377Z

iTrader: (1)

The problem with being ignorant is that you don't know you are. Stock 400 rods are 5.565", not 5.7" long--a fact that I was blissfully unaware of when I ordered my engine kit from PAW several years ago. I recently took the motor down to the short block and when I learned this I checked, and sure enough the TRW flat tops are L2256's--350 pistons--so I have the gimp rods.

Question 1: Is this a big deal? I end up with a wonderful rod/stroke ratio of 1.484. Plus, I've heard these pistons, which are 610 grams IIRC, are on the heavy side. Finally, since forged pistons need more clearance, I'd assume side loading/rocking may be an issue. With all this in mind, what is the practical RPM limit of the bottom end? crank is cast, though I have a feeling the rods would go first. Also, I know I'm giving up some power/torque here, can anyone offer a practical guess how much I am missing out on?

Question 2: The piston-deck height is 0.036" in the hole; I've heard its to be 0.025" nominal with new parts (block and pistons are new). Whats the tolerance on this (and is this a stupid question)? Seems I'll need to run the thinnest head gasket I can find to maintain proper quench clearance of 0.045," or is the proper quench height larger for forged pistons?

Thank you

377Z

iTrader: (1)

/\Bump/\

RB83L69

They're fine. The main issue is, that with the short rods, you are forced to use the external balance parts, assuming no Mallory metal ($$$$$$$$$) in the crank.

The RPM limits are the same for any other stock type rods, i.e. mostly limited by the hardware in them; 6000 is about as far as they should go on any kind of regular basis.

I'd estimate you're giving up about 1.36 HP and 2.04 ft-lbs of torque.

I would not use a thin head gasket just to get "quench". It is too much risk of failure. A motor with less than optimum "quench" and head gaskets that actually seal will produce more power than one with blown thin head gaskets and theoretically perfect "quench", every time.

377Z

iTrader: (1)

:sillylol:

Judging by the woodies some people get over long rods, I figured this wasn't too unreasonable to ask, oh well. I'm glad to hear that they will be okay, but if I could do it all over again I'd still go with longer rods--I glanced at some prices and 5.7 rods & corresponding pistons aren't more expensive.

RB--whats the thinnest head gasket you would go with? Is the Fel-Pro 0.015" installed height gasket too thin? The reason I ask is that I am concerned with detonation and I've read that the having the proper quench height is important. In your experience, is it? Whats the largest quench distance you've run, & at what compression?

With a piston deck height of 0.036, 6cc piston reliefs, std bore, 3.75 stroke, 67cc head, a 0.015 gasket gives 10.2:1 compression. Going to a regular gasket with a 0.040 installed height gives 9.63:1 compression. The heads I have are iron, 10.2 is probably a bit high for that, but is 0.076" quench clearance fine to run or am I splitting hairs here?

I did notice the dish on factory pistons, doesn't this basically render the quench area useless?

Please bear with me on all the questions, sometimes its hard to tell between fact and rumor--& I'd like to avoid as many mistakes as I can.

Thank you

gmsmallblockguy

Hello all,

I am considering a simmilar setup and was worried about the same side loading issue.
As with everything the short rod/long rod debate seems to be give and take. With the shorter 400 rods you have more clearance for a larger cam but you have a slightly weaker rod and wider range of stress on the piston when opposed to 5.7 or 6" Hbeam rods.

377Z: You said you recently took it down to the shortblock. Was this after running it for a while? Did you see any uneven wear on the bore or outer piston skirt?
With the 5.565" rods did you have to grind any rod bolts for clearance?

RB83L69: With the short rods which were originally designed for the 400 crank, and (lighter?) pistons would it not be possibly to internally balance the assembly via removal of material rather than Mallory metal?
Sorry if this is a dumb question I have never balanced a crank. I allways left that to the machine shop.

RB83L69

Right, the dish pretty much invalidates any possibilty of quench. That's alot more of an issue with a higher-compression flat-top motor; mostly it helps prevent detonation, by making the mixture circulate around in the cyl so much that no uneven concentrations of fuel can occur. With the large flat deck surface meeting a large flat piston, that can be an issue; but with dished ones it's really not.

The super-thin gaskets are steel shim stock. They don't have the ability to make up for very much movement or dimensional change in the heads and block at all.

I'd suggest just using the regular 1010 gasket, .039" IIRC.

RB83L69

gmsbg:

The problem is the crank counterweights. There isn't enough room for them to be the full cylindrical shape necessary to counterbalance the rod big end and journal. Think about what happens as the crank rotates: when the rod journal is at TDC, the counterweight is at BDC so to speak, opposite the journal; when the piston is at BDC, the counterweight is at "TDC", right immediately underneath the piston. With the shorter rod, there isn't enough room at that point for enough counterweight to completely balance the engine, even with the big hole drilled out fo the rod throw; it has to be "flat cut" across the top of the counterweight, otherwise the bottom of the piston will hit it. I don't think there's any way with steel rods to make the assembly light enough to do that; maybe with aluminum rods, but I wouldn't run those in anything that I didn't tear down and throw them away on a very regular basis. And it wouldn't make any real sense to build a motor with short aluminum rods anyway.

377Z

iTrader: (1)

Thank you RB.

gmsbg: I didn't have to do anything to the rod bolts, however this seems to be hit or miss from what I've seen (pun not originally intended, but its pretty bad so I'll leave it in). I didn't take the shortblock apart so I can't comment on cyl wear, but its got so few miles on it I doubt anything would be seen yet.

gmsmallblockguy

Sorry maybe I am just being stupid today. Why do the shorter rods work in the 400 SB without balance or clearance problems but not in the 350. Does the 400 piston have a shorter skirt? The only thing I see as being different here as far as mass that would require balancing different than the 400 is the piston. Am I missing something? Did the 400s use all external balancing from the same issue?

RB83L69

The 400 block casting is slightly relieved at the base of the cyls to allow rod clearance. Since this isn't necessary in a factory 350, it often must be supplied by the user.

When used in a 400, the short rods require external balance. All 400s are externally balanced from the factory.

The reason they used short rods in the first place is because when they designed the motor, they had to put the wrist pin some place; the 350 "compression height" (1.56") is where the piston is moved up in the piston to the farthest point that still allows wide rings with factory spacing etc. In order to use a longer rod in the 400, the wrist pin will intrude into the factory design ring space. The solutions are: (1) move all the rings up toward the top of the piston; (2) reduce the spacing between rings; (3) use narrower rings; or (4) allow the wrist pin bore to be overlapped by the oil ring. None of these compromises were acceptable to the factory, so they shortened the rod, which in turn requires external balance.

The reason that the balance is different in a 400 from a 350, is because the stroke is longer, so the mass of the rod throw is farther from the crank centerline. Remember, balance depends on 2 things: weight and radius; not weight alone.

gmsmallblockguy

Ok thanks that all makes sense now. So if I use a 400 crank and 400 rods in a 350 then I will be using external balancing and require limited internal clearancing but if I went with 5.7 or 6" rods I would be able to do internal balancing and have to do more internal clearance work.

Mark W. Winning

Last thursday night, I clearanced my 350 block. I used a Summit 383 crank and 1995 LT1 rods. I have to remove very little material from the block. Now, I believe this was impart due to the LT1 rods. They seem to be a little less bulky them the last motor I did with "X" rods. I could be wrong, maybe someone here can answer that.

As for using the 5.7 or 6.0 inch rods with the combo you listed above, it would still be externally balanced, but you could change to internally balanced with the 5.56, 5.7 or 6.0 inch rods. I do not believe rod lenght has any bearing on internal or external balance.

Mark

RB83L69

Rod length definitely has a bearing on whether you can use internal balancing or not.

You're not listening, and need to go take a good hard look at a stock 400 crank, to learn what the real deal is. Set a 350 crank and a 400 crank side by side and look at the shape of the counterweights. Basically it is impossible to internally balance a SBC with 3.75" stroke and 5.565" (stock 400 length) rods, for the reason I outlined above; the piston comes so close to the crank centerline at BDC that there isn't enough room for enough counterweight, because it has to be shaved off to miss the bottom of the piston.

You're right however that you could continue to use external balance with the longer rod combinations if you want to. You just can't go the other way.

The PM rods have a somewhat different bolt, and a little bit less metal around it, and so would not require as much work to make fit.

camarojoe

This is true about not being able to use a short rod w/ a 3.75 crank and I can testify to it. When my "senile" former machine shop guy sold me my rot. assembly, he sold me 400 rods, matching TRW pistons, an INTERNALLY balanced Scat crank, a 400 balancer, and a set of rings. The pistons were coming in contact with counterweights, like stated by RB83L69, and then this guy advises me to shave the piston skirts! Being gullable and not knowing much at the time me and my father took his advice. After the install and when we first started the engine, all H*** went loose. The engine was so far out of balance that it probably could have shaken anyones teeth out after so long, even the Terminators! I obviously have to pull the engine back apart and do, and that's when I took the rot. ***. to another shop and find out that they had to take out about 400grams on each side of the crank to make it balance out. And that of course was with a 400 ext. balancer, the same one that you have to run with the short rods.

Anyway, there's my horror story, and right now I'm going to have to pull the engine again, do to ring failure, because of this machines shop I used to go to, which I find out is the worst in town. My cylinder bores are probably all 3 degs off from being perfect. But my reccommendation is to forget the 400 (junk) rods and go with the longest rod you can afford. Which probably will be a max of 6". Also get the lightest parts you can buy. 600+ grams for a perf. piston is a joke. I guess it's alright if you're only spinning to 5 grand, which a lot of people with 400's get away with because of the tq. But it all depends on what you're goal is for the engine. If you want a engine that screams up top go with 6" rods, if you want it to max out at 6000 rpm then 5.7s would be alright.

Quench is very important when running flat top pistons. Check with the manuf. but I believe most perf. piston companies, suggest a quench of about .040. So you'll most likely need to get the block decked which helps out with detonation, anyway.

Mark W. Winning

Thanks man, that is something I did not think about with the 5.56. I was trying to think why a rod lenght would have a bearing on balance, but was only thinking about direct cause and not the indirect cause. The piston skirt makes very good sence.

Mark

377Z

iTrader: (1)

Hence my concern. With a piston deck height of 0.036" and a standard 0.039" gasket I'll get 0.075" quench clearance. With the flattops I have compression will come out to 9.65:1. So in theory I have excessive quench height and I'm on the high side of iron heads' tolerance for compression on pump gas. We all know theory sometimes doesn't hold up in the real world, so I wonder if anyone has run a similar quench height, and what the rest of their combo looks like. I'd rather not find out the hard way that this will knock like crazy.

Even with 0.015" shim gaskets (which many people consider sketchy for street use), quench height is 0.051." Don't know if this is okay, and with these gaskets compression would be closer to 10.2:1.

I'm going to double check the piston deck height measurement, and if it still checks so large, I may consider getting some dished pistons (if I'm going to have to disassemble the shortblock, I want to lower compression to about 9:1) and having the block decked.

camarojoe

Deck your block .025" Don't use that thin of a gasket. Run the Felpros #1003, I think they're .041" compressed, which would be good. I'd also suggest having someone experienced to smooth out your combust. chambers a bit also have them unshroud the valves if needed. This and decking the block will help out big time against detonation.

RB83L69

The problem in a 400 isn't the piston skirt, it's the pin boss.

Remember that the rod is .145" shorter in a 400 than in a 350; and the the stroke is also .270" longer, which means the crank throw is .135" longer. The top of the stroke is in the same place (top of the block), so the increase in the stroke is all downwards, which means that the bottom of the piston is therefore the sum of those 2 numbers, or about .280", closer to the crank centerline at BDC in a 400 than at BDC in a 350.

In a 350, the bottom of the pin boss misses the crank counterweight when the piston is at BDC, and the crank counterweight is at "TDC" so to speak, by about 1/8", or less. Therefore, in order to fit a counterweight under that farther-down piston in the 400, you lose at least 1/8", actually more nearly 1/4", of metal off the top of the counterweights, in a curve that's a much larger radius than the circumference of the counterweight. The radius of that cut across the top of the counterweight looks almost like a big "flat spot" across it, which is why people call it "flat cut"; it ends up removing so much metal (weight) from the counterweight that it's no longer possible to balance the motor internally. However, if you use a longer rod, it is possible to internally balance it, because that way there's room for the full-size counterweight; but to gain that benefit, you compromise the ring package.

It's hard to explain in words. A drawing or some pics would be really helpful, but unfortunately I don't have any.

Don't worry about "quench" with a dished piston, it's meaningless. Ignore all that. Use the Fel Pro 1010 gasket, it's the right one for your bore size, and the right thickness.