That sucks that u have a vibration. Rule out the bad cylinder and then move on to see if those counter weights are in place. On the front and rear of the engine. An aftermarket harmonic balancer will have a weight bolted to the inside of the ring. A stock balancer will just have a casting with a chunk added to the inside of the ring.
your flywheel will have an 8 inch plate of steel welded to the inside of the starter ring. Make sure that you check both areas for the weights. If one is missing you have a conundrum. Was the engine externally balanced and someone forgot to add the weighted part or is the engine internally balanced and they added the weighted part (flywheel and/or Hormonic balancer) ? You may need to drop an oil pan to find out because you can tell from the counter weights on the crank which set up do you have.

 

Someone was really grinding down those connecting rods but it looks like they were doing it for clearance issues rather than balancing. Can you get a pic of the large counter weights?

 

3951529 is casting # for OE sbc 400 crank which fits a 2-piece rear main seal. It is a Cast Iron piece; Not forged.

OE 3951529 requires BOTH external balance damper SPECIFIC for OE sbc 400 ... AND external balance flexplate SPECIFIC for OE sbc 400

do NOT use big block parts

While an old 400 crank w/ main journals turned down to fit a +0.030" overbore 350 block 's saddles was the original way to make a 383 ...

... You still don't know what block you have; do you?

 

Here is the pictures of my crank lobes and my damper it is smooth all the way around with no weight bolted on


a friend said my crank numbers go with a 400 crank this is my crank number 3951529

 

my block number is 3970014

 

OP
just as I wrote above ... 3951529 ... your friend is correct

block 3970014 is typical 350 block from early-mid seventies with 2-piece rms.

some 400 dampers have the "imbalance" cast into the OUTER inertia ring ... it'll be sort of "scalloped" on one section of OD of the outer ring ... pics don't show it all

some of the balance-holes drilled into counterweights appear factory ... some maybe not ...
... it's unlikely the 350 4" bore rod-piston combo weigh same as OE 400 ...
... let's hope the assembly was balanced as a 383 in machine shop.

 

Download Pioneer damper catalog

pns DA-400 & DA-400HD ... pics on page 77 show the outer ring.

http://www.pioneerautoinc.com/wp-con...06/HB-2011.pdf

 

Download Pioneer flexplate catalog

pns FRA-104 & FRA-104HD ... pics on page 149

http://www.pioneerautoinc.com/wp-con...2/FRA-2017.pdf

 

so what is my next couple of steps in finding out what I need to do to get the vibration out The guy I bought the car from doesn't know who built the engine he bought it that way at a swap meet he said and according to the casting numbers there are no high end parts used so I thinking they didn't spend the money to have it internally balanced if you can even do that The vibration is there all the time at idle and gets heavier as you increase rpms the front balancer has no indents or cut outs any where on it the flex plate has no weight welded on it the crank has 3 holes drilled in the front counter weight one looks factory not sure about the other two any advice will be appreciated

 

OP RR
If you continue to run a motor with a major imbalance, it will break the crank; often destroying entire motor. It can cause an awful wreck / oil fire.

What to do? ... Buy both the damper and flexplate for 400 sbc as shown above and install them both ... and hope that's it. Racing parts not required here.
If new damper & flexplate don't smooth it out, motor will require auto machine shop to balance it. Shop will require you to bring them entire rotating assembly ...
... that is correct damper, correct flexplate, crank, rods, rod bearings, pistons, pins, rings and probably $150 - $300.

fyi ... even an External balanced motor ... when being "built" or modified with different pistons etc ... needs to be balanced by shop ... it'll still be External balanced.
On the other hand, an External balance motor can be Internal balanced" by a shop but ...
... but that often involves very expensive crankshaft, light $ components and addition of expensive "slugs" of mallory metal aka heavy metal aka tungsten ...
... $ slugs pressed into holes drilled $ front-to-back through crank's counterweights.

JMO ... grain of salt ... Your new toy may become a moneypit, timewaster or both ... if your family is not on board ... this can become a long slog (i.e. rancor).

 

Ok ill purchase a flex plate and damper for a sbc 400 if that doesn't do it im going to rebuild the engine would like to know if my block is a good base to start with and what would be a good rotating assemble to purchase if I have to build one I want some thing that will last not the most expensive but good parts

 

https://www.speedwaymotors.com/Chevy...m_campaign=SMI - Shopping (CSE) (Bing)&utm_term=4577404346894473&utm_con tent=All Products (Jun26_2019) flex plate. https://www.speedwaymotors.com/Small...m_campaign=SMI - Shopping (CSE) (Bing)&utm_term=1100309956890&utm_conten t=Speedway (Jun27_2019) balancer. if still vibrates, tear engine down and see what is in it. if cast pistons toss reciprocating assy in the trash and buy a correct 383 rotating assy. OR buy a 350. OR buy a 350 or 355 rotating assy. those 30 extra inches ain't the be-all end-all some think.

 

damper $49 delivered
https://www.ebay.com/itm/Pioneer-DA4...gAAOSwSZFZzJCD

flexplate $30 delivered
https://www.ebay.com/itm/Pioneer-Fle...QAAOSwcSFbNFJc

 

Thank you very much I ordered the flex plat and harmonic balancer hope that fixes it would really like to drive my new toy If that doesn't fix it Im looking at a scat or a eagle rotating assembly any thoughts on those two is one better than the other or are there something you would recommend

 

OP RR
Hopefully, proper damper & flexplate will correct your vibration/imbalance ... if they don't, I'll be glad to help where I can.

I can make several practical suggestions including some you may not've considered yet.

However, and so long as it seems practical, I'd like to see you fix what you have before I get into Q&A about other recommendations (including budget).

Keep us in the loop?

 

How did you check the stroke?
Do you have ANY information as to the internal components of the engine?
Not many street 383 engines are internally balanced as the crank alone for internal is close to $1000.......
I would say that you need an external flexplate and balancer.......Pioneer products makes both and are OEM quality.

Jebby

 

FWIW
Internal Balance 383 cranks do cost more than External ... but ...
... you can buy an eagle CAST 383 crank that'll Internal balance for around $275 - $300 ...
... about $100 more than eagle's External balance Cast 383 cranks.

 

I have installed the flex plate and the balancer I haven’t fired it up yet I need some advice does it matter which way I put the drive shaft back in it looks exactly the same on both ends I put u joints in sense I had it out

 

OP RR
Hopefully you've installed driveshaft by now and have driven car.

? Did the new parts correct the vibration/imbalance ?

 

The flex plate I received has two bolt pattern 3 of each my old flex plate has 6 bolt and my torque converter has 6 places for bolts Am I only supposed to use 3 bolts with the new flex plate and is that strong enough 3 my transmission is a th400