402 piston question.

[Jcentsr6]

I just pulled the heads off my 402....getting ready to install the closed chamber heads per my low compression problems. ( I had open chamber 049 heads on there) ANyways....I saw a part # on top of the piston...it says.....

274-03-06-8 and also says

.030 402

Who can tell me who makes these pistons...and does this mean my 402 is bored .030 over?????

Thanks

[The Big Green]

Yes, its .030 over, as for the make, who knows, you may call a speed shop and they may know. I know a shop that will know if you can't find out

[Jcentsr6]

Thanks....so When its bored over....what changes? I know that the piston is bigger and how its bored over..but What else changes. Do all your other performance parts have to change?

[79gmcsierragrande]

The cylinder wall gets thinner and your piston gets bigger, .030 is about the thickness of six sheets of printer paper. But its .030 overall so basically its .015 off each wall so 3 sheets of paper, very minimal. It just means your engine was rebuilt and they bored it out to clean up the walls.

[Jcentsr6]

Were the stock Pistons domed on a 402 Motor?? I have domed pistons in there now and need to find out if Closded chamber 215's head will fit?

I have open chamber heads now with very low compression.

[FIVE 0]

You need to make sure the dome size is compatible with the closed chamber heads your going to. Domed pistons will work with either open or closed heads if they are made for them. A pic would help, but you can usualy tell by the shape of the dome. The dome on an open chamber piston will cover at least half the piston. The dome on a closed chamber will be only raised across the center third of the piston and be flat on each side of it. This is only a rule of thumb however and should be checked more carefully.

049 heads are good heads. If your bullets were made for open chamber heads bolt them back on a run them.

[Jcentsr6]

Here is a picture of The Piston....

[FIVE 0]

That is a closed chamber piston and would work fine with your 215 heads. Not knowing all the sizes I would say that with those pistons you would yeild a touch over 9 to 1 comp. You were probably around, or less than 8 to 1 with the 049's. This of course is just a guess knowing the cc's of both heads and a wild guess of the piston dome. Alot of factors can change it some.

[Jcentsr6]

Thanks alot..Good info. I have has some local guys check it out and game me near the same info as you...so thanks again

[Green Machine]

Chevy made the 402 by boring the 396 .030 oversize right from the factory, so a .030 over 396 piston would have been the standard piston. Whether they marked them as std or .030 I don't know.

If you can measure the bore -
4.096 = 396 ..... std 396
4.125 = 402 ..... std 402 or 396 .030 over
4.155 = 408 ..... 402 .030 over or 396 .060 over
4.185 = 414 ..... 402 .060 over or 396 .090 over
BBC have pretty thick cyl walls so don't worry about a small overbore.

[LONGHAIR]

I know what you are saying Green Machine, I thought the same thing, when I first saw this.
But, aren't all factory pistons standard? It only makes sense, where-ever that bore started from was the "standard".
Overbores are "repair" tactics to be able to recyle the engine, aren't they?


I realize that those pesky 402s do confuse things though. Between the 396-402 and the associated mis-leading badges.....who knows?

That Dot-Matrix looking stamping on the pistons in that pic are definately not factory.

[PanelDeland]

You may want to put a small lump of clay on the piston,bolt on the new head,no gasket and rotate the engine 720*.This will cause the piston and valves to run to their closest proximity.The thickness of the thinest part of the clay will be the piston to valve clearance.I don't remember the specs but if you are running an unknown cam and pistons with a new head it might be good insurance to know how much space you have between them and look online for the minimum.

[Jcentsr6]

Originally Posted by PanelDeland

You may want to put a small lump of clay on the piston,bolt on the new head,no gasket and rotate the engine 720*.This will cause the piston and valves to run to their closest proximity.The thickness of the thinest part of the clay will be the piston to valve clearance.I don't remember the specs but if you are running an unknown cam and pistons with a new head it might be good insurance to know how much space you have between them and look online for the minimum.

Well we know what pistons now....and the cam is a Lunati Voodoo

Lunati Voodoo Cam and Lifter Kits
Cam and Lifters, Hydraulic Flat Tappet, Advertised Duration 268/ 276, Lift .542/ .554, Chevy, Big Block, Kit

and the heads are CLosed chamber 101cc port. (215's)

I will try the clay trick....Thats a good idea. (also made by member 70rs/ss today as well) Thanks everyone..I think After everything...I just had the wrong heads on it all together. I will be installing the heads friday night and will post up my results.