[GoreMaker]

I swear back when I did a 1978 W72 400 many years ago, the stock pistons were dished. The pistons in this 1977 W72 are not dished:



However the bores are definitely 4.12", they haven't been worked in any way. In fact, I'm kind of surprised that they have exactly zero wear and zero ridge after 77k miles.

So am I just remembering wrong and these are in fact regular W72 400 pistons?

[PAUL K]

Those pistons look like correct W-72 pistons for sure.

[GoreMaker]

Great, thank you! I must be misremembering. It was almost 20 years ago when I last worked on a W72

[J.C.you]

Anybody know the appropriate ccs of the bevel?

[Rocky Rotella]

Pontiac began using pistons with the beveled circumference in 1975. As best I can tell, the bevel displaces approximately 3.3 cc on a standard bore 400-ci piston (4.12-inch).

[ponyakr]

This thread got me to thinkin about it.

The only factory dish pistons I remember seeing were 428 pistons. Seems they had a small dish in the center, so they could run 400 heads.

So, for you guys who know, what other Pontiac V8 engines had factory dish pistons ?

Did some 421 engines have dish pistons, so they could run 389 heads ? No experience with 421 engines.

[GoreMaker]

I looked at my old photos, my '78 W72 did not have dished pistons. I must be remembering the L67 3800 Supercharged V6 from my old '99 GTP.

However it did have different pistons with concentric valve reliefs:



No way to know how original that was. That 1978 W72 was definitely modified when I got it and there's a good chance the pistons had been part of that regimen.

[ponyakr]

Those appear to be the aftermarket "8-eyebrow" 400 pistons, which nobody like.

[GoreMaker]

That would explain why one of them split in half inside its bore, hence the teardown at the time. Mind you, I beat on that thing for over 100k miles before it finally broke.

[Jay S]

Speed pro use to make a replacement piston for those pistons. The part number was L2440F. The total cc’s for the bevel and valve reliefs was listed as 10.1 cc. The OEM piston forged replacements the compression height was about .006” shorter than the original pistons. I have 2 sets of pistons from 77 W72 4 speed cars and the compression height I measured was 1.72” instead of 1.714”.

I use to think that those pistons were introduced when the 500557 blocks were introduced. But I have a 1975 or 76 400 YZ from a Firebird and it has the earlier style pistons with out the bevel. It is kind of a weird block, it has 75 cast as the year but it the Julian code says Jan of 76. I assume the car was a 75. I thought all 500557 block had the pistons with the bevels until I took that one apart. Still getting schooled…

[MarkS57]

Originally Posted by ponyakr

Did some 421 engines have dish pistons, so they could run 389 heads ? No experience with 421 engines.

Often wondered the same thing...

[J.C.you]

Originally Posted by Rocky Rotella

Pontiac began using pistons with the beveled circumference in 1975. As best I can tell, the bevel displaces approximately 3.3 cc on a standard bore 400-ci piston (4.12-inch).

Ok thanks, I’m doing a nice standard bore 75 block, with those pistons, with some 13 heads. Figuring out what I need to do for a pump gas application

[77 TRASHCAN]

What would the non W72 pistons look like???

[74Grandville]

Originally Posted by 77 TRASHCAN

What would the non W72 pistons look like???

The Same. the difference was in the heads/cam not the pistons

[KEN CROCIE]

In the 60s,Pontiac had " export pistons" with a dish. 389s definitely, 421s not sure. C/R was 8.6:1

[MarkS57]

Originally Posted by KEN CROCIE

In the 60s,Pontiac had " export pistons" with a dish. 389s definitely, 421s not sure. C/R was 8.6:1

Like these, early 60's 389's were available for regular gas. 8.6:1. I've looked at 421 pistons and they appear to be flat head 4 valve relief like a premium fuel 389..

https://www.ebay.com/itm/25491660366...UAAOSwUuFW10Hm

(BTW, they've been for sale forever).

[FrankieT/A]

Originally Posted by 77 TRASHCAN

What would the non W72 pistons look like???

Exactly the same...

[tom s]

AS I remember 62 had the dish option pistons,after they went with 2 diff chambers sizes much like 350-400 48 heads.Tom

[MarkS57]

Originally Posted by tom s

AS I remember 62 had the dish option pistons,after they went with 2 diff chambers sizes much like 350-400 48 heads.Tom

Hi Tom, most 389's in 62 had the 4 valve relief pistons, I know as my 1st engine rebuild was a 1962 389 in a Bonneville 'vert my aunt bought new; the 303 HP 389 so common back then. Even the 2bbl 389's in Cat's had the 4 valve relief pistons and 10.25 compression; my granddad's car; no joke! I'm still questioning those 421's though, with the same heads, the CR would be much higher. Maybe that's why the early 421's were so nasty. So it seems the 64 GTO and 421 HO had the same casting # heads?

[tom s]

I think there was a 8.6 economy 2 bbl engine with dished pistons and a smaller 2 bbl carb.Will try to look it up tomorrow.Tom