Hot Rod Forum banner
1 - 11 of 11 Posts

·
Registered
Joined
·
671 Posts
Discussion Starter · #1 · (Edited)
Wondering if the 6 inch pistons have less weight vs the 5.7 variants?

About to order a 383 kit and not sure - this is NA only. no spray/boost...

thanks

to quote what ive read:

"Google what Smokey Yunick says about long rods. Long rods make more torque and horsepower. The rods are the levers,,,longer levers can do more work. Long rods decrease piston side loading. Long rods also allow the piston to dwell at TDC for a longer period allowing the combustion pressure to build and generate more downforce on the piston crown. The downside is the ring-pack intrusion and camshaft clearance issues. The rod bolts might hit a normal base circle cam. Long rods will not rev as high, but won;t be an issue on a street engine. Smokey wrote the book on long rods."

"There you go, justifies em right there. My reciprocating assembly has Scat $280.00 at the time, 6" rods which are 20 grams more than the 5.7" rods. The Probe piston I run is well over 100 grams lighter than the 5.7 type. Net gain 80 plus grams. Thats easier on the assembly and revs snappier to boot.
The lower friction thing is cool too! "
 

·
Registered
Joined
·
2,271 Posts
With all defference to Smokey , the connecting rod is not a lever , crank pin to crank centerline provides the " lever". Ts a piston rod ,not a crank rod ..Capscrew rods don't always have to be clearanced . how many years ago did he write that , ? Things change you know ..
 

·
More for Less Racer
Joined
·
20,943 Posts
Yes, when comparing 5.7" to 6.0" rod 383 pistons in most companies offerings, the 6.0" piston is usually a significant amount lighter..

Another nice plus about the 6.0" rod is many 3.75" stroke cranks for this stroker package are available with capability to be internal balance, something that is not so commin with 5.7" rod versions.
 

·
Registered
Joined
·
15,322 Posts
Pistons for 6 inch rods are lighter than those for 5.7 inch rods where 6 inch rods don’t gain much if any weight over the 5.7 rod so the net result is the 6 inch piston and rod assembly is lighter than the 5.7 assembly. This often is enough to get 350 style internal balance which allows the use of the 350 damper and flex/fly. Now there is a little difference at that and for a two piece rear seal crank and a one piece. The two piece seal crank carries a little outside balance off set on the crank that the one piece seal crank doesn’t. For the one piece seal crank that little nub of weight is moved off the crank and put on the flex/fly. You can’t mix these up because the one and two piece seal cranks use a different bolt center so flexplates and flywheels don’t interchange across the rear seal types.

Block castings especially those prior to 1986 are a looser tolerance on cross section thicknesses than 86 and newer castings and the pre 74 castings are not only looser on cross section tolerance but are generally thicker as well which can be more desirable especially if you tend to over-grind material off. So grinding for clearance at the pan rail and where the cylinder wall is blended into the crankcase can vary block to block so you want not to cut more material than necessary to get about .050 clearance between moving parts and the castings. To that end aftermarket cap screw rods generally require less block clearancing, often none. They, also, usually provide better cam lobe clearance without grinding on the big end which is on the bolt head of rods using a bolt and nut which gives me the creeps even though failures are far and few between, but for a cap bolt style where the big end is threaded and the bolt’s threaded end is facing into the rod shank usually no grinding is required and if it is it‘s not being done on the bolt‘s head.

The longer rod length eases the side thrust loads on the piston skirt but given that the longer stroke dictates the skirt be shortened and the pin be moved up for clearances at both the top and bottom of the stroke so while there is less skirt area against the cylinder wall the unit force per area of contact doesn’t really change much.

Bogie
 

·
Race it, Don't rice it!
Joined
·
8,822 Posts
rod lenght makes little to no difference in a street application however the longer rods net less weight and is easier on the crank and piston pins.
We can get into technicalities with long rods dwell at BDC vr TDC but again, no need to worry about it as thier are already lots of compromises in a street driven car it won't make a hill from beans.
 

·
Registered
Joined
·
671 Posts
Discussion Starter · #7 ·
well after spending hours reading on 5.7/6 rods, i came down on a sweet deal on a balanced eagle 5.7 setup and ordered it at 230 am - i prob would have wanted to try the 6" but im so green at this as its my first 383 too..

studs/bolts?
Another question is on alu heads on iron blocks, will stock bolts be ok (considering theres enough thread of course), do i use washers for alu heads, and should I avoid studs for my build? Im aiming for 500hp NA build, also went with hyper pistons as well.. no plans to boost or spray ever on this motor.. ( I have a wideband for tuning)


Also - do valve springs have to sit perfectly around the guide boss or is it ok if theres a small gap?

fyi i do gave a valve height install tool
 

·
Registered
Joined
·
2,271 Posts
Spring I.d. is quite a bit larger than guide o.d., you either use spring cups or internal locaters to locate the spring . you need to know the spring pocket size , the specs on the springs you wish to use as well to know which locater to use.
 

·
Registered
Joined
·
15,322 Posts
My responses are in BOLD.

well after spending hours reading on 5.7/6 rods, i came down on a sweet deal on a balanced eagle 5.7 setup and ordered it at 230 am - i prob would have wanted to try the 6" but im so green at this as its my first 383 too..

For those of us that are not new at this 6 inch, cap screw rods would have been the choice. The problem with prebalanced assemblies is if you find yourself having to carve on the rods for clearance sake then your prebalanced assembly isn’t anymore. Plus now you need a 400 style damper and flexplate or flywheel depending on the transmission if these don’t come with the crankshaft kit.

studs/bolts?
Another question is on alu heads on iron blocks, will stock bolts be ok (considering theres enough thread of course), do i use washers for alu heads, and should I avoid studs for my build? Im aiming for 500hp NA build, also went with hyper pistons as well.. no plans to boost or spray ever on this motor.. ( I have a wideband for tuning)

Aluminum heads require bolts selected for aluminum, here you need to consult with the manufacturer of the heads as most aluminum heads raise the upper deck the springs ride on by at least .1 inch this requires longer than stock valve stems and usually affects bolt length, plus aluminum requires a ground and hardened washer under the fastener head whether that being the bolt hex head or the nut of a stud assembly so even of the fastener base is the same dimension as the factory head from the block‘s deck the thickness of the washer requires a longer thread length be added to the fastener to get the same “bite” length into the block. Also, with the SBC the head fasteners penetrate into the cooling passages of the block so the threads must be sealed and with a sealer that mimics the lubricant used by the manufacturer that established the torque values. This lubricant sealer must also appear between the head or nut and the washer, the washer is dry against the head if aluminum. Aluminum heads also require attention to the head gasket. Because the aluminum has a larger coefficient of expansion that iron they require a composition gasket that absorbs this movement without rubbing the head‘s gasket surface. At the same time you need to manage the squish/quench dimension between the TDC piston crown and the step of the combustion chamber. We like to keep this dimension between .035 to .045 inch. Since the SBC piston is already .025 inch below the deck, hitting the ideal squish/quench range is hard enough with a iron head. For aluminum we usually use a zero decked block or raised crown pistons to eliminate the factory clearance thus buying some freedom in gasket selection. These are some of the reasons why back in the start of your blog I said you need to build the engine on paper and report them to us. The good old days of using high compression and a big cam to hide the poor engineering are long gone, these days to get the best from a build requires a lot of attention to the details, lot of guys still don’t understand this. Iron heads are simpler to deal with in the regards I sight above they just don’t need a bunch of design time from the builder to figure these complexities that travel with aluminum out. To that end most aftermarket head manufacturers also cast their ideas in iron. The L31 Vortec from GM is the most sighted but it requires a unique intake bolt pattern, most aftermarket heads will accept either or both the pre-1987 6 bolts per side or the 96 up L31 Vortec 4 bolt per side pattern. Be aware that the 87 through 95 use the 6 bolt per side pattern but the bolts along the plenum change their angle of incidence to the head.


Also - do valve springs have to sit perfectly around the guide boss or is it ok if theres a small gap?

fyi i do gave a valve height install tool

Here again aluminum is more complex requiring a spring seat to protect the softer aluminum. These can be a cup or a locater. The cup positioning the spring from its OD the locator positions from the guide OD and remains flat to its OD under the spring. Either needs to provide a little clearance about the guide but also has to lay flat in the machined spring cup. If you’re buying aftermarket bare heads which most of us will recommend save buying top end valve train parts till your grubbing fingers have actually touched the heads.
Bogie
 

·
Registered
Joined
·
671 Posts
Discussion Starter · #10 ·
thanks bogie - my heads have both patterns for intakes. Im going to mock up a piston and see what i have in the hole and deck the block accordingly for the squish without the use of a shim like i have for iron heads to target the 40 sweet spot..

I always use sealer on the oem builds i do- no issue there. I will either get arp studs or bolts-

thanks for the tips.

on another note - my xr288hr cam from amazon turned out to be a muther thumper flat tappet so thats immediately going back in the box - and i might be back to the drawing board cam wise-

i plan on running this on 89 and have 68cc heads - so hopefully with the alu heads and nothing too wild timing wise i will hopefully not ping! static going for 10.5..

The kit i bought does require very light clearancing on the rail - nothing major, and does not require a small base cam - all these aspects i triple checked -its almost a drop in -

I have the correct flexplate (with the weight for the 1pc rear) and damper from summit from my 350 build - so no issues there either.
 

·
Race it, Don't rice it!
Joined
·
8,822 Posts
On the rod bolt deal clearance grinding and pre balance deal, you will not remove enough material to warrant any other balance issues unless you grind all the way to the bearing tangs. Prebalanced just means the rod weights are checked at the manufactuere and bundled in set that are pretty close to each other in big end weight, small end weight and total weight. They cant balance to the crank or the pistons because they have no idea what they will be attached too.
Keep in mind that balance isn't even a exact science nor is all of the dynamic even understood so grinding off 2 or 3 grams for clearance and feeling like a re balance is needed is complete BS.
 
1 - 11 of 11 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top