[steve25]

Can anyone confirm or deny for me that these motors where made with flat top Pistons and no valve notches as I have read at times?

Also it seems another question for these mid 70s 400s and 455s would be if the 1976 motors where built with Pistons with valve notches and chamfers.

Any help would be great, and thanks in advance!

[Grand73Am]

Here's a factory 455 with block date of July 15, 1975.

001 by grand73am, on Flickr

[MarkS57]

FWIW, John Witzke's history of the W72 claims the W72 & the standard L78 400 used the same piston.

http://pontiacformula.free.fr/schema/W72.pdf

[Region Warrior]

I still have a wz out of a totaled TA(had 1100 miles) back in summer of 78.
Has valve notches and chamfers around pistons.
Been apart after bout 10k miles sitting in corner of garage since 83. Never sold it after realizing vin # was punched and restamped...

[track73]

Mine had Chamfered pistons with valve reliefs. I put #62 heads on it and the compression was around 9:1

[bluebandit]

My 78 W72 also has the flat tops with 4 valve reliefs and chamfers.

[PAUL K]

This is an original 79 TA 6.6 piston. I've built many 78 and 79 400 220hp engines and all the pistons are visually the same.

[77 TRASHCAN]

Originally Posted by PAUL K

This is an original 79 TA 6.6 piston. I've built many 78 and 79 400 220hp engines and all the pistons are visually the same.

What about the lesser HP version of the 400, like a 77 180 HP version???

[Cliff R]

I don't remember ever seeing different pistons in any of those engines. The big difference were the heads, 6X-4's vs 6X-8's, 92cc vs 101cc. Going by memory the later engines also used better blocks stamped XX and had the thicker mains instead of the thin 557 block variety......Cliff

[PAUL K]

Originally Posted by 77 TRASHCAN

What about the lesser HP version of the 400, like a 77 180 HP version???

Pretty sure they were the same. I've seen lots of them and they look identical.

I've never checked the numbers but next time I'm at the shop and think of it, I'll check. I'm sure I have some laying around.

[Jay S]

Photos of pistons of a late 75 400. If the link works that is...Looks like what Paul posted.

https://imagizer.imageshack.com/v2/3...921/AFazX8.jpg


I cc’d a set, same as the one in Paul K photo. But from a 77 W72. Been a little while but if I recall the valve releifs were 5cc and the bevel was just 2-3. The shallow valve reliefs with the bevel were about a wash with the older style 400s with deeper reliefs and no bevel. The measured compression height fairly tall though... 1.72. Higher compression than their speedpro replacements.

Interesting photo on the oem 455. Tiny valve reliefs.

[steve25]

Thanks all for the info and the photos!

I guess I should have asked about the general compression height seen also as mentioned in the last post here, because the change from a dash 8 head to a dash 4 does not seen to account for the added compression gain these motors where stated to have.

[Jay S]

This link has a ton of info on the W72 and the L78

http://www.originalho.com/1979TA/ima...2Tech_2006.pdf

According that links research the W72 engines blueprinted they were suppose to have a -.003 piston deck height and a .051 thick head gasket. I when I run compression numbers with the pistons the smaller vr pistons with the chamfer I come up with very close 8.5ish scr for the 6x-4 (93.74cc) and 8.0ish scr on a 6x-8 (98-101cc).

FWIW. The two W72s I have both had the piston down in the hole at .015. And IRC head gaskets were thinner than .051-.052. IRC, they were .044-.046. I probably still have them somewhere. I don’t remember the exact number. One of my engines was a low milage engine that had never been apart. My head cc’d at 93. The compression on my WA-4speed w72 calculated out to about 8.24.

[track73]

*During period dyno testing, the National Hot Rod Association rated the limited-availability T/A 6.6 high-output Pontiac 400 engine at 260–280 net horsepower, which was significantly higher than Pontiac's conservative rating of 220 hp.

*Wikipedia

[ponyakr]

"...Interesting photo on the oem 455. Tiny valve reliefs."

X2

I've never seen those before. But, if you think about it, with the HUGE 124cc chambers those late '75-'76 455 heads had, and the small 067 cam, it's not likely that any valve reliefs at all were necessary.

http://www.wallaceracing.com/enginesearch4.htm

http://www.angelfire.com/on/geebjen/heads.html

The '74 350 GTO engine had an 066 cam & much smaller chambers than the 455's, but no valve reliefs.

So, why would even those tiny reliefs have been needed. IMO, they were totally unnecessary. In fact, it would be interesting to know just how big of a cam could have been used, without needing any reliefs.

I'm just guessing that the '75-'76 455's were the only 455's that had those pistons. Assuming that all other 455's had the larger reliefs. For '76, NHRA shows a chamfer & notches. They don't even list a substitute piston, for the '75-'76 455. Probably because nobody has wanted to race a '75-'76 455, under NHRA rules.

http://www.classracerinfo.com/Engine...7&MAKE=Pontiac

For all non-SD455 engines, before 1975, NHRA shows that the common L2359F TRW pistons are legal subs for the other 455's.

I have a 500813 block. But I didn't see the pistons that came in it, from the factory. I suppose they looked just like those shown.

[Grand73Am]

Here's another 455, block date 10-18-71:

THE 72 455 by grand73am, on Flickr

[Cliff R]

I agree that 220 HP is a pretty conservative rating, even SAE Net at the back of the trans with all the accessories in place, etc. I've driven pretty new stone stock 78-79 TA 4 speed cars and they are no slouch and they respond very well to a few minor improvements making them even better in every area as far as engine power and vehicle performance is concerned. These include free-flowing exhaust, richer secondary metering rods, and a few degrees additional timing. One can really wake them up with a better cam choice. Those cams were really low lift from what I can remember and not taking full advantage of the head flow from the excellent 6X heads........Cliff

[Held for Ransom]

I'm posting pictures of pistons from Trans Am engines. A '75 400/4speed, 1978 400/auto and a '78 400/auto W72.

[Jay S]

Originally Posted by Held for Ransom

I'm posting pictures of pistons from Trans Am engines. A '75 400/4speed, 1978 400/auto and a '78 400/auto W72.

Hmmm....top photo is a w72 piston.

Bottom photo looks like a early 75, from one of the last 988 blocks?

Middle photo is what again? Out of what car???

Is this also a Warsak test?

[track73]

Originally Posted by Cliff R

I agree that 220 HP is a pretty conservative rating, even SAE Net at the back of the trans with all the accessories in place, etc. I've driven pretty new stone stock 78-79 TA 4 speed cars and they are no slouch and they respond very well to a few minor improvements making them even better in every area as far as engine power and vehicle performance is concerned. These include free-flowing exhaust, richer secondary metering rods, and a few degrees additional timing. One can really wake them up with a better cam choice. Those cams were really low lift from what I can remember and not taking full advantage of the head flow from the excellent 6X heads........Cliff

I can attest to what Cliff said. I put a Summit 2800 cam and headers along with his carb and some 4 tube headers and it was a hoot to drive.