed to make up the difference.

[77 TRASHCAN]

If the Pistons you intend to use do NOT have the groves machined in them for floating rods,

AND you intend to use floating rods (there's benefits to floating rods)
You will need time find someone that can machine the grooves in your intended use pistons.

I say all this...not many shops can or will perform this type of work these days!!!

[b-man]

Along with the above posted information the 389 and 421 do not share the same bore size.

The 421 is 4.093” bore with the 389 being 4.062”.

Another difference is the large valve reliefs used in 421 pistons to lower the compression so the same heads could be used on both 389 and 421 and maintain the same compression ratio, in the same vein as dished 428 pistons versus flat top 400 pistons. The 389/421 heads used either 1.92” or 1.88” intake valves depending on engine options and valve lift was .406” so there’s no reason other than adding volume to lower compression for the larger 421 valve reliefs.

Original factory 421 and 389 pistons for comparison, both are from 1964 engines.

[blackadder]

Thanks for the info. I was using this as my source for the bore sizes. I expected that there had to be some difference and the pin to dome hieght was the only difference I could think of if bores were the same. Thanks again.
https://sites.google.com/site/kensau...c-engine-sizes