Piston & connecting rods

clintonvillian

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Okay, talked to Molnar today (Not Tom, he wasn't in). Put my order in, said it will be May before they are in.

Just to make sure my information is correct, he told me the rods were 6.210" Long, with 2.123" and .975" ends.

So now I am back to finding pistons.

(If this goes through, Ill have a nice set of 240 rods for sale....maybe)
 

pmuller9

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Those are stock 300/4.9 six 1969-1996 rod dimensions.
If you are looking for forged pistons to fit those rods you will have to go with custom pistons.
 
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CNC-Dude

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Yeah, that was the whole point in getting a custom length rod to fit off the shelf pistons. Now the rods and pistons will have to be custom.
 

Pontus

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Sorry to butt in here, but I'm about to start my own build and am just gathering info and want to get this straight. I'd like to open up the airways on my 4.9L EFI airways moderately (RV cam, porting, EFI exhaust, better intake/carb) and build for torque and longevity (and carbureted, not EFI, going into a '69 van). I have no problems running premium (already required in my other cars), so I'd like to counter the potential loss in low end torque and drivability by increasing compression.

If I buy C5AE 240 rods and off the shelf forged 351W pistons, with the stock 351W pins, that is a bolt on mod? I can get that for about $600, so that's looking like a pretty good investment that gets me the following...?
- 30% lighter
- less side loading and less cylinder wear
- less bearing load and less wear
- higher and faster spin

And if I get flat tops with the EFI head, that's a CR of 9.8:1 by Summit's calculator with 7cc of valve dish. But will those pistons sit at the same height in the bore? What is the deck clearance? I was hoping to increase CR with the stock pistons by simply milling the deck 0.030" but it seems that is not recommended at all, so bringing it down if needed with a potential custom gasket might be the better option. I was thinking around 9.2 might be better? Thanks for any advice.
 

pmuller9

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I'd like to counter the potential loss in low end torque and drivability by increasing compression.
What would be causing the loss of low end torque?

There are no off the shelf pistons that have the correct compression height to work on a 240 connecting rod on a 300 crankshaft.
The CH can not be greater than 1.200"

Using an EFI head is counterproductive. It does not breath very well without modifying the combustion chambers.
 
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Pontus

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What would be causing the loss of low end torque?

There are no off the shelf pistons that have the correct compression height to work on a 240 connecting rod on a 300 crankshaft.
The CH can not be greater than 1.200"

Using an EFI head is counterproductive. It does not breath very well without modifying the combustion chambers.
DOH. I totally read the original post wrong, sorry. And then I read you talking about the advantages of a 240 rod in 300 and glazed over the fact that the 302/351 pistons only work on old 240/300 rods respectively, not universally. Autotec pistons aren't too badly priced at $500 I guess. Now my eyes are bleeding after pouring over piston specs.

I don't want to derail this thread, but it's my understanding that opening up the airways in this motor causes a loss of low RPM torque and drivability, which is why Offy built the DP manifold. There's lots of talk about "high velocity" and such. I figured that increasing the compression would go a long ways to helping that if not eliminate it. Are these anecdotes and/or assumptions wrong or am I misunderstanding the issue?
 

pmuller9

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I don't want to derail this thread, but it's my understanding that opening up the airways in this motor causes a loss of low RPM torque and drivability, which is why Offy built the DP manifold. There's lots of talk about "high velocity" and such. I figured that increasing the compression would go a long ways to helping that if not eliminate it. Are these anecdotes and/or assumptions wrong or am I misunderstanding the issue?
Stay with information that is 300 specific found within the pages of this forum.

A big valve ported 300 head is still undersized for the 50 cu in cylinders of the 300 engine.
We find no loss in torque from an idle using small to moderate hydraulic cams.
There is no loss in torque from 1000 to 1200 rpm with the largest street hydraulic cams in the 220 to 230 duration range.

The Edelbrock DP shows no low end advantage and is a restriction to upper rpm power.

Raising the compression to the point of having to run premium pump gas leaves no margin for tuning and there have been piston and ring damage as a result. Don't push the DCR limits of the 300 engine.
 

benchracer

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if when we turn down our crank for BBC rods we stroke it will this move the pistons high into cylinder so we keep block from needing cut as much?
 

pmuller9

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if when we turn down our crank for BBC rods we stroke it will this move the pistons high into cylinder so we keep block from needing cut as much?
No because the BBC small journal rods require custom pistons that are made for zero deck height.
 

clintonvillian

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So, talked to Molnar (been several weeks maybe a month), anyways I was told that the RPMs play a more significant role in failure than anything else, and unless I was going to really wind my motor it was probably overkill.

I started digging, and I have found at least 3 running builds. Some of those are members here. One guy was running a 240 rod and had it bend. This happened under detonation. The other guys are running stock 300 internals, and pushing 16+ psi of boost and have been doing so for years and thousands of miles. Keep in mind this is on E85 and retarded timing.

At 400 HP/TQ is there a need to spend the money on the rods? With the Molnar rods being dimensionally the same, it doesn't seem like a necessity. I'm thinking stock rods with a good set of pistons.

What are your alls thoughts? Also does anyone know if the aftermarket hypereutectic pistons have struts like the factory cast pistons? (I am assuming no, because they don't tend to expand......)
 
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pmuller9

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There have been two bent 240 rod engine failures, one at 20lbs of boost with detonation and the other was just high boost, no report of detonation but who knows?
There haven't been any bent 300 rods reported yet.

The 240/300 high rpm rod failure is with those that have the oil spit hole which is where big end breaks.

Broken Spit Hole 2.jpg

Just remember that 400 HP/TQ at the flywheel with a supercharger doesn't include the HP/TQ that is required to run the supercharger.
The actual power that the crank, rods and pistons see is 400 HP/TQ plus the power to run the supercharger.
The turbocharger doesn't take any addition power from the rotating assembly so it is much easier on the engine.
Even if you add a turbocharger to a supercharger, once the turbocharger takes over the power taken to run the supercharger is very low.

The stock rods will handle 10 lbs of boost with a supercharger.
I would have them magnafluxed and take the time to polish the forging lines off the beams to minimize the chance of fractures starting.
Resize with ARP bolts.

The hypereutectic pistons do not have the struts. They will work fine as long as you never have detonation.
Detonation will crack the upper ring land off exposing the top of the upper piston ring.
A custom forged piston for use with boost will have a thick upper ring land and have a much better chance at survival because of the more malleable alloy.
There may still be some piston/ring damage but you may be able to drive home from a long distance meet.
 
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pmuller9

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Congratulations on getting the last set.
Inventory is disappearing on a lot of different parts like cam cores.

What would you like to do for pistons?
 

clintonvillian

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Tom posted on the facebook page that they will have the others really soon. They are in the states. Should I hold off for those?

Also, is there any reason I can't install the press fit pins at home? It seems easy enough to heat the rods up, and make/use a jig to center everything....
 

sixtseventwo4d

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The fixture to hold the piston in place should have an adjustable pin stop; so you get the pin placement correct. Otherwise you could go further and end up with the pin improperly centered, or worse.
 

clintonvillian

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The fixture to hold the piston in place should have an adjustable pin stop; so you get the pin placement correct. Otherwise you could go further and end up with the pin improperly centered, or worse.
I understand that, but other than setting up the jig and heating the rod, there isn't much to it. I mean is there any kind of "precision" that would keep you from being able to do it?

I'll either buy, or build a jig.

Also is there any limit to how many times a press fit rod can be heated and the pin replaced?
 

sixtseventwo4d

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There's a yes and no answer that will go to the quality and type of the rods, and some discretion. I wouldn't want a rod that's been heated 20 times over. Depending on the rod metal there's heat margins I would the careful not to exceed. That would mean asking the manufacturer
 
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