cj92firebird

Will the rotating assembly out of a 3.1L fit in a 2.8L block without any special machine work (Crank, Rods, and Pistons)?

Project 3.4 Camaro

99% sure it will, only thing I can think of is that the 2.8 is externally balanced, if the 3.1(idk if it is or not offhand) is internally balanced make sure you use a 3.1 flexplate as well, harmonic balancer doesn't matter which I believe.

Maverick H1L

iTrader: (8)

Need to correct a couple things as were said above... Pre-88 2.8 engines were externally balanced. Anything later was internally balanced. However, if you're not sure, when you pull the engine, take the wheel off of the back of the engine and look at the engine side of it. If there isn't a weight, you can use it with the 3.1 internals. If there is a weight, you need the right wheel. Otherwise you'll shake the engine apart and need expensive machine work on the block AND another rotating assembly.

GTA50

I bored and stroked my '89 Firebird (2.8 to 3.1 rotating assembly). But again, internally balanced, so no problema.

cj92firebird

I have a brand new flywheel for a 3.1L. I'm going to use it with the 3.1L harmonic balancer on the 3.1L crank. Everything is 3.1 except the block. Shouldn't have any problems then right?

Project 3.4 Camaro

Correct.

cj92firebird

One more question. My 2.8 that I had in my car has flat top pistons and I would like to use them in the new motor that I'm building. Would the 2.8L pistons and rods work on the 3.1L crank without any clearance issues?

cj92firebird

Anyone able to answer the question above about the 2.8 pistons and rods on a 3.1 crank? I need to know because I am going to begin reassembly soon. My block just got back from getting dipped and new cam bearings.

Maverick H1L

iTrader: (8)

Not having done it, or knowing anyone who has, even though there are kits out there for stroking the 2.8 to a 3.1, my guess would be you'd need pistons. The rods are the same length, 5.7", but the stroke length of the 3.1 is 3.31" versus the 2.8's 2.99". With the piston being only .017" below the deck at TDC on the 3.1 and .025" under at TDC on the 2.8, my guess would be that the 3.1 piston uses an offset wrist pin location to make up for that .15" difference in stroke height.

I'll look up one of those kits and verify this... I'm pretty sure they also come with pistons besides the crank.

:edit: Just looked it up. Yes, those kits come with a full rotating assembly, pistons, rods and crank. Site: http://www.engine-parts.com/GMV6/gm2831stkr.html

Project 3.4 Camaro

if you already have the 3.1 rotating assembly and 2.8 pistons, just try putting one on and see if it works or not imo, generally the best way to find out parts interchangeability between diff engines of the same platform, unless someone is around that knows the answer, and if maverick doesn't I can only think of 1 or 2 others who might.

GTA50

I'm thinking back to when I stroked my 2.8 and if I recall correctly, the machine shop said you need the full rotating assembly of crank, rods and pistons. The 2.8 pistons don't work - I seem to recall asking as I had a spare set of 2.8 rings that are now collecting dust.

Maverick H1L

iTrader: (8)

You'll want to check to make sure the crown of the piston doesn't come above the deck surface of the block. The head gaskets are only .060" thick max (for the late model 3100 engines, ours are .040" thick as advertised), which means that the head is the thickness of the gasket above the deck. My guess would be, as said above, that the piston will come to a stop .125" (1/8") above the deck.

The pistons won't work, but, GTA50, the rings will (same piston diameter unless block has been bored). So, if you need rings...

Six_Shooter

iTrader: (2)

Holy crap....

Where has all the knowledge gone from this forum?

The 3.1L pistons are different than the 2.8L pistons for a few reasons:
-The increased stroke neccesitates the wrist pin being moved nearer the top of the piston, so that at TDC the piston is at the same hight in teh bore bwteen the 2.8L and the 3.1L.
-The dish in the piston is added, or increased depending on how you want to look at it to maintain the same static compression ratio.

The 3.1 rotating assembly will slide right into a 1985.5 and later 2.8L block. It does not need to be a previously internally balanced block. The 198.5 and newer block for the most part is the same as the .3.1L block, same bore, same journal dimensions, same oiling, etc.

Don't waste your time with a "stroker kit", it's all stock 3.1 parts, or made for off the shelf 3.1 applications.

The harminic balancers are all nuetral balance, all the way from 1982 to 2008.They were different on the FWDs, especially the later years, but still nuetral balance, none-the-less.

FWIW, I "stroked" my 1986 2.8L block to 3.2L, actually I stroked it to a 3.1L, but the block was already bored .75mm over, making it a 3.2L. This should twist your mind, I used a FWD rotating assembly, slide right in. I also used a FWD top end, so that I could maintain the SCR I wanted, and get the superior flow.

This is basic 660 knowledge that's been posted for years now.

cj92firebird

Here's how it is..

I bought my car with a 3.1L with a spun rod bearing in it for $300, and got a motor with the deal, but it was a 2.8 that had sat for over a three years, which I didn't know at the time. I put the 2.8 in my car and it ran like a scalded *** ape for 6 weeks, then it started knocking one night after it started ticking severely. Well, I tore into the 2.8 and the intake lifter on #4 blew apart, which I think parts of the lifter caused it to start knocking, cause of shrapnel and **** like that. Well, I somewhere along the line, got some wrong information.. I went to a junkyard and bought a 3.1L out of an Isuzu Amigo pick-up truck that was TBI, people told me that the motor would go right in my car aside for having to change the intake system. I get the motor to my house and start swapping **** over, the block mounts wouldn't bolt to the 3.1L block, and the mount holes were offset ~ 2 inches toward the front of the block. I was pissed.. Mind you I only paid $150 for that motor, so not to big of a deal. So basically I have a full 3.1L engine, in a block that won't fit in my car. I have a 2.8L engine that has 3 spun rod bearings, a spun main bearing, a piston oil ring broke in half and an exploded lifter.... The block isn't hurt at all. I was just wondering if the 2.8L rods and pistons would work on the 3.1L crank because the 2.8L pistons are flat tops.. More compression = more power = more money spent on gas, oh well, we all like power... But I guess that won't work, so I'm probably just going to use the 3.1L pistons and rods instead of buying new ones. Thanks

Project 3.4 Camaro

If the block was run like that I would still have it honed or bored a little...I wouldn't trust a block that had broken internals, but thats just me, if it wasn't run with those issues too long though then its probably fine.

Maverick H1L

iTrader: (8)

I'd definitely get that block checked and machined if a ring blew... If it hasn't been done already by the machine shop. In any case, you'll need new pistons for the oversize bores, and neither the 2.8 or 3.1 standard pistons will work.

Six_Shooter, nobody said anything about the harmonic DAMPER (it has nothing to do with balance on most engines)... It was the balanced flywheel/flexplate we were referring to. And the SCR in the 3.1 is LESS than the 2.8, 8.5:1 as compared to the 9:1 in the 2.8. And if he were sliding the newer assembly into the externally balanced block, he would require a neutrally balanced flexplate/flywheel because the balance weight on the existing wheel would shake the engine apart, requiring even more machining or complete short block replacement (line boring and crank replacement at the very least).

cj92firebird

It was a faint knock when I was at the gas station that I pulled off at. I had it towed home, and the only time it ran after that was to run 15 feet into my garage. You could barely hear the knock without sticking your head down by the distributer. The thing that gets me is with all the bearings scored (not actually spun, just scored pretty badly) there was no visible damage to the 2.8 crank. All the journals are still as smooth as a baby's ***, well as far as my fingers can tell anyway.

EDIT: The ring that blew was the 3mm oil ring on the bottom of the piston. It was still attached to the piston, and did not fall apart till I got the rod and piston out of the block.

Whats the most you can get these blocks bored without affecting cooling?

Six_Shooter

iTrader: (2)

Really?

Also there are a few engines out there that do require a properly matched harmonic balancer, that does effect the balance of some engines, the GM 660 is not one of them.

As far as SCR goes, it depends on which source you go by, I trust the sources that show that both use a 8.8:1 SCR, because that's what I've also measured and calculated. Also depending on the year, like older 2.8s they had SCRs as low as 8.5:1, but the later, such as fuel injected 2.8Ls and early "HO" 2.8s had the 8.8:1 SCR. No factory 2.8L genI or II had 9:1 SCR, 8.9:1 was the highest in the FWDs.

Yes, the flex plate/flywhell information is correct, but everyone makes a mountain out of a mole hill here, all 3.1s were internally balanced, so why is ANYONE talking about the externally balanced flexplate/flywheel, especially when the OP says he has a BRAND NEW flywheel for the 3.1?

bl85c

iTrader: (2)

Here's an old thread to help. And please check out 60degreev6.com when you need info on these motors.

cj92firebird

@bl85c Thanks for posting that thread. I have a question though. If I use the .060 head gaskets for a FWD 3.1 with the 2.8 pistons on the 3.1L crank, how much would I have to get the pistons milled down to keep the compression ratio at about 10:1?

Project 3.4 Camaro

You can't use a FWD block without a some modification and fabrication. And if thats 10:1 scr you might get away with it, if thats the dynamic though you def won't, but unless you have an aftermarket cam I wouldn't run 10:1 scr.

cj92firebird

I'm not using a FWD block. All this is going into a 2.8 RWD block. In the thread that bl85c posted it said that you could use 2.8L pistons on a 3.1L crank if you used a .06 head gasket and got the crown of the pistons milled down. I'm just wondering how much you would have to get them milled down to keep the compression ration at ~ 10:1 and I already have a cam.

bl85c

iTrader: (2)

You don't want to mill them any further. You want to keep some quench height to decrease the chances of detonation. You're more likely to run into detonation without enough quench than with proper quench and higher compression.