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Discussion Starter · #1 ·
Trying to figure out the best way to go. I have a stock bore 4 bolt main block, with a forged stock stroked 350 crank,that is 10-10. Stock rods. With a new set of forged dished pistons. The pistons are 1.190 X 4.030, comp ht is 1.1900, the bore is 4.030, piston dish is -38 cc., the dome rise is 246,deck thickness is 468, the depth from TE is 275 -38cc. Since Chevrolet sbc's use mostly the 5.7 and the 6" rods. Trying to figure which way to go. I know you can get other size rods beside the 5.7 and 6 ". Like a 5.565 for the sbc and the 6.135,6.200,6.340,6.385 for the big block chevys. Mostlikey your think why the deep dish. -38. The motor has a 8-71 2 780s, that puts out 12 lb. Any ideas. Or what combo would you go for ? Stroke etc.
 

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Nominal block deck height of a virgin SBC block is 9.025", so you want to put a stack of parts together that will fit into that space. Sounds like what you have to start with is the pistons at 1.190" and are looking for a crank radius and rods to fill that space. Deducting 1.190" from 9.025" leaves a space of 7.835". If we used an Ohio 3.625" stroke crank (part #43503625, $495) with a radius of 1.8125" and a 6.000" rod, the stack would equal 9.0025", leaving a theoretical piston deck height of 0.0225" (1.190" + 1.8125" + 6.000" = 9.0025"). You could then cut the decks to whatever you need to in order to match up with the gasket thickness you will run to set the squish that you want to run. This would make a 370 cubic inch motor. Back in the day, this used to be referred to as a "5/8ths stroker motor" and there was a ton of 'em built as circle track motors.
http://www.ohiocrank.com/chevysb_cranks.html

Ohio also makes a 4.000" stroke crank, but only shows it with 400 main journals. If there were such a crank with 350 mains, you could use a rod length of 5.850" and pop the piston out of the block by 0.015", using a thicker gasket to set the squish with no deck cutting. (1.190" + 2.000" + 5.850" = 9.040"). Oliver makes the rods and others probably do too. This would make a 408 cubic inch motor, but I'd talk with Ohio about what kind of clearances would have to be cut out of the block to clear the crank and rods. You would probably encounter rod big end to cam lobe interference also, but there are reduced base circle cams that will help to alleviate that problem somewhat. The Ohio techs can address any counterweight to piston skirt issues.
http://www.oliverconnectingrods.com/downloads/TechSWS.pdf
 

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More for Less Racer
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You could also do a 3.48 stroke crank and 6.125 rods(gives a 9.055" stack) if you were willing to cut .040" off the top of the piston(the rim around the dish)in a lathe or mill to give a stack height of 9.015" or cut .035" for a stack of 9.020". Pistons should handle this no problem unless they have a really high(.200" or less from crown) ring placement. You would have a 6.125" rod 355 this way. I have done this before with dished pistons.

You could also cut .045" and use a 6" rod with a 3.75" stroke for a 383 cube engine, makes a 9.020" stack.
 

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Car Craft (Aug. '09) recently did a 408 SBC based on a 4" bore block. They used an RPM crank ($500) w/Probe pistons ($500, 1.175" comp height) originally spec'ed for a stroker Ford 302/331. Uses Eagle 5.85" rod ($475). I'm thinking there are definitely other cheaper parts for this deal.

In any event, with a 9.025" deck height, this combo will yield a zero deck. Large cams might need valve reliefs cut, the reliefs as they come do not line up w/SBC valves (Ford pistons).

408 cubes, via the back door.
 

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Discussion Starter · #6 ·
First guys thanks for all the replies and info. Let me describe my engine. The reason why I want to use the deep dished pistons -38 cc is because its a blower motor. If I mill them like .035-.045 won;t they be like flattops with higher comp. My first motor I used flattops, and after break end. I like really nail it. And ended up blowing both head gaskets. I like to run about a 10-12 lb boost. he rods I have not got yet, because of not knowing lengths. But hopefully I can find something in a H beam. The crank I have a stock stroke forged one, but not sure if I could stroke it. The crankshaft was mentioned, It maybe wrong.
But what I was thinking of is a cam thats 0.533 in 0.519 ex. lobe lift 0.355 intake 0.346 ex. Rockers 7/16 1.6
The heads, Some say I should use a 72-76 cc. Because of the low comp. But to me the really don't have the flow as a 64-67 cc. WOW ! guys several mention a 408. Cobalt you were one. You said you built one or someone built one for you using almost the same piston. Do you know the stroke of your crank,rods and etc. Also did this comb you used have any clearance problems with the block,rods crank,cam and etc. Thanks again ! Keep your ideas and thoughts coming. I will get this motor built yet !
 

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With the 355 that Eric described, cutting 0.040" off the crowns would yield 33.5cc pistons. Using a 67cc head, zero deck (decks cut to 9.015" block deck height) and a 9.5cc gasket (Gasket Works copper 0.043" with stainless compression rings), you'd have a squish of 0.043" and a static compression ratio of 7.60:1. This would allow 9.6 lbs of boost on 93 octane gasoline according to Blower Drive Service charts. Any more boost than that would require more octane or a crutch such as water/alcohol injection. This combination would be the cheapest way out.

Using 75cc heads instead of 67cc heads will give you a static compression ratio of 7.16:1 with the same squish. This would allow 11.3 lbs of boost on 93 octane gasoline.

I'm doing these calculations with an 0.043" copper head gasket and stainless compression rings from Gasket Works. Call them up on the phone and discuss with them what you're doing and have them make a recommendation. I used 0.043" because you need a squish of between 0.035" and 0.045" to help prevent detonation.
http://www.headgasket.com/faq.html
 

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whynot said:
WOW ! guys several mention a 408. Cobalt you were one. You said you built one or someone built one for you using almost the same piston.
See TI's post #2 and my post #5.

I have not built anything like that- I would go w/a 4-1/8" bore to get to 400+ cid, w/a shorter stroke if I were doing it from scratch.

The idea was to point out some often overlooked options- not necessarily viably options, mind you- but options that are available to those w/the money and desire to do it.

The main problem w/a 4" stroke in an OEM 350 block, is at anything approaching "high" RPM, that big crank will be the tail that wags the block's dog. A 4-bolt, splayed cap conversion of a 2-bolt block would help, a 4-bolt OEM block would be better than nothing, and I wouldn't do it at all w/a 2-bolt block.

Also, in the mag article, they half filled the water jackets w/Hard Blok to stiffen it, plus grinding for clearance in the pan rail area was required that might have gone into the water jackets.
 
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