I’m reading Dave Vizard’s SBC book. Maybe I’ve not arrived at this part yet, I have a question.

6.0” rods offer a lower rod angles as crank turns. Meaning less stress on the rod bearing and thrust forces on the cylinder wall. When compared to the stock 5.7” rods. My questions are:

Do the 5.7” and 6.0” rods run on the same crank?

Won’t the extra rod length cause the piston to travel higher in the bore? Or are special lower height pistons required for 6” rods? I assume with 6” rods, if you want to keep compression down to 9.5 or so, are dished pistons or lager cc combustion chamber heads needed to offset increased compression?

thx.

 

Yes the.piston pin height changes with rod length.. The late John lingenfelter recommended a 5.850 rod length on a 383 which he thought was the best combination. I'm inclined to agree with his findings. Not to take anything away from david Vizard.
However, finding Pistons for a combination with a 5.850 rod length would probably be a little bit more difficult than finding it with a more common 6-in rod.

 

Thanks for your response. I suspected a possible lower piston pin bore on the piston. That makes sense.

Next question, aside from cleaning up the deck or squaring it up, why machine more than that off the deck height? Is this to achieve a desired, improved quench area? Thx.

 

Do they both run on the same crank?.....Sort of. There are practical concerns. Common aftermarket 3.75" stroke cranks are produced for "internal balance" or "external balance". Don't over think the rod length. It just isn't the most important choice you have to make.

The internal balance is preferred for strength and durability. The counterweights are larger. You can use your existing internal balance flexplate or flywheel and internal balance harmonic damper. Most of the time you will need 6" aftermarket rods to clear the larger balance weights on the internal balance 383 crank when the pistons are at bottom dead center. The piston pin holes are higher up on a 6" rod setup.

External balance 383 cranks require external balance flywheels/flexplates and external balance harmonic dampers. You can use a 5.7" rod and pistons with matching pin heights to fit the 5.7" rods on these cranks. Some people will use a bolt on crank weight to help with the balance. It is POSSIBLE to internal balance an external balance crank BUT it is not financially practical. The slugs of mallory metal are expensive and drilling the crank to install the heavy metal costs more too.

The preferred block deck height is where you can achieve a .040" final quench height. The easy way to get this is to pre-assemble the short block and measure each piston hole to see how far down in the hole each piston is at top dead center. You might find some holes are ,028" down and some holes are .022" down for example. The deck heights will show you how far off they are from square. Your machinist will need to know how much to knock off each corner of the deck so write it down on the side of the block at each corner. Need this for #1, #2, #7, #8.

Choose your pistons and head chambers to get you to the compression you are aiming for. I built my 383 with external balance, 5.7" rods, Mahle -5cc forged flat tops, SCAT external balance crank, SCAT 5.7" floating rods, 67cc Dart iron heads, .040" gaskets, zero deck height, and a final CR of 10.7, Mike Jones custom cam. Runs amazing on pump gas.

 

Great responses. Thank you.

I will be using a 350 block that will need 383 modifications.

To clarify. Assemble engine block with 383 crank and new main and rod bearings in-place. Assembly lube on all. Install pistons. With or without rings? Torque all bearing caps. Turn crank manually. Use dial indicator to verify piston is at TDC at each corner of the block. Measure piston depth down in the hole with a dial indicator.

Only concern I have is possible piston scuffing if pistons are installed without rings, then turning the crank.

Then remove pistons and rods crank. Send block back to machine shop for decking. Sound about right. Thx.

 

Yep.


I use a straight edge and feeler gauges but whatever works for you. You need the rings to stabilize the piston in the bore. You need to assemble the rotating assembly anyway to clearance the block for the rods. I bought SCAT rods that were already clearanced so it was easy.

 

suggest do Not mock-up with 'NEW" bearing inserts. If there's any filing/grinding to follow (often is when building a stroker), that grit will get into inserts' soft overlay, rendering them unacceptable for service other than mock-up. You can wipe that schmutz off journals; but not All of it from inserts' overlay.
If you're increasing Gen 1 sbc 350 stroke, suggest longer rods with shorter pistons to match; suggest 6". FWIW, a 6" rod + piston combo is typically a bit lighter than its 5.7" counterparts; while lighter is typically both easier and cheaper for machine shop to balance. And, the lighter the RA is, the more quickly + more efficiently it can be accelerated.
* suggest do Not select old-fashioned, farm implement style THICK rings (Nor pistons w/ matching WIDE grooves).
Decades back, ALL carmakers switched to thin metric ringpaks; nowadays OE (and aftermarket) top & second rings are often Less than One millimeter thick. All for Very good reasons. Many of GM's current Performance Gen 1-based sbc are likewise. Thin is simply, but noticeably, more efficient. And, thinner rings seal + wear as well; and usually much better. I can't see everything, but don't know of any contemporary passenger ICE that does Not have thin ringpaks.

 

Thinner rings do have some advantages (less friction) and disadvantages from what ive read.(like a small increase in blowby) There is a forum member here who has had issues with his caddie xlr that might disagree with some of those benefits.

I looked into it when I built my 406 but for me the additional cost increase for the required thin ring pistons was pretty considerable and being that the HP gains were on average about 10hp on a 450-500hp engine which is less than something like my electric fans (often criticized here as not being worth it) gained me it wasnt worth it for my application.. Going to a roller cam however was justified for me so YMMV
I am all about adding the modern improvements to my old school SBC I added an ls1 firing order cam for its performance improvements, EFI, electric fans and of course the roller cam and rockers so please dont take my comment as a negative about going to thinner rings I am simply pointing out it really is just another optional step in improvements in trying to make the sbc perform more like an ls1 and the question I now ask myself is why didnt I just swap in an LS1 at this point?

 

Good question.

Wouldn't a 377 be more like an LS1?

 

Seems "classic" 377ci is Gen 1 OE 400 Block bored + thirty over to 4.155" with a 350 OE stroke crank (3.48"). I still have a 377 like that.

LS1 is a Gen III architecture platform; apples and oranges.

 

An LS1 is a measly 345 cubic inches versus even a 350. You’ll have to get an LS3 at its 376 ci to even get close.

Still, I know which one I would pick.

 

My point was that the 377 was the classic way to get a high-revving race engine. 400 ci bores, with a 350 crank. The LS1 (and sequels) is the new way.

I seriously looked into building a 377, since I miss the high-revs more than anything else. Then I priced junkyard LS engines. A well maintained aluminum engine (LS1, LS6, L33, etc), should be good for 750 hp, stock. Iron block? 1200 hp or so.

GM took a look at what it would take to turn a Gen 1 SBC into an LS, and built the LS instead.

 

You can still have your high revving LS cake. Here’s a thread about a 4.8 crank in a 6.0 block. 332 ci, and 1092 HP at 7,600 RPM!

 

Back when NASCAR actually ran Gen 1 sbc in CUP, there was the usual displacement limit (recall it was about 358-360 ci). That era's Chevy CUP combo was typically, again, a 400 block bored to about 4.155" paired with a 327 sbc crank (3.25" stroke), and that crank's rod journals would be turned down to smaller diameter to fit earlier 2" OD sbc size or even smaller using a Honda rod bearing about 1.88" OD. Those reduced OD journals (also inherently lighter) were also offset ground and that could Increase stroke aka lever arm and eventually approach the aforementioned mandated displacement limits. With solid flat tappet lifters, +10K rpm endurance motors were de rigueur. All that was a LONG time ago.
And, IIRC, it was FOMOCO that First developed what is commonly referred to today as an "LS Firing Order".

Also, MANY of today's so-called 383 are not at all 383ci. Many "383" crates are sold as New with New 4" bore blocks paired with a 3.75" (some 3.8") stroke crank. That 4.00 x 3.75 pairing yields about 377ci but that "383" moniker sells product.

Of course, with the Gen III prevalence and superior intake-exhaust ducts, it's cheaper to shove a junkyard LS in em and go fast.

 

I was thinking more along the lines of installing all bearings pistons, rings,rods, caps and then torquing. Then measuring all pistons for deck height at TDC. Then deciding on a head and pistons to ensure I get the compression ratio and quench I want. All parts would be removed again before any grinding was done.

Is there a better way to achieve this? My understanding is that the deck height is needed to choose the combustion chamber size and piston top style, to ensure the compression ration. I want to run pump gas. My understanding is quench and compression ratio are important. Thx.

 

Yes, they are critical. I would suggest you determine the compression ratio target first before ordering pistons and heads. How did you arrive at your target C/R?

I would not worry too much about the bearings during mock up. Torque is not critical here at all. You have to clearance the block first or the rotating assembly will not rotate. Bearings will be sealed up in the rods. Everything needs to be cleaned up after the grinding is done anyway.

 

if you have your old inserts, or some used ones from somebody else, use old inserts. Or use whatever inserts you think you want to hear.

If no grinding, use the fresh, correct size inserts when you measure how far your piston crown is below deck. During mock-up, just snug cap bolts; true No torque required during mock-up.

Best to select parts After you've measured for what's needed.

 

Yes.
You need to check all this a couple of times.
First on paper.
Then with a parts mockup.
Then the machining gets done.
Then deck height checked one last time.

That is the way race motors are built.

Every part matters when you deviate from stock.
piston compression height (pin height), block deck height, head gasket thickness, head deck cut, block deck cut, rod length, crank stroke, offset journals, etc.etc.

A completely stock GM Gen 1 block of this era starts around .025" piston deck height (down in the hole).
My buddy frequently decked the block and juggled pin height to get it to ~.002" down.
That's the way my BB was done. The block was decked .015", .125" longer rods, and carefully selected pin height pistons were used.
If you stick with common "combos" there are pistons available without going full custom.
A 383 is pretty common. You should be able to find ones you need.

 

I arrived at a target C/R after reading and researching. Pump gas is my goal for a street car. General consensus seems to be 9.5:1 for pump gas so that’s where I am aiming. I also am under the understanding that a properly machined quench area will allow higher C/R and still control detonation. For example 10:1 or maybe 10.5:1 can run fine on pump gas with the right quench clearances.

At this point I am unsure exactly what quench clearance I should aim for. 10.5:1 at 0.020” quench? 9.5:1 at 0.030” quench.

I am still reading and researching. Which includes asking questions here.

Here’s kinda what I’m thinking: Scat or Eagle 383 stroker reciprocating kit with 6” rods. Flat top pistons. Block bored at .030 over stock. Mock-up bottom end. Measure deck clearance of all 8 pistons. Assume .045” head gasket squished thickness. Get machine shop to mill deck. Then choose a head and appropriate combustion chamber size to get to between 9.5 to 10:1 C/R.

Is this sounding like a reasonable plan? Thx.

 

sort of:

Do you want to run regular gas or premium?
9.5 CR is the "old" goal, from way back, or the regular octane goal.

If premium you can go up to a low to mid 10 CR, as long as you have ~.035-.045" quench, and a DCR from low to mid 8s with aluminum heads.
The cam duration comes into play with DCR.
A 1/2 pt lower CR buys you a little comfort for tuning without costing hardly any power.

Do not buy pistons until the machine shop has determined what overbore the block needs: .030" or .060"

A common felpro head gasket gives you about ~.039" , and they tend to seal better than the thinner metal ones.

Much below .040" quench is too tight. and too far past .050" and above you lose a lot of the quench effect. The mid 70s L48 ran ~.065" or higher from GM, and had pinging problems. But that was only one of it's issues, the very low CR was the main one ~7.5 CR.

You need to buy the right piston dome to get the right CR.
A Scat kit is a good idea. But check with them on recommended specs like deck height and head ccs.

The heads are where the power is, like 70HP worth. So if you want the best, start with AFRs and work from there.

I have not built any 383s, but the basics are the same.
Someone else will chime in I am sure.

"The standard nominal deck height for a production Chevrolet Small Block (SBC) engine is 9.025 inches, measured from the crankshaft centerline to the cylinder head deck surface."
"Builders often deck (machine) the block to 9.000 or 9.010 inches to ensure a "zero deck" surface (where the piston sits flush with the block deck at top dead center)."

Most pistons are designed around the above measurements.
Here is a Wallace Racing Compression Calculator, but these specs give a 10.9 CR with .048" quench, with a .010" deck.
With this piston dish, 72cc heads work better, at 9.9 CR.

You need to work at this.
And then run everything by your machine shop.
Or one of the engine builders online here.

link

I would suggest you run a few head, & piston combos thru that until you find one at a price you like.

IMO 6.0" rods will just gain you more expensive pistons, with a higher pin height/compression height. Piston comes first, before rod length, in my book. But there are probably lots of 6" rod "kits" available, scat lists like 56 versions.

A bigger "elephant in the room question" is: do you want cast or forged crank, forged or hyper pistons?
How hard are you going to lean on it or rev it?