Figured I would start a thread, I have had several emails regarding the use of a 3.8" stroke crank in the LT1, and to my knowledge, nobody has used one. So hopefully this answers everyones questions.

I already have built 1 LT1 using the 3.8" crank. For me, I located my first rotating assembly from a 300 mile ZZ383 crate motor that had a bad block. Price was right (free). These engines come with a 4340 cola crank from GM. I ended up having to replace one connecting rod and piston, which are both available through GM dealers... the rest were perfect. I chose to replace mine with 4340 I beam scat forgings since the GM rods were rather pricey. The pistons were $35 each. They use a metric ringpack like the LT1 as well. These pistons and rods are press fit only. The 4.00" bore and 3.8" stroke makes 382 ci, which GM rounds up to make the 383. The pistons are skirt coated hypereutectic with an 18cc dish... the rods are powdered metal capscrew style., assuming you use GM components.

The crank I had came with an externally balanced flexplate and damper. The LT1 uses a neutral balance front damper and external balance rear. So for the crank to work right, internal balancing the front was needed, which required 3 slugs of mallory (heavy) metal drilled into the counterweights, while leaving the rear alone. This process cost me $300... where you are, it may cost more. This allows the use of a neutral balance front damper like the LT1. Adding weight to the stock damper is NOT an alternative.

This combo fit fine in the zz383 block, but did not clear the LT1 in a few places, most likely because of the rods being different. The GM rods were capscrew rods and the ones I used weren't. Slight additional clearancing of the block and factory windage tray were required. Most was in the front, interference between the block and front counterweight, which was very easy to fix. Other than that, everything bolted in fine.

I used Clevite 77 'H' bearings in my build, because they have chamfering that in common on 4340 cranks that many factory cast cranks dont have. Clevite 'P''s fit and I started block assembly with them, but quickly changed them when I picked up on the difference.

I am going to be fortifying my engine in the near future when I return from the desert. The block I used was a std. bore 2 bolt main block the required nothing more than a hone, fluxing and tanking; I will be adding 4 bolt mains with studs, and replacing the pistons. To date, the only piston I found that I could use to replace these without going custom was the TRW forged L2491F, which came almost to exactly zero deck;1.430" compression height, 3.4cc 2 valve relief flattops, floating pins, .030 only available, $330 a set. With LT1 heads the compression was very high...I will be doing some work to lower my compression some, but those with AFR heads should be OK.

For the record...this crank came with 5.7" rods because it was originally externally balanced...the internally balanced versions require 6" rods because the counterweights are larger, so they need to space them further away for piston to crank clearance at BDC.

Any more info I dig up I will post here. I found this as a very cheap alternative to a 396, and I liked the fact the I didnt need an overbore or fat rings to do the job. I had no issues with the cam, tuning, or driveability at all. Still used low friction rings, flexplates, flywheels and other components like the factory LT1.

I will soon see how one of these runs with a single plane, 3" true duals and one of brets cams on a dyno. I already have a set of lloyds heads now.

Pistons with a compression height or compression distance of 1.425 will fit right at zero deck without decking. Pistons for this stroke cannot be looked up by anything other than compression height since compression ratio and other factors will be different than listed due to having a different stroke than listed. Use 1.425 CH for 5.7" rods, 1.125 for 6" rods. Other CH pistons can be used, but try not to stray more than .020" from the deck at TDC or performance may start to suffer. Figure your compression height by using this formula:

deck height (stock is 9.025)-rod length-half crankshaft stroke+ compression distance to deck. Then subtract the compression height for your pistons and you know how far in the hole you are at TDC.

Example: 9.025-5.7-1.9=1.425
1.425-CH (1.425)=0 deck.

Stock HT/ZZ383 GM slugs work, but they are the same price as TRW forgings...why not just go forged? At this stroke at zero deck, you will need to pay more attention to compression, and most likely need a dished piston. This with .040" Flatout head gaskets at .040 will give an ideal quench.

Any other ?'s, please post them here.

 

Is there any way you could use this crank and not have such a high compression because I'd like to build a nitrous motor, but with the compression, the 3.8 doesn't seem fesible.

BTW...thanks for making this. I really needed it.

 

You could get some custom pistons to do that. The GM hypereutectics they have for the zz383 look well made just like some keith black's, but I wouldnt give them too much of a dose of nitrous. I do recall some builds with the crate motor that used some nitrous though. Just make sure you dont go lean.

I'd get some custom forgings then. Dont cost too much more than off the shelf pistons.

I got a 10.3:1 CR with LT1 castings and impala ss gaskets with a stock deck height and GM pistons.

I still plan on using a little nitrous...what made this idea so attractive was that I could get a few extra cubes without having to bore/weaken an otherwise stout block. I can keep bore intergrity at it's maximum for nitrous or a supercharger this way.

 

How would I go about getting custom piston heads for nitrous, and who would I contact to do it? What company would you recommend for a nitrous shot. I'd like to put a fairly sized shot into the motor and wouldn't like to screw it up when I do it.

 

how much different would a 4" crank be clearance wise than the 3.8"?

 

As far as who can do custom pistons for you, look up mahle, JE and SRP. There are others out there who will also do them for you. Chences are, you will either have to email them and wait or call them to get a price quote. I have never had to use custom forgings before so I couldnt tell you. Some of the guru's on the board could answer those questions for you better than I can.

If you are planning on using nitrous, make sure you tell them. Pistons that see heavy nitrous or boost are usually made a little differently (for example, more distance from the piston top to the first ring land).

As for the 4" stroke, it really depends. Some guys have used 4" cranks in their LT1's, and I have heard of guys going as far as using 4.125" strokes (I wouldnt do that, makes an oversquare engine). As far as the 4" stroke is concerned, you would just have to try it to find out if your block will work with it. Chances are you will end up in a water jacket. With an engine like that, you should consider a little bit of hardblok, a small base circle cam, and bigger heads and better intake to take advantage of the 4" stroke. If you dont do that, there really isnt any point of going 4" IMO.

If it were me, I would also consider keeping as small of a bore as possible, just because I'd be worried about rod angularity compromising the cylinder walls. I'd also go with as long as a rod as I could, which would most likely be 6" considering the crank and deck height. .030" isnt worth dependability to me.

 

updated post with more new info.