i just got my 383 shortblock delivered today, but i noticed a decently bad problem... it has a flywheel for a manual transmission, but i need a flexplate for an automatic th400!!!

is the issue simple enough that i can just buy a flexplate spec'd for a 400 off some internet site, or is it an important part of the balancing process that i cant just switch out? i must have forgot to specify that it was for an automatic when i ordered the motor...

that's wierd... now that i get to looking at it, it looks like a flexplate, although the box says flywheel on it. it has the distictive 6 big holes cut in it around the middle... i dont see any flywheels like that lol...

that's wierd... now that i get to looking at it, it looks like a flexplate, although the box says flywheel on it. it has the distictive 6 big holes cut in it around the middle... i dont see any flywheels like that lol...

A flywheel is typically about an inch thick, while a flexplate is about 1/8 inch thick. A flywheel also has a lot of weight to it, a flexplate does not.

If you see "big holes", it's a flexplate.

yeah its only about a quarter inch thick, it looks about the same thickness as the one that is on my other motor (350) and thats a flexplate on there, so im gonna assume that it is the right part

What is the base block for the 383? If it uses a one-piece rear main seal, a 400 flexplate is going to present a problem with balance. We'll also need to know how the crank was balanced (internal, with Mallory, or externally).

im assuming its a 2 piece rear main seal because it has a boss for the fuel pump, and i didnt even notice if it was a 1 or older 2 piece seal... the three 4 bolt caps say hecho en mexico, so im assuming it started life as a mexican crate 4 bolt 350, which is what i just pulled out of the truck in the first place lol... they took care of the balancing at the shop over in california (i live in omaha nebraska lmao!!!), but im assuming if i use the balancer and flexplate they supplied, that it will be ok...

im assuming its a 2 piece rear main seal because it has a boss for the fuel pump, and i didnt even notice if it was a 1 or older 2 piece seal... the three 4 bolt caps say hecho en mexico, so im assuming it started life as a mexican crate 4 bolt 350, which is what i just pulled out of the truck in the first place lol... they took care of the balancing at the shop over in california (i live in omaha nebraska lmao!!!), but im assuming if i use the balancer and flexplate they supplied, that it will be ok...

A one-piece seal block will have an aluminum "fixture" behind the flexplate, and the crank flange will be completely contained within. The two-piece block doesn't have the aluminum seal mount, but it does have a counterweight on the crank flange.

One-piece blocks have the pump flange, too.

yeah ill have to pull the plastic off and see what it is, though im assuming the flexplate they sent me with the motor will work or they wouldnt have sent it lol... i heard the newer 1 piece blocks are much better at preventing oil seepage.

How do you get around the problem of making a 383 1 piece rear main seal block? Buy a newer flexplate or um?

my block is a 1 piece. figured out is a '87 350. dunno what it was doing in a 85 suburban

yeah i think its a two piece, as in the end with the area for the oil pump has two half-pieces. also found out the crank is made by a company called "CAT." there is a leopard carved into the end lol and a part number
CK-383. I looked it up and "sadly" it is a cast iron crank at the bottom of the barrel. i dont care though, im still gonna go for 500lb/ft. and if it breaks, ill build my own damn 383 lol. at least they put the hypereuctetic pistons i wanted in there along with the comp 274 Xtreme Energy cam. that saves me from having to degree the cam and stuff, which im sure i would screw up pretty good.

How do you get around the problem of making a 383 1 piece rear main seal block? Buy a newer flexplate or um?

my block is a 1 piece. figured out is a '87 350. dunno what it was doing in a 85 suburban

There are adapters that you can buy that will adapt a two-piece crank to a one-piece block.

There are adapters that you can buy that will adapt a two-piece crank to a one-piece block.

I found some aftermarket cranks that are made for 1 piece blocks. If I got one of those I wouldnt have to adapt anything right? just buy a stock 400 flex plate and balancer and have it extrernally balanced.

if you are using a 350 block to get to a 383, then make sure that the journals are the right diameter, and if it's one piece i think you are right about the aftermarket... i think i saw a 1 piece stroker crank with the 350's journal size and the 3.75 inch stroke that is needed.

I found some aftermarket cranks that are made for 1 piece blocks. If I got one of those I wouldnt have to adapt anything right? just buy a stock 400 flex plate and balancer and have it extrernally balanced.

Nope, the flange bolt patterns are different on the one-piece and two-piece cranks, plus ALL one-piece cranks are externally balanced. You will not be able to use a "stock 400" flexplate - you will have to obtain one specifically for the crank.

Nope, the flange bolt patterns are different on the one-piece and two-piece cranks, plus ALL one-piece cranks are externally balanced. You will not be able to use a "stock 400" flexplate - you will have to obtain one specifically for the crank.


I get it now, thanks silicon :)

Were any 305 4 bolt mains?

No.