[PAUL K]

Originally Posted by Stan Weiss

Stan

That 224/234 cam is a good choice for an 8:1 455 using a tight converter and numerically low gears. Crane offered that cam and a more modern version 222/234 467/494 lift that worked very well, also especially with a 1.65 rocker on the intake side.

[25stevem]

Here’s some more info.
Stock 6X head wise ( valve spring installed height) will not work with that .467” lift, you will be into coil binding those stock springs .

The stock spring / installed height set up many times is really only good for .425” lift before the stock spring is .060” away from coil bind.

Even if a freshen up valve job was done you would likely be too close to coil bind for used stock springs to hold up, you would also only have at the very best 105 psi of seat pressure, and with used high mileage springs more like a pity full 90 psi.

Block / piston wise 455s are spec out to have from the factory a deck clearance of .013”.

The compression height of the factory piston is 1.497” to 1.510” from what my notes I have taken over the years have shown me.

Math wise if you wanted a 9 to 1 compression out of your .060” over motor you would need a total CC volume with the piston at at TDC of 121 CCs.

Assuming a stock deck height of .013” would make for a CC volume of 3.77 CCs.
The usage of a Fel pro head gasket makes for 9.7 CCs, so now we are at 13.47 CCs.

If we add that to your cut 6X heads of 93 CCs we are at 106.4 CCs.
This leaves 14.6 CCs left out of that 121 CCs that we are shooting for to have a 9 to 1 compression for your 455.

Now the CCs of your piston notches and chamfer need to be determined, and hopefully that will not exceed 14.6 CCs.

[Stan Weiss]

Bore = 4.21"
Stroke = 4.21"

Total Volume against CR

Stan

[25stevem]

Don’t forget that as seen in his photos his block still has some intake chamfers left even after a .060” overbore and I did not factor in if he has any dish in his valves being used and I did not factor in 1.5 CCs in ring land volume.

All of this I would estimate to be a easy 3 CCs.

[MDGOAT69]

Had some time to get out to the garage, and my big question was how far in the hole the pistons may be. To my surprise they seem to be flush to the deck surface to me, so Is this zero decked?

[Stan Weiss]

Originally Posted by MDGOAT69

Had some time to get out to the garage, and my big question was how far in the hole the pistons may be. To my surprise they seem to be flush to the deck surface to me, so Is this zero decked?

If in fact the top of the piston is flush with the deck than they are zero decked.

Stan

[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?

[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.

[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.

[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.

[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.

[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?

[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.

[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.

[25stevem]

Who knows at this point!
Maybe the 455s made without the chamfer where for slightly increased compression for High altitude applications , lol!