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Old 09-16-2006, 11:54 AM   #1
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Default Forged Replacement Pistons for the LN3

Just thinking out loud again, and thinking ahead to the first overbore.

Anyone given any thought to replacement pistons for the 3800. So far, I've found the one that has been discussed here before, Diamond. All the rest of the manufacturers seem to focus on the older Buick GN 231cid. I imagine JE, Wiseco, and others would make anykind of piston ya want for a price.

But, what about using GN 5.960" rods and GN pistons in a LN3? Seems like that would kill two birds with one stone: forged pistons with thicker wrist pins and stronger rods to go with them. The selection of piston crows is pretty good too..dished, flattops, and domes depending on who'* 231 piston you are looking at. Rod choices are the stock I beams and H-beams.

I guess the question would be will it fit in the block and what is the stock diameter of a GN rod journal (3800 is 2.250")

Thoughts anyone?
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Old 09-16-2006, 12:27 PM   #2
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The first question I'd ask is how many cars here (including those with high boost and high power) have ever broken a piston, wrist pin, or rod?

The second question after that answer comes back as Zero, is why spend the money on more durability when you don't need it?
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Old 09-16-2006, 01:07 PM   #3
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Not sure how to answer this one...

The 3800 SC from 91-95 Used the Same crank as my 91'*... From what we have seen on the rods, the SC uses a floating pin, and the pistons are supposedly hardened...

I have only seen 1 3800 LN3 pop a rod( Owner ran it low on oil ... )

The rods in the 3800 seem to be pretty beefy when you compare them to say a Ford 4.0 60* V6...

But I am sure there are items out there for them... Maybe someone with an 88 service manual could tell us what the Rod journals were on the Old LG3 3.8 V6

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Old 09-16-2006, 01:24 PM   #4
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Quote:
Originally Posted by willwren
The first question I'd ask is how many cars here (including those with high boost and high power) have ever broken a piston, wrist pin, or rod?

The second question after that answer comes back as Zero, is why spend the money on more durability when you don't need it?
Fair enough. I'm hitting the motor with nitrous oxide first (first run is next weekend, as I am currently installing it today), followed by a blower (next month or the following month), followed by nitrous oxide and the blower together (when I'm comfortable trying the two together.) Sooner or later something is going to "pop". The pop part I don't mind very much, always carry the means to have it towed home.

That'* the train of thought...after the pop, what can I use to rebuild it with to increase the likelyhood of it surviving the next round. First thought that comes to mind is the rotating assembly...hence the question what is available that is stronger than stock.
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Old 09-16-2006, 01:32 PM   #5
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Curt you make wme wonder how much of a diffrence there are in the pistons between the LG3 and LN3... Both of them have the identical 3.40 in stroke and 3.80 in bore, I don't think the deck heigth shrank any in the LN3 over the LG3... Almost makes me wonder if the same rods were used, and possibly could the LG3 pistons be used in the 3800 LN3 application... If I remember right there are pistons galore for the old 3.8

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Old 09-16-2006, 02:23 PM   #6
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Quote:
Originally Posted by jr's3800
Curt you make wme wonder how much of a diffrence there are in the pistons between the LG3 and LN3... Both of them have the identical 3.40 in stroke and 3.80 in bore, I don't think the deck heigth shrank any in the LN3 over the LG3... Almost makes me wonder if the same rods were used, and possibly could the LG3 pistons be used in the 3800 LN3 application... If I remember right there are pistons galore for the old 3.8
Don't know, that'* why I'm asking. Deck height of the GN block vs the LN3 would be very important to find out. I tried searching the Turbo Buick sites, couldn't find a number for deck height or rod journal diameter. The bore and stroke numbers are the same, so the difference in rod lengths would have to be accomodated by either the wrist pin hole placement in the piston, the deck height, or some combination. Either way, it seems I would have to use both the GN piston and it'* longer rod as a pair.

Is the rod journal on the L67 the same as the LN3? Over in one of the forced induction threads, as an argument against putting a blower on the LN3, was the comment that the L67 pistons and rods were stronger than their normally aspirated brethren. But are they also interchangable? (i.e. do they share the same 2.250" journal diameter) Based on your comment that the L67 crank was the same as your 91, then they must be interchangable...and that would open up another option of using just better stock replacement pistons (or forged copies) with L67 rods.
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Old 09-16-2006, 02:31 PM   #7
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Quote:
Originally Posted by clm2112
Is the rod journal on the L67 the same as the LN3? Over in one of the forced induction threads, as an argument against putting a blower on the LN3, was the comment that the L67 pistons and rods were stronger than their normally aspirated brethren. But are they also interchangable? (i.e. do they share the same 2.250" journal diameter) Based on your comment that the L67 crank was the same as your 91, then they must be interchangable...and that would open up another option of using just better stock replacement pistons (or forged copies) with L67 rods.
The rod journals on the LN3 and L67 Series 1 are the same. The cranks are interchangeable. Keep in mind that the L67 uses a bushing/thrust bearing style wristpin, and the LN3/L27 use a floating wrist pin.
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Old 09-16-2006, 02:32 PM   #8
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I would only replace them if you were doing an engine overhaul.

If you HAVE to replace them, yes, then go forged for insurance against failure and that extra reliability when you wind it out to 6500 rpm.
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Old 09-16-2006, 02:35 PM   #9
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Quote:
Originally Posted by 2000SilverBullet
I would only replace them if you were doing an engine overhaul.

If you HAVE to replace them, yes, then go forged for insurance against failure and that extra reliability when you wind it out to 6500 rpm.
He'll never wind out that far. 5000-5500 is going to be tops for him. The Series 1 doesn't make any power up there. It'* all down low in the RPM band.
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Old 09-16-2006, 02:44 PM   #10
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Quote:
Originally Posted by clm2112
Quote:
Originally Posted by jr's3800
Curt you make wme wonder how much of a diffrence there are in the pistons between the LG3 and LN3... Both of them have the identical 3.40 in stroke and 3.80 in bore, I don't think the deck heigth shrank any in the LN3 over the LG3... Almost makes me wonder if the same rods were used, and possibly could the LG3 pistons be used in the 3800 LN3 application... If I remember right there are pistons galore for the old 3.8
Don't know, that'* why I'm asking. Deck height of the GN block vs the LN3 would be very important to find out. I tried searching the Turbo Buick sites, couldn't find a number for deck height or rod journal diameter. The bore and stroke numbers are the same, so the difference in rod lengths would have to be accomodated by either the wrist pin hole placement in the piston, the deck height, or some combination. Either way, it seems I would have to use both the GN piston and it'* longer rod as a pair.

Is the rod journal on the L67 the same as the LN3? Over in one of the forced induction threads, as an argument against putting a blower on the LN3, was the comment that the L67 pistons and rods were stronger than their normally aspirated brethren. But are they also interchangable? (i.e. do they share the same 2.250" journal diameter) Based on your comment that the L67 crank was the same as your 91, then they must be interchangable...and that would open up another option of using just better stock replacement pistons (or forged copies) with L67 rods.
Yes... The Cranks are the same... The LN3 like its L67 Cousin uses a 737 or 4737 Casting number for the crank...( 91 LN3, 91-95 L27 and 91-95 L67 should all use this crank ) I think the rear diffrence for 90 was the fact that it still used a rope seal for the rear main as the 91'* went to the one piece...

Piston Pin Diameter for the 91 LN3 is 0.9053-0.9055 in
Connecting rod journal 2.2487-2.2499

I am not 100% here... But when GM went from the 87-88 Vin 3 3.8 V6 to the 3800 Vin C V6, there were changes in the piston design... I believe that the pistons got shorter, and the rings were moved up to help aid in reduced emissions... So there may be some diffrences there... As for the heigth and location of the Piston Pin within the piston I couldn't say...

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