[android 211]

Pontiac installed the beveled pistons in the 75 and 76 455s. They went into a lot of Trans Ams, Grand Villes and station wagons.

[PAUL K]

Originally Posted by android 211

Pontiac installed the beveled pistons in the 75 and 76 455s. They went into a lot of Trans Ams, Grand Villes and station wagons.

I have disassembled dozens of those engines and never seen a factory beveled 455 piston. They have less than 8:1 compression with a flat top as it is. Do you know any specific applications or block codes they were used in?

[ts3189]

Doug,slap a set of low compression heads on it and send it. You already know you will be building something else!

[Jay S]

Good point Paul. I think all the beveled pistons I have seen in 455s are aftermarket cast oversized. They show up a lot in stock reman’s.

I was thinking Lee Atkinson on the forum had a OEM set in a 455 with the beveled pistons. But I was mistaken, his are aftermarket oversized with that beveled. FWIW, he has very low compression with his 66 heads and a bigger cam with tri-y headers, and has done multiply pulls on his wheel dyno with it. It runs well, if it is just temporary anyway, the 66 head can work too. Lee’s makes over 300 at the rear wheels.

[MDGOAT69]

Originally Posted by android 211

Pontiac installed the beveled pistons in the 75 and 76 455s. They went into a lot of Trans Ams, Grand Villes and station wagons.

Originally Posted by PAUL K

I have disassembled dozens of those engines and never seen a factory beveled 455 piston. They have less than 8:1 compression with a flat top as it is. Do you know any specific applications or block codes they were used in?

Originally Posted by Jay S

Good point Paul. I think all the beveled pistons I have seen in 455s are aftermarket cast oversized. They show up a lot in stock reman’s.

I was thinking Lee Atkinson on the forum had a OEM set in a 455 with the beveled pistons. But I was mistaken, his are aftermarket oversized with that beveled. FWIW, he has very low compression with his 66 heads and a bigger cam with tri-y headers, and has done multiply pulls on his wheel dyno with it. It runs well, if it is just temporary anyway, the 66 head can work too. Lee’s makes over 300 at the rear wheels.

I really just don’t understand why these pistons exist with the bevel, these 455s were already low compression so why lower it anymore on a rebuild?

[John Milner]

My '76 WX 455 from a 4 speed Trans Am had factory beveled pistons. I think I have those stored away. I'll post up a picture of one this weekend if I can locate them. My '75 YW 455 has them as well. It is a low mileage standard bore engine from a Grandville.

[Murf]

Originally Posted by MDGOAT69

I really just don’t understand why these pistons exist with the bevel, these 455s were already low compression so why lower it anymore on a rebuild?

I always assumed they were aftermarket & the bevel was to reduce the comp gain from the overbore. Of course,I’m usually wrong but I could be right today cause it’s my birthday! 😁

[25stevem]

I do not know what to believe in terms of 455 pistons with chamfers.
In 75 I have info that shows the 200 hp 455 as having a flat with valve notch piston and a 121.5 cc head.
The stated factory compression was 8.05.

In 76 it shows the 455 with the same head CCs and then the piston also has a chamfer and yet the same compression of 8.05 is stated, and both of these motors used the same .045” head gasket.

[John Milner]

Here is a standard bore piston from my 76 WX 455. It is the piston on the right. The piston on the left is from a 78 400. Both pistons are stamped “GM” inside the skirt. My 75 455 has the same pistons as the 76.

[PAUL K]

Originally Posted by John Milner

Here is a standard bore piston from my 76 WX 455. It is the piston on the right. The piston on the left is from a 78 400. Both pistons are stamped “GM” inside the skirt. My 75 455 has the same pistons as the 76.

I have recently disassembled two 1976 WX code engines and they both had flat tops. Is there a part number cast in the underside of the 455 piston? Do you know which heads were on that engine originally?

[PAUL K]

Originally Posted by Murf

I always assumed they were aftermarket & the bevel was to reduce the comp gain from the overbore. Of course,I’m usually wrong but I could be right today cause it’s my birthday! 😁

HAPPY BIRTHDAY!!!

[PAUL K]

Originally Posted by 25stevem

I do not know what to believe in terms of 455 pistons with chamfers.
In 75 I have info that shows the 200 hp 455 as having a flat with valve notch piston and a 121.5 cc head.
The stated factory compression was 8.05.

In 76 it shows the 455 with the same head CCs and then the piston also has a chamfer and yet the same compression of 8.05 is stated, and both of these motors used the same .045” head gasket.

Steve I thought I've seen literature stating the 75 & 76 engines having 7.6:1 compression ratio... Where are you seeing 8.05? I'm wondering if the 455's topped with heads other that the 6H used beveled pistons.

[Joe's Garage]

Originally Posted by Murf

I always assumed they were aftermarket & the bevel was to reduce the comp gain from the overbore. Of course,I’m usually wrong but I could be right today cause it’s my birthday! 😁

H
A
P
P
Y

B
I
R
T
H
D
A
Y
!

[Murf]

Thanks Joe. It’s pretty happy so far. Just got a free meal & a couple cold ones at the Pontiac Uprising in Great Bend Ks.

[John Milner]

Originally Posted by PAUL K

I have recently disassembled two 1976 WX code engines and they both had flat tops. Is there a part number cast in the underside of the 455 piston? Do you know which heads were on that engine originally?

I will take a look and see. This engine had 6H heads, still have them. I knew of the car it came out of and the engine had never been into to my knowledge. It still had the GM bearings in it. My 75 455 that has this style of pistons came out of a 1 owner Grandville that some older people sold to a local salvage yard. It also had the factory GM bearings, factory cam, etc. It had 5I heads. I did a cheap hone/bearing job on it over 10 years ago. Maybe Pontiac used both style of pistons in 75-76.

[Stan Weiss]

Originally Posted by PAUL K

Steve I thought I've seen literature stating the 75 & 76 engines having 7.6:1 compression ratio... Where are you seeing 8.05? I'm wondering if the 455's topped with heads other that the 6H used beveled pistons.

Paul,
My old Chilton shows '75 and '76 as 7.6:1

Stan

[John Milner]

Here is a picture of the part number for the beveled edge 455 Pistons. Appears to be 497044.

[25stevem]

Paul my info was from McCathys first book and if his compression listed is from the factory then we know that’s always .5 higher then reality so your 7.6 is likely right.

My other info in terms of pistons list part number 499052 for all 455 motors from 75 to 76.

1970 to 74 other then the SD motor used number used 485804 pistons.

It’s amazing that we are still uncovering anomalies 46 years down the road!
I mean heck, we know of at least one documented case of a 455 having in a 76 car having one 5 series head and one 6 H head.

[PAUL K]

Looking at John's beveled 455 pistons it appears that the valve reliefs are small in volume. I'm guessing the factory did this to make up for the loss of compression from the bevel. I'm also guessing the engineers found better emissions by adding the bevel to the piston. it would be similar to moving the piston ring closer, towards the top of the piston. What I'm confused by is there doesn't seem to be a specific date they changed the piston design. John and android211 have seen them in 75 & 76 units and I have seen the flat tops used in 76 engines.

Interesting

[77 TRASHCAN]

Originally Posted by 25stevem

Paul my info was from McCathys first book and if his compression listed is from the factory then we know that’s always .5 higher then reality so your 7.6 is likely right.

My other info in terms of pistons list part number 499052 for all 455 motors from 75 to 76.

1970 to 74 other then the SD motor used number used 485804 pistons.

It’s amazing that we are still uncovering anomalies 46 years down the road!
I mean heck, we know of at least one documented case of a 455 having in a 76 car having one 5 series head and one 6 H head.

It looks like we are uncovering some more somewhat insignificant tidbits of Pontiac engine Lore.

I wonder if they maybe wrote down several comp ratios on tiny pieces of paper, threw them all in a hat. And had one of the engineers children, draw out of the hay, to use as that years published Comp ratio for each respective cubic inch engine.....

It seems as the Pontiac engineers were a sharp bunch, although I am unsure how so, compared to other GM divisions.
All these small things we notice or uncover had effects on the Pontiac engine, although most were small.