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View Full Version : calculating BBC (402) compression ratios -

12-10-2003, 12:32 AM
I need to know what the expected crush depth and diameter bore on a head gasket is for a BBC. :unsure:
I was expecting ~.030 like a SBC? Is this right?

I am looking at a complete fresh short block (402 BBC) with some big ol' domed pistons that appear to give 10.58:1 ratios with 109.4 cc heads

The domed pistons are 38.3cc with a .335 dome.

Anyone... anybody? Anyone at all???

I have either some 97 cc heads /forums/images/graemlins/crazy.gif giving me I THINK 11:1
Or I have some 112 cc heads giving me some 10.2:1 :blink:

I only want 9.5:1 or LESS!!! HELP!

Can I trim the pistons? Can I trim (bowl) the heads?

I THINK I need ~ 123cc heads... /forums/images/graemlins/smile.gif

LMK what you engine gurus think.


12-10-2003, 05:33 AM
Find a local machine shop, they can help a lot with the answers. The compressed head gasket thickness can vary a lot depending on which gasket is used.

The piston tops can be milled a bit....

The head chambers can be polished to gain some cc's if required....

Have someone on here look up the casting no. on your heads to give you a rough idea of the chamber cc's. The best way is to cc them. This can be done yourself. Often the advertised cc's of a head very a bit from the actual no. found....

The only difficult part of this project is the 402 BBC has fewer piston choices available than the more popular Chevy builds. This only means you will have to dig a bit deeper for the results you desire....

The deck heighth should be surfaced which will effect the compression also....The heads can be milled too. If milling the heads, the intake should be done as well....

You need to talk with a machine shop....

12-10-2003, 09:39 AM
Thanks for the reply.

I am looking at a complete shortblock (402) that has .060 over pistons TRW-L2242F60

I have three sets of heads to choose from

casting....yr....type intake....chamber....type engine's

4092360....Unk.....unknown.....unknown....454...un known

3904390....67......oval..CLOSED..396, 427, 98cc chamber

3933148....69-84...oval...OPEN...'69 396/265hp, 366T, 427T, 112cc chamber

I got these numbers from (

I can't find anyone who specifies the head gasket thickness.

I was looking things over in Desktop Dyno, trying to calculate the compression ratio.

I am looking at
head chamber volume = 112cc
head gasket bore = 4.250 ??? I am not sure on this
head gasket thickness = .100 ?? I think these are normally .030
Piston down from TDC is .335 (from TRW on the pistons)
Volume above piston is 38.3cc (From TRW on the pistons).

Oh well. I guess I really know the answer, it's just such a good deal I don't want to pass it up. But there is a lot of work to make it work on 87 octane gas.


12-10-2003, 10:18 AM
The first casting number is 14092360 86-88 year, small oval port, open chamber at 118 cc's 454 truck head. If you do not mind the small oval ports those are the heads you will need. All a 402 is a 396 bore .030 over from the factory. The other heads would work great on a 454 with a set of flat top pistons which should yield about 9.0 compression on the 454. The head gasket compressed thickness of the FEL-PRO: 8180-PT-1 is .038".
The GM head gaskets compressed thickness range from .022 for steel head gasket to GM part No. 12363413 composite type head gasket with a compressed thickness of .041". One more thing with those pistons you will have 11.6 compression with 106 cc combustion chamber heads. The other two sets of heads have large oval ports which flow very good at higher rpms. Save those for a 454 with flat tops and they turn a low compression 454 into a real screamer. Sorry for being so wordy. Hope this helps.

James Startz

12-10-2003, 11:02 AM
Often the advertised cc's of a head very a bit from the actual no. found....

[/ QUOTE ]

I'll second that remark. My "67cc" S/R Torquers actually cc'd at 72cc's. Strangely enough, it worked out perfectly with the pistons I wanted to use. /forums/images/graemlins/thumb.gif

mudjunkie 82
12-10-2003, 04:40 PM
You do not want to mill the pistons,that will weaken them.You will have to CC" the chambers,subtract the dome volume,then add gasket volume,The more important thing with gasket thickness is piston to head Clearence.You might be better off with a set of flat top pistons,nice set of oval port heads and good cam to make a nice motor with a nice powerband.The heads are the most important part of building a good motor,Do not make the mistake of going to the big square heads,these are designed for all top end and they suck at low rpms because the velocity in the big ports is Too slow at low rpms.

12-10-2003, 04:45 PM
If it is a screaming deal, then just install new pistons....It will probably still be a good deal. I would snatch up a 'close' 402 in a heartbeat if it just needed a bit of tweaking to run....

12-10-2003, 07:37 PM
Thanks to everyone for their replies.

I figured I should swap out the pistons for some 9:1 KB Hypers. Perhaps I can sell the TRW forged ($355 in summit) for the cost of the hypers ($255 in summit)?

I found the short block, no cam (plan on running a Crane 266) it has been line bored, decked, cylinders cut for ~$800. He is including my choice of heads (not the sqaures he's keeping those for his 427 corvette). He is also giving me all the accessories, dampener, and an aluminum intake. It needs an oil pump $30 - $60. It sounds like a good deal.

Is it?

Of course itís just a matter of getting my standard / standard 396 tall deck to a machinists and waiting. It needs .030 over Ė line bore, deck. I donít even know the condition of my crank. But I wonít get it back until AFTER Christmas. L

On the one I am looking at it has pistons stamped a .060 for a 396. The different manufacturers list a 402 with .030 - are these the same size pistons?


12-10-2003, 10:11 PM
The FelPro Prematorque head gaskets range in thicknesses from .039 (9.7cc vol) to .041 (10.9cc vol) for typical 4.25" & smaller bores. This will give lower compression than you were calculating.

12-10-2003, 10:19 PM
On the one I am looking at it has pistons stamped a .060 for a 396. The different manufacturers list a 402 with .030 - are these the same size pistons?

[/ QUOTE ]

A 402 is simply a 396 bored .030" from the factory. GM screwed up and had to bore them so they could put them togther and sell them AFAIK. So, a .060" over piston for a 396 is the same as a .030" over piston for a 402.

12-11-2003, 05:52 AM
You are correct, same piston....

Yes thatb is a good deal, for a grand you will have a fresh built BBC ready to install. Many people spend about that for a motor they have to rebuild....Good luck selling the pistons. It can be done, but might take longer than anticipated....

12-11-2003, 11:43 AM
Kewl - Thanks for the information.

Thanks to everyone for their help.

This 396 with the pistons would still need a head job... ~$350
so it will still be about $1400 - $1500 when it's all said and done. Is it still a good deal. Of course, I still need to assembly it all. But then I can sell the other pistons and hopefully get $200+ back.

Thanks for the information from everyone. The gasket info helped a great deal.

WOohooo It will be ~350 hp (at 5k) with ~445 lb/ft torque (at 3200).

Just needs some headers and tune in my TBI.

12-11-2003, 07:43 PM
Just a bit of info for you and anyone else who reads this. GM never made a tall deck 396, tall decks were either 366 or 427 truck engines.

12-11-2003, 07:55 PM
Just a bit of info for you and anyone else who reads this. GM never made a tall deck 396, tall decks were either 366 or 427 truck engines.

[/ QUOTE ]

How many years were 396s offered in trucks?

12-11-2003, 08:00 PM
I have no idea how many years they were in trucks but the first 396 was in 1965 and I think GM stop making them in 1969 or 1970 when GM screwed up and overbored them and now called them 400's or 402's

12-11-2003, 10:14 PM
Just a bit of info for you and anyone else who reads this. GM never made a tall deck 396, tall decks were either 366 or 427 truck engines.

[/ QUOTE ]

That's interesting. I was just going off the ( website:

My block at home is a
3969854....402.......70-72...2 or 4 bolt

Oh man, I can't wait for this year to be over.

For some BIZARRE reason, both blocks (the one I have and hte one I was going to buy) are the SAME casting number.

I thought FOR sure the one I have at home was a T casting.

I am going to go crawl under a rock and wait for the end of the year... /forums/images/graemlins/1zhelp.gif

12-11-2003, 11:22 PM
So what exactly do you think you have and or aren't sure about now? Sometimes GM used the same casting number on different displacement engines.

12-12-2003, 01:24 PM
I know what I need to do. I just don't know if I WANT to do it.

What I was trying to avoid was waiting for the machinists to do the work.

That's why I thought the 396 (with 060 over pistons) was a good deal. But they have too high a compression that I want to run. So I will have to change the pistons and have the heads redone.

Or I can redo MY block needs machine work, rods, pistons, and head work done.


Thanks for everyone's help.