[archerytony]

Got a buddy with a 455 and 6n heads he wants to rebuild it What Pistons would get it to 9.5 cr? He doesn't want to go crazy on it but at least get the cr up

[steve25]

No such thing as casting number 6N, they are a 6H casting on a late 455 motor ,and as such they have a massive 124 CC combustion chamber!
The size of the piston dome you would need to get 9.5 compression would in and of itself a limit on power!
Try to turn up a 6X casting with a dash 4 stamping on one of the tall bosses below the valve cover on the Exh side of the head.

If you go to Wallace racing they have a diagram to show you where to look on the head.

[archerytony]

Originally Posted by steve25

No such thing as casting number 6N, they are a 6H casting on a late 455 motor ,and as such they have a massive 124 CC combustion chamber!
The size of the piston dome you would need to get 9.5 compression would in and of itself a limit on power!
Try to turn up a 6X casting with a dash 4 stamping on one of the tall bosses below the valve cover on the Exh side of the head.

If you go to Wallace racing they have a diagram to show you where to look on the head.

Steve I did look it up too after I posted this and I didn't see it eather. I haven't seen the heads myself but I'm with you on 6h he talked to me about it on the ph. Sounds like the guy wants to stay original that's why I asked about Pistons I guess it's up to him Thanks

[steve25]

Is he showing the car that he has to stay original?
As far as I know in 1976 there where some 455s fitted with large chamber 6X heads of the size of the 6H heads.
He could run the smaller chamber 6X-8 head and still be legit while picking up a good chunk of compression!

[ponyakr]

I agree that the 6x-8 heads is the best way to go.

But, if he wants to keep the 6H heads, there are some things he can do to increase the CR slightly, while keeping flat top pistons.

(1) Buy some Auto-Tec pistons, with the 3.8cc valve reliefs. They can be ordered with a different pin height, in order to get zero deck height, without cutting a lot off the block. I've even heard of lots of engines running the piston top SLIGHTLY out of the hole. So, if the piston is .005 above the deck, he could run the Butler .045 thick gaskets, and retain .040 quench distance.

http://www.yellowbullet.com/forum/sh...d.php?t=624697

http://www.ebay.com/itm/AUTO-TEC-SMA...JW4Kyl&vxp=mtr

There are also guys who post here who can order these pistons.

(2) Have some shaved off the heads. Some heads can be safely shaved .060. Some of the engine builders here may know if the 6H heads can be shaved more, safely. Make sure to shave the exact same amount off the intake surface of the heads, as is shaved from the chamber surface. This should insure that an uncut intake manifold will still bolt up, without machining it.

(3) If for some reason the shortblock ends up with the top of the pistons down in the hole a bit, he can go with thinner Cometic head gaskets. For example: if the deck height is .005, then a Cometic .036 thick x 4.2 bore gasket will increase CR slightly, as compared to using the Fel-Pro 1016 gaskets, which are .039 x 4.3. It will also keep the quench distance down to .041. Most consider that keeping the quench distance down near .040 will help prevent detonation. I've read that if Cometic gaskets are to be used, there is a slightly different machine finish required for proper sealing. Maybe some of the engine builders here can give more details on this.

"...WHAT SURFACE FINISH IS REQUIRED TO USE AN MLS HEAD GASKET?
A surface finish of 50 RA (roughness average) or finer, is recommended for a proper gasket seal. Anything rougher may conflict with the gasket design..."

(4) Run a Voodoo cam. This will increase cylinder pressure, on a lower compression engine. Which one ? There are MANY, MANY different ideas about cam choice. I'd say that a 262 would be a good choice, for a MILD, cast rod, low rpm street 455, for good street manners and power to 5000, or just below. But, many don't wanna leave any upper end power on the table, and recommend much bigger cams for any 455.

http://www.lunatipower.com/Product.aspx?id=2354&gid=287

[archerytony]

Originally Posted by ponyakr

I agree that the 6x-8 heads is the best way to go.

But, if he wants to keep the 6H heads, there are some things he can do to increase the CR slightly, while keeping flat top pistons.

(1) Buy some Auto-Tec pistons, with the 3.8cc valve reliefs. They can be ordered with a different pin height, in order to get zero deck height, without cutting a lot off the block. I've even heard of lots of engines running the piston top SLIGHTLY out of the hole. So, if the piston is .005 above the deck, he could run the Butler .045 thick gaskets, and retain .040 quench distance.

http://www.yellowbullet.com/forum/sh...d.php?t=624697

http://www.ebay.com/itm/AUTO-TEC-SMA...JW4Kyl&vxp=mtr

There are also guys who post here who can order these pistons.

(2) Have some shaved off the heads. Some heads can be safely shaved .060. Some of the engine builders here may know if the 6H heads can be shaved more, safely. Make sure to shave the exact same amount off the intake surface of the heads, as is shaved from the chamber surface. This should insure that an uncut intake manifold will still bolt up, without machining it.

(3) If for some reason the shortblock ends up with the top of the pistons down in the hole a bit, he can go with thinner Cometic head gaskets. For example: if the deck height is .005, then a Cometic .036 thick x 4.2 bore gasket will increase CR slightly, as compared to using the Fel-Pro 1016 gaskets, which are .039 x 4.3. It will also keep the quench distance down to .041. Most consider that keeping the quench distance down near .040 will help prevent detonation. I've read that if Cometic gaskets are to be used, there is a slightly different machine finish required for proper sealing. Maybe some of the engine builders here can give more details on this.

"...WHAT SURFACE FINISH IS REQUIRED TO USE AN MLS HEAD GASKET?
A surface finish of 50 RA (roughness average) or finer, is recommended for a proper gasket seal. Anything rougher may conflict with the gasket design..."

(4) Run a Voodoo cam. This will increase cylinder pressure, on a lower compression engine. Which one ? There are MANY, MANY different ideas about cam choice. I'd say that a 262 would be a good choice, for a MILD, cast rod, low rpm street 455, for good street manners and power to 5000, or just below. But, many don't wanna leave any upper end power on the table, and recommend much bigger cams for any 455.

http://www.lunatipower.com/Product.aspx?id=2354&gid=287

Thanks for the info I will let him know

[steve25]

Another possible fix might be this.
TRW use to make a piston for the 455SD motor in standard and .030" over, it's part number was L 2394F.
It had a 25.8 CC dome, and give a 8 to 1 compression with a 111 CC head.
If a set of these could be tracked down and his 6H heads milled .060" he would have 9 to 1.
These Pistons would need to have the pin boss spread milled wider though to take a non 455SD rod end.
In the end it's still a lot to do and a 6X-8 head, a 5C-8 head, or a number 96 or 7K3 would be the better way to go!

[steve25]

Another possible issue with re-using the stock 6H heads is replacement valves, as I do not think any off the shelf ones can be had .

[GRX]

Originally Posted by steve25

Another possible issue with re-using the stock 6H heads is replacement valves, as I do not think any off the shelf ones can be had .

Good point. Have had trouble sourcing valves for 5C heads which have the same deep chamber.

[steve25]

Well you mean valves for the 5C heads with a chamber size for the 455 , there are plenty of valves to be had for any of the other heads including the 5C-8 and 6X-8!

[GRX]

Referring to all heads with the large 125cc chambers that used 4.710" valves. All stamped #6 if memory serves. Which included some of the early 6X 455 heads. Rare but they do exist.

[steve25]

Yup that's right!
Ferrea could make them up but it's a big addition charge these days and by the time your done your over half way to the cost of a aftermarket head by just getting the needed valve in hand!