[voodoohalfbreed]

Greetings, currently my 66 2+2 tripower has a stock 428 engine with 093 heads and 66 tripower. The engine is tired and I need to put the original 421 engine back in. Only option for a 421 piston is foreign junk or a custom made piston. In my search I have found 428 pistons more readily available and the price is not too bad. The machine shop said I need at least .012 bore to clean things up. A 428 standard would work perfectly. I am not sure what the compression ratio is with the 428 engine with 093 heads. The 421 was 10.75:1. The 421 heads are 68cc and the original 428 heads are 72cc. I would estimate the 428 with 093 heads at probably 11:1??? I am going to run a stock spec cam as well. I am not sure what cc pistons to use, also not sure how that works, higher cc lower comp??? Lower cc higher comp??? Ideas, suggestions, I would like to hear it!
Thanks
Scott

[steve25]

For a long life motor if you have the funds I would ditch the 093 heads and find a set of 6X-8 heads and have them rebuilt and run the off the shelf 428 Pistons that you can buy.

Your 421 with a 428s stock Bore will become 426.6 cid.

When I run the numbers to figure out the motors compression I get about a 8 to 1 compression in general as there are a lot of veriations that can effect that end result compression number.

For instance, if you turn up a set of 6x-4 heads or other large valve heads with about a 89 to 91 CC chamber then you would have closer to 9 to , which is a good number.

Conversely if you mill .060" off of the 6X-8 heads you will have about that same 9 to 1 compression.

I used numbers of 9.7 CCs for the head gasket, a deck clearance of .023", 98 CCs for the chamber volume ( 8 to 1 comp) 1.5 CCs for piston ring land volume and 10CCs for the 428 pistons dish.

[voodoohalfbreed]

Originally Posted by steve25

For a long life motor if you have the funds I would ditch the 093 heads and find a set of 6X-8 heads and have them rebuilt and run the off the shelf 428 Pistons that you can buy.

Your 421 with a 428s stock Bore will become 426.6 cid.

When I run the numbers to figure out the motors compression I get about a 8 to 1 compression in general as there are a lot of veriations that can effect that end result compression number.

For instance, if you turn up a set of 6x-4 heads or other large valve heads with about a 89 to 91 CC chamber then you would have closer to 9 to , which is a good number.

Conversely if you mill .060" off of the 6X-8 heads you will have about that same 9 to 1 compression.

I used numbers of 9.7 CCs for the head gasket, a deck clearance of .023", 98 CCs for the chamber volume ( 8 to 1 comp) 1.5 CCs for piston ring land volume and 10CCs for the 428 pistons dish.

If I used the 093 heads, 68cc and the other numbers you gave what would be the comp ratio?

[grandam1979]

You could run all your 421 original parts with a 30cc dish piston and be well under 10-1 Go to the butler site and use there compression calculator. I came up with about 9.4

[John Milner]

Originally Posted by voodoohalfbreed

Greetings, currently my 66 2+2 tripower has a stock 428 engine with 093 heads and 66 tripower. The engine is tired and I need to put the original 421 engine back in. Only option for a 421 piston is foreign junk or a custom made piston. In my search I have found 428 pistons more readily available and the price is not too bad. The machine shop said I need at least .012 bore to clean things up. A 428 standard would work perfectly. I am not sure what the compression ratio is with the 428 engine with 093 heads. The 421 was 10.75:1. The 421 heads are 68cc and the original 428 heads are 72cc. I would estimate the 428 with 093 heads at probably 11:1??? I am going to run a stock spec cam as well. I am not sure what cc pistons to use, also not sure how that works, higher cc lower comp??? Lower cc higher comp??? Ideas, suggestions, I would like to hear it!
Thanks
Scott

If you are running the stock dished 428 pistons and 093 heads, your compression is likely around 10:1. If you are running a flat top with no dish, you are likely around 11:1. If you are running a stock style 428 piston with the dish, your valve reliefs won't be in the correct location for the 093 heads. You can run it that way but you would just have to be careful in what camshaft you choose.

[mgarblik]

It's important to decide what your performance goals and are and how important the "proper parts" are to you. If you have a tired 428 in the car niow and wish to put the "original 421" back in, I take that to mean you want to return it more to the original parts and look which would include the 093 heads. You also mention using the stock camshaft, which to me means you are not trying to increase the HP. If my assumptions are correct, the 093 heads will support 400 HP with just very minor clean up and a proper valve job. Being closed chamber, and pretty small at that, a 428 piston with a little dish will get the compression where you want it. On pump gas, with 093 heads, 9.2-9.5 to 1 CR is about all you can run with the crap gas around here. Should make about 460-470 lbs. torque and 360 HP or so. If that's enough, all the stock parts will get it done with a slight drop in compression.

[voodoohalfbreed]

Originally Posted by John Milner

If you are running the stock dished 428 pistons and 093 heads, your compression is likely around 10:1. If you are running a flat top with no dish, you are likely around 11:1. If you are running a stock style 428 piston with the dish, your valve reliefs won't be in the correct location for the 093 heads. You can run it that way but you would just have to be careful in what camshaft you choose.

I currently have the stock cam which is the same for 428 and 421 #68. I intend to keep same cam specs and keep the comp between 10 and 11

[ANDYA]

A 66 2+2 is a pretty desirable car. If you have the original 421 block, heads and intake then that is what I would use. Not many guys running around with a correct 421. From what I understand the valves in pre 67 heads will interfear with off the shelf 428 pistons due to the valve angle in those heads. Talk with one of the experienced Pontiac builders on this sight and be sure and get the correct pistons even if they need to be customed ordered. Quality machine work is important and work out the compression ratio with your engine builder. It will be worth in the end.

[leeklm]

"currently have the stock cam which is the same for 428 and 421 #68. I intend to keep same cam specs and keep the comp between 10 and 11"

Do you plan on running E85 or something with higher than 92 octane? An 11:1 CR with aluminum heads is fine, but could be an issue with iron heads and readily available pump gas.

As mentioned earlier, 9.0 to 9.5 CR is a good target for typical 90-92 octane fuel. At times when traveling, you may not be able to find anything above 89.

Sent from my SM-G965U using Tapatalk

[voodoohalfbreed]

Originally Posted by leeklm

"currently have the stock cam which is the same for 428 and 421 #68. I intend to keep same cam specs and keep the comp between 10 and 11"

Do you plan on running E85 or something with higher than 92 octane? An 11:1 CR with aluminum heads is fine, but could be an issue with iron heads and readily available pump gas.

As mentioned earlier, 9.0 to 9.5 CR is a good target for typical 90-92 octane fuel. At times when traveling, you may not be able to find anything above 89.

Sent from my SM-G965U using Tapatalk

The 9 to 1 is fine, I figured from the original specs of the 428 engine and the original numbers from the 421 both 376 hp, both around 10.5to 1 or 10.75 to 1 things would be okay. It runs pretty good on pump gas as bbn it is other than being tired

[steve25]

99% of all the pre 1971 factory stated compression ratios as far as normal production Motors where concerned are .5 higher then they actually where!

[voodoohalfbreed]

https://www.google.com/url?sa=t&sour...uHw9giGWqWj543
So I found this link to a kit for 428 rebuild, seems to have good name parts and price. Any opinions???

[steve25]

I could go with most all of that for a street only rebuild, but for the lifters!

[b-man]

For reference, my ‘64 421 HO build thread:

https://forums.maxperformanceinc.com...d.php?t=771741

[KEN CROCIE]

Use the 428 pistons. have a shop extend (expand ?) the 428 dish. The piston will then clear the valves on the 093 heads. take this opportunity to lower the C/R to pump gas compatibility.

[geeteeohguy]

Years ago I built a .060" over 389 and used standard bore flat top 400 pistons with the valve reliefs in the wrong spot for the 093 heads I used. I installed a Sig Erson cam with about 479 lift and had no issues. If the cam isn't too radical, it usually is not an issue. With dished pistons and an 068 cam, I can't see it even being close to an issue. That 389 ran strong for years and years.

[mgarblik]

Originally Posted by KEN CROCIE

Use the 428 pistons. have a shop extend (expand ?) the 428 dish. The piston will then clear the valves on the 093 heads. take this opportunity to lower the C/R to pump gas compatibility.

Agree! Do this to lower the compression and you will have a nice engine you can live with. I would personally buy all the upgrades listed on your parts list kit. Especially the plasma molly pings. Plain cast iron rings are for a back yard ball hone rebuild only IMO.