A little history first.... Not too much due to it being a fresh build.....

• First I got a fresh rebuilt 350 bored .040 over with standard size rod bearings and a poorly turned crankshaft resulting in no oil pressure.....

• Second I purchased a crankshaft with specs stated below and installed along with a fairly new but still used 400ci harmonic balancer......

Chevrolet 383 Cast Steel Crankshaft 3.750'' Stroke
2-Piece Rear Seal (Early Style)
External Balance
1870 Bobweight

Now onto my issue that is driving me crazy. My newly installed 383 stroker, vibrates like nothing I have ever felt..... I have read through many post on here mentioning things like flywheel and missfire, I don't believe it is either of the two listed..... I am not sure what balancer to use the 350 or 400? I figured the 383 crank would still require the balancer that it was originally designed for, but should I use the old 350 balancer...? Also if I am overlooking a missfire, what's the best method to discover said miss? I am losing my mind tracking this issue, any help would be greatly appreciated.

 

If its an aftermarket crank, consult the manufacturer to find out what flexplate/flywheel and harmonic balancer are required.

If its an ACTUAL 400 crank from an actual 400 engine, you'll need a weighted harmonic balancer. (and also a weighted flexplate, I think)

Since its a two-piece rear main, my knowledge is fuzzy, but I believe a two-piece RMS, neutral balance flywheel has NO counterweight. (whereas my 1-piece RMS, internal balance crank does have a counterweight).

 

If it was spec'd as "external balance", it should use a 400 damper and flexplate/flywheel, both with a weight on them.

The best way to look for a misfire is by tracing each spark plug wire from the distributor cap to the the spark plug. The driver side cylinders are 1-3-5-7 from front to back, and passenger side 2-4-6-8 front to back. Firing order is 1-8-4-3-6-5-7-2. It must not be too far off if it is at least running. There is a (very) slim chance you have a 4-7 swap cam, in which case the firing order would be 1-8-7-3-6-5-4-2.

 

Ok, update.... I called Jegs, they said it is likely to be the flywheel since I have the correct balancer and my symptoms are still as radical as they are..... Don't our thirdgens use neutrally balanced flywheels? I believe they are. So I am going to try having my flywheel machined to balance it for a 400ci. That being said.... onto my next issue: I built a nice solid 383, and didn't think about the transmission not being stout enough to handle it.... any budget friendly ideas?

 

3rd gen 2-piece rear main seal flywheels are neutral balanced. 400 flywheels are not.

You can have your flywheel unbalanced for the 3.75" stroke (if the shop knows what its doing).

I assumed you knew your T5 was weak, and were planning on babying it - my bad. Your choices are aftermarket like Tremec, or a T56 from an LT1.

 

Yeah I am gonna baby it as much as possible, the good news is it just came back from getting a check up and new synchros, they told me that it should be fine as long as I don't ever launch it..... So what choices with keeping it a stick require the least modification? I wasn't planning on building this potent of a motor just yet..... so I didn't make any plans on finding the proper transmission.

Update..... I spoke with summit and they advised me of a plate that bolts behind the flywheel that will counter balance the 400 crank roughly $50 to your door.... I will post my success or failure after I get my parts either sat or mon.....