406sb rod,piston help

Example: a 4.155 x 3.750 (bore x stroke) .030 over SBC 400 block, zero decked will yield about 10:46 compression with a .040 thick head gasket at bore size, or about 10:58 compression with a .035 thick head gasket at bore size, using a 64cc chamber and the following pistons:

http://www.cnc-motorsports.com/product.asp?ProdID=18761&CtgID=18757

I have these rods on order and flat top KB pistons for my 400, because my heads have larger chambers.

http://www.cnc-motorsports.com/product.asp?ProdID=1125&CtgID=

Edit: These KB pistons have a 1.133 compression height which is .008 higher then some other available pistons, so you have to take that into consideration BEFORE having the block machined. Get the crank kit before any machine work is done...

With that said and the block zero decked for the above pistons (9.008 deck height). The compression examples are correct.

for what you are doing call speedway motors order stock 5.7 rods(already resized,new bolts shotpeened and magna fluxed) order a set of flatop speedway hypers for the 5.7 rod and you should be good to go for a couple hundred bucks.

barnym17 said:

for what you are doing call speedway motors order stock 5.7 rods(already resized,new bolts shotpeened and magna fluxed) order a set of flatop speedway hypers for the 5.7 rod and you should be good to go for a couple hundred bucks.

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Typical flattops would put him WAY over his desired compression target... like the mid 11 range.

so a flat top is to much CR? i want to use a rod that i dont have to do clearance work on. any suggestions?

pal said:

so a flat top is to much CR? i want to use a rod that i dont have to do clearance work on. any suggestions?

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Only stroker engines need rod clearancing to clear the cam or block.

I'd use the stock length rods and easy to get 406 pistons.
IMO you would be better off with a flattop and a larger more efficient chamber.

You could run whatever head you wanted with reverse dome pistons, but they might be expensive. This would be ideal for chamber quench.

i keep reading that using 5.7 rods in a 400 with high lift cam need the tops of rod bolts trimmed or use small circle base cam. am i getting my wires crossed?

pal said:

i keep reading that using 5.7 rods in a 400 with high lift cam need the tops of rod bolts trimmed or use small circle base cam. am i getting my wires crossed?

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IF the rod bolts interfere, it is only TWO bolts that need about .050 clearancing, unless you have some ridiculous lift.

Again, why 5.7 rods???? The difference in performance is inperceptable in the real world.

i need to buy pistons. it seems the selection of pistons arent so great for the stock rod. also by the time i have my rods re-done, it seems i can have a set ready to go for not much more money. i want 10.5:1 with 64cc heads. so i need pistons. my rods are original, with stock bolt and at least 1 rebuild under there belt.

pal said:

so a flat top is to much CR? i want to use a rod that i dont have to do clearance work on. any suggestions?

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Yes with 64cc chambers the compression would be over 11:1.

pal said:

i need to buy pistons. it seems the selection of pistons arent so great for the stock rod. also by the time i have my rods re-done, it seems i can have a set ready to go for not much more money. i want 10.5:1 with 64cc heads. so i need pistons. my rods are original, with stock bolt and at least 1 rebuild under there belt.

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I think your on the right track.
What heads do you have?
How much cam do you want to use?

If you use a rod with cap screws you should be alright. If not your machine shop can take care any rod/cam interference issues when they balance the rotating assembly. Pick the pistons you want to use and then buy the appropriate length rods.

xntrik said:

Again, why 5.7 rods???? The difference in performance is inperceptable in the real world.

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After a quick glance through the Summit online catalog it appears that there is a wider selection of pistons designed for use with 5.7" rods in the 500 vs using the stock 5.565" pieces. They appear to be a better choice because of the greater piston selection and not so much because they offer a better rod ratio.

One of the great things about a low rpm 400sbc is that with stock rods you can run a TINY cam and make good hp. As soon as you start stretching rod length to make a low rpm torquer rev you lose the high air speed endemic in these engines.
An early intake closing and narrow lca make low rpm torque in any engine, but in a 400 this combo still makes good mid range due to the high air speed around 100deg after tdc. Best of both worlds - just don't rev it hard.

My reason for saying 5.7 has more to do with reliability it is far stronger than the 5.565 rod. just extra ins. I missed the part about 64cc heads, unless the 64 cc heads are something special go to 76 cc heads .or order cast rebuilder flatops the way they drop the comp. height it should work out . usually .020 down in the hole.

For that matter a 6" rod will lighten the piston so the rod has less mass to control. And the bore wear is lessened.

thanks everyone for a lot of good info. took some time on sunday to look things over and decided to work with what i have. pretty much stock rebuild done about a year ago. everything looks to be in very shape. stock rods, .030 24cc dish pistons .025 down in the cylinder. fairly fresh set up so i should use it and live with the about 9.5:1 cr with 64 cc 2xhump heads. still want to run a solid lifter cam. another thought is how well do the dart iron eagle small chamber heads work? they have 49cc heads with 215 or 230 runners. are these only for small cube engines?

Solid cams numbers do not coordinate to hydraulic cam numbers. Talk tech to specific cam tech lines for their recommendations. :thumbup:

You are going to have ZERO quench with that piston/head combination. I suggest you rethink what you are putting together at 9.5 compression.

That compression would indicate a 224* single pattern cam with 106* LSA. But it would probably need 91+ octane anyway and careful timing.

IMHO it would be a pinging gas hog with soggy midrange power and a limited top end.

You should look into larger chamber heads and flattop or reverse dome pistons for effective detonation protection and efficient fuel burn.

i dont understand quench? what you are saying is even with the heads raising the current cr of 9.5:1 they will kill the engine? high octane and some type of octane booster is okay. gas milage of no concern. what cr would those heads push engine upto? what about the world torquer 58cc head? at this time all i have is some double hump heads that i will need to spend some money on. i can either spend money on the bottom end, or new heads but not both at this time.

Then I would build a bullet proof bottom end because it's by far easier to change a head than change a rod and piston without getting into money.

A killer head might break a bottom end that get's really expensive really fast.



Dart 49cc heads are designed for class rules that mandate a flattop piston without a compression rule. Those are not in you best interest. A flat top with 76cc chambers will get you where you need to go.