What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

400 sbc rod length

TruckNutzDude

TruckNutzDude

1/2 ton status
Joined
Mar 31, 2003
Posts
3,530
Reaction score
0
Location
Agawam, MA
I'm in the process of building a 2 bolt main 400 sbc with a +.030 bore. It's got a fresh grind on the crank (still has the grind sizes in marker on it) and I'm sending the block to get checked in the next few weeks. The engine was running when I got it, with a stock intake and cam, but smoked, I found a burnt piston skirt but no damage to the cylinder wall.

I do not know how to measure the rods for length. I've heard there are 5.7" and 6.0" rods for the 400's. I took a quick measurement from eyeballed center of the wrist pin to eyeballed center of the crank bearing and found about 5.7. Worst case, if I'm too dumb to figure it out, I'll just get new rods and pistons when I buy my rebuild kit and cam. Are 6.0's better than 5.7's?
 
SBC 400 rods are 5.565" ALL others are 5.7" length.

The longer the rod the less side loading on the cylinder walls BUT a 6" rod in a 400 will put the wrist pin into the oil ring and for a street driven engine that's to see lots of mileage that isn't recommended. Also, if you wish to run a longer rod you must also change the pistons to accomodate the longer rods otherwise the piston will be out of the top of the deck.
 
I need new pistons anyways, that's the point of my question so I get the right ones. I figured I'd upgrade the rods "while I was in there" though there's nothing wrong with the ones I have.
 
Keep in mind that if you go changing parts such as rods you NEED to rebalance the rotating assembly. If you don't you're just asking for trouble.
 
About any piston company will pistons for a 400 with long rods. When I built mine 10 years ago, they were the same price as stock rod 400s. Mine pulled like a freight train off idle to ran out of road.....

While your at it, be money well spent, get a new flexplate and balancer, and like 4x4 said, get it all balanced.
 
Yes! Nail on the head there. A balanced engine is critical for getting good longevity. And you cannot just say that the weights on the balancer and flexplate/flywheel make it "Balanced."
Take everything down to a good machine shop and have every final part to weigh, down to the spiral locks and have it balanced properly.
In my findings, I have never seen a longevity issue from having the pin up in the oil ring groove. As long as the piston is designed well, and the rings and oil support rail is installed properly, I have never seen an issue. All of the trick LS series stroker hits have it that way.
I am not saying that it is not a problem in certain applications, or others engine builders findings, just what I have personally seen.
 
i have built 400's with both 5.7 and 6 inch rods and longer as well. it really depends on what im building the engine for as to what rods i use. if you use quality parts the pin being in the oil ring land wont affect reliability. make sure you balance everything.
 
The crank was balanced when it was ground about a year before I got it, but one of the pistons had burnt up due to running lean. I noticed the rods had been stamped on the caps to mark location, however none of the locations matched their markings. If I understand you guys right, "balancing" is making sure the crank/balancer/flywheel are zeroed so there are no heavy spots (like balancing a tire) and the rods/pistons need to weigh the same amount when assmebled. So my crank is balanced with these components and the only thing changing will be rods/pistons. I will make sure they weigh the same before I install them. I will either re-use the stock rods and get new pistons or get go with 5.7" rods/pistons.

This is going in my '39 chevy pickup 2wd backed by an m-21 4 speed with an undetermined gear ratio (haven't located a decent rear end yet) :doah:
 
The pistons weighing the same as the others is important, but that is not it. It is balancing the crank, (balancer and flexplate) to the weight of the new pistons. If the new pistons/rings/locks/wrist pins/rods weigh exactly the same as the ones that came out of it, then the old balance job may be O.K. But that is a fat chance. A balance job is getting the counterweights of the crank balanced to what is hanging off of the other side of the crank. (The piston and assembly.)
So just because something was balanced once, does not help when you put new parts in it. Especially if you are changing rod length.
 
Thanks for clarifying that.

I'm thinking the stock rods should work so I'll start searching for some decent stock-ish pistons along with a gasket set, bearings, a decent cam, timing set, oil pump and some new nuts/bolts/studs.

This is a very low buck truck. I'll weigh the old pistons and the new to see if I need to re-balance the rotating assembly before I put everything together. This won't be a high horse, high RPM engine, just something to cruise with and maybe the occasional squeel off the line... I'm just getting my license back after a 2 month slap on the wrist for the last 7 years worth of "faulty equipment + no inspection sticker" tickets, two more tickets in two years and I lose it for a year plus get a $500 fine to boot... I've got to be on my best behavior from here out. :doah:
 
If the pistons/pins/rings/locks are not within 20 grams, you will want a quick re-balance. It does not matter the rpm that much. It will make the engine last SOOOOO much longer. Also easier on clutches, input shafts, bearings, rods, etc.
 
WannabeRacing said:
If the pistons/pins/rings/locks are not within 20 grams, you will want a quick re-balance. It does not matter the rpm that much. It will make the engine last SOOOOO much longer. Also easier on clutches, input shafts, bearings, rods, etc.
Click to expand...

20 grams :eek1: , when balancing an engine we get down to within 1/4 of a gram.
 
It absolutely can be done. Around here, we don't go that crazy unless the specific engine will pull at least 15 inches.
However, for many years, the standard from the factory was a 20 gram overbalance. That was also a common deal among basic crate street engines. Partly for changing out of parts for the future, and also because the piston and rod combo carry around quite of mass of oil in standard running conditions. Not to mention the possible build up on top of the piston.
As I see it, they guy is trying to save every dollar he can. For a 3000 rpm deal, It is my opinion that if he is within 20, he is good to go and save that 80 dollars and put it somewhere else in the build. It is not perfect, for sure! And if he has the means and the time, it is worth going to extra mile and getting it balanced, but we live in a world where there is no such thing as perfect, and $80 may be real hard to come by. Plus, I know the clutch or converter is not within a half a gram of balance.
 
I'm shooting for less than a few grams of weight. My friend builds some wicked slant-6 engines and said I should shoot for less than 1/4 g. I may send the assmebly out with the block when I pick out some pistons and have them shot peen the rods and rebalance everything. It's just a budget engine, if I really wanted to save money I'd just replace the one piston that's burnt... the other 7 show no signs of wear for the year and a half they've seen inside the block.
 
TruckNutzDude said:
I may send the assembly out with the block when I pick out some pistons and have them shot peen the rods and rebalance everything. It's just a budget engine
Click to expand...

something to think about, since your talking about reworking your stock rods & buying new pistons, you could get pistons for a 5.7'' rod in a 400sbc & then rob the 5.7'' rods out of a salvage yard 350 for cheap. then have those rods reconditioned & shotpeened & then have the ''new'' rotating assembly balanced.

i don't hear many people talk about it, but longer rods improve torque #'s, just like putting a cheater bar on a wrench to make it longer to get more leverage on a nut or bolt to break it loose, the longer rod will give the piston more leverage on the crank, therefor creating more rotational torque.

when your doing a budget build, everything you can do to squeeze some more out of it gives you more bang for your buck, and having it balanced after you gather all your parts up, will help you have a more dependable engine that will serve you well for quite awhile.
 
blazin_blazer said:
i don't hear many people talk about it, but longer rods improve torque #'s, just like putting a cheater bar on a wrench to make it longer to get more leverage on a nut or bolt to break it loose, the longer rod will give the piston more leverage on the crank, therefor creating more rotational torque.
Click to expand...

Thats not quite it but close. The longer rod allows for more dwell time at both TDC and BDC which allows for more cylinder filling on the bottom of the intake stroke, and for more push on the power stroke at TDC. This happens because the piston sits there for a fraction of a second longer as the crank goes through its revolution.

The other part of this equation is that a longer rod gets more of the weight of reciprocating assembly closer to the crank and then has less parasitic drag. Means it takes less power to turn the engine.
 
i got a long rod 400. long rod means i put rods out of a sbc 350. keeps skirts from breakin. simple and easy to do. if u need more info google long rod 400.
 
You must log in or register to reply here.

Latest Posts

Top Bottom