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01-04-2009, 01:24:18 AM
Recently bought a 73 Z with the original but tired L-82 350. I pulled the 350 to save it for future rebuild to return the car to original someday and installed a small block 400 in it's place. That is where the questions come in.

1. The SBC 400 runs very strong but vibrates hard as RPM increases. In the 67 Camaro with Turbo 350 trans the SBC 400 ran smoothly so am suspecting that it is not matched to the stock 11.5 inch clutch that came behind the 350 in the in the 73. I seem to remember something about SBC 400 being internally balanced/SBC 350 externally balanced. Am I right, and if so, what flywheel do I need to eliminate the vibration?

2. The temperature sender taps in the 202 heads on the SBC 400 are 1/2 inch, so the 3/4 inch senders from the 350 won't work. Which senders do I need to enable my temp gauge to work again?

3. We installed later model electronic ignition with the SBC 400 and have not attempted to reconnect the factory tach. Is it simply a matter of attaching the factory tach lead to the TACH terminal on the electronic ignition distributor cap or is other modification needed?

Thanks in advance!

01-04-2009, 01:33:26 AM
The Sm Blk 400 was externally balanced and the harmonic balancer is different. All 350's were internally balanced.
What you might need to check besides the balancer is if the right flywheel is on it.

If you can find a good NAPA store with a knowledgable counterperson who can read parts books, they can check the Echlin books to see what sending unit with the right size threads, match the ohms of your 350.

Attaching the tach to the TACH terminal on the HEI is just what you need to do!

Good luck!

01-04-2009, 02:04:43 AM
Might locate a tap on the intake for that temp sensor? Sometimes there is one not used near the thermostat housing in front.

My tach on the 70 works good connected to the tach terminal on the HEI unit from a mid 70s car.

If your flywheel is an aftermarket billet piece you might cut the weight off the flexplate and weld it on the back side of the flywheel, careful to get it lined up right. Probably not possible to weld on a factory cast/forged piece. Or maybe try this Ebay piece:



01-04-2009, 10:40:27 AM
just throwing this out there - not related to your vibrating problem.
do your 350 heads have steam holes in them? i had to have my 350 heads drilled to match the steam holes on a 400 i built years ago.

01-04-2009, 10:47:13 AM
your 400 is most likely externally balanced, which means there needs to be balancing weights on the balancing damper, and the flywheel.

Your 350 is internally balanced so the damper and flywheel do not have weights on them.

By chance did you use your 350 flywheel and damper for your 400 swap? If so, there lays your problem.

01-04-2009, 01:14:14 PM
The damper was not changed after pulling the SBC 400 from the 67, but the flexplate from the automatic was changed out with the flywheel from the SBC 350 to match the 4 speed, so apparently therein lies the problem as I suspected.

Is a counterweight as pictured available separately to apply behind the stock flywheel, or is there a specific part number/application for a SBC 400 11.5 inch clutch flywheel to quote to a supply house, or must I modify the 350 flywheel to add the appropriate counterweight?

On the temp senders, the intake would have been my first option but I have no additional tap available there so have to find alternates. The Z-28 has two, one on each head, one being the type with the tab (idiot light sender I am guessing) and the other being the stud with nut (gauge sender I am guessing) so I need to deal with both.

I appreciate all the helpful info, will post a picture if someone can tell me how!

01-04-2009, 01:16:57 PM
Ooops, yes, forgot to mention that when I had the 2.02 heads done I had the cooling holes bored to fit the SBC 400 also; good catch! This 400 has dish pistons stamped "30" so I am guessing it was bored .030 when built before I bought it. If so, what CID would it have now? Thnx

01-04-2009, 01:21:54 PM
are you sure it is 11.5" clutch. all of the flywheels for SBC I have seen at either 10.5" or 11".

assuming it is an 11", below is a link for an externally balanced flywheel which is what you need.


I would NOT run it anymore with the incorrect flywheel...

01-04-2009, 01:28:14 PM
Maybe you're right about the clutch size, I had noticed that it was bigger than the clutches I am used to seeing so assumed it was an 11.5 but will verify when I pull it before I buy anything. And no, I won't be driving it anymore until repaired; it is back up on jack stands already. The cold front came in this morning, just in time to make it more fun pulling the transmission again...

01-04-2009, 01:28:53 PM
also, reading your temp gauge issue.

all early sbc heads that I have ever seen either come with a 1/2" NPT tap for a temp sending unit or after about 1986, they came with 3/8" NPT.

I assume you are looking for a 3/8" sending unit.

If so, then a GP Sorenson part # TSU64 should work in the 3/8" NPT tap.

also you referenced 202 heads. what are 202 heads? Do you mean the intake valve is 2.02? do you know the casting number on the head?

01-04-2009, 01:50:18 PM
You have to have the 400 ext. balance flywheel. It could kill the bearings in a hurry without it.

01-04-2009, 03:02:05 PM
Take the correct 400 flexplate and your current manual flywheel to your local machine shop with a balancer. He will be able to match balance the flywheel to the original counterbalanced flexplate. If he can't, find a shop that can. Over the years our shop has balanced a bunch of 350 flywheels for 400 use, I don't believe GM ever offered a 400 in a manual trans vehicle, if they did it wasn't common.

01-07-2009, 06:17:27 PM
Thanks, all. A local machine shop here in San Antonio tells me that the stock flywheel from behind a BB Chevy 454 is counterweighted exactly as needed for the externally balanced SBC 400 and is a direct bolt on replacement, and a lot cheaper than some of the other alternatives to boot, FYI...

01-08-2009, 02:57:52 AM
Not exactly, but probably "close enough". If you compare a 400 and 454 flexplate, the weights are different.

01-08-2009, 10:59:15 AM
I would buy the exact part for the 400, if it were me. Rustbucket79 is right. The counterweight grams are different between a 454 and a 400, because the 454 is swinging the heavier rod/piston farther than a 400. Plus, if you get the 454 flywheel, install it and find out it's wrong, the transmission has to come out to replace it. That would suck.

01-08-2009, 11:17:08 AM
I think that is very good advice and I am holding out for the 400 flywheel. I need the car to be right the first time, and pulling that trans is no fun.

Another little problem remains, I am having trouble getting the factory tach to work with the HEI for some reason. It moves at the bottom of the scale, around 500 RPM, but not normally, and the factory temp gauge and ammeter are dead also. The ammeter wiring was not touched during the engine swap. Is there a reason they would all go on the blink at the same time?

01-09-2009, 09:04:21 PM
I ordered and installed an OEM-style SBC 400 flywheel and that took most of the vibration out, especially at low to midrange RPMs, but the vibration comes back at the higher end, though noticably less than before. Still haven't figured out how to get the tach working so am not sure exactly what RPM the vibration is kicking in.

Should a SBC 400 damper/harmonic balancer have obvious weights added, as the flywheel and flex plate for the SBC 400 do? Maybe someone put a 350 balancer up front? Frustrating...

01-21-2009, 12:27:26 AM
After going thru 3 so-called SBC 400 flywheels with very poor results I ordered the counterweight plate and installed it today behind the stock SBC 350 flywheel. Up on jack stands it appears to run very smooth, as it did before I put the 4 speed behind it. That is a relief; live and learn.

Found an open fusible link at the starter for the ammeter gauge so that works again, now if I can just get the TACH working I will really know what is going on under the hood...

Thnx all.

01-21-2009, 12:29:42 PM
Glad it all worked out for you.

01-21-2009, 08:46:14 PM

GM did offer the stick behind a 400 SB b/c I have stock GM f/w behind my 406.

I wanna say like in an earlier Monte or the Box Vans back then?


Doug Jaynes
01-22-2009, 10:51:35 AM
not that it matters in your case but someone said all 350's are internally balanced and that isnt true, late model one piece seal 350's (1987 up ? ) are EXTERNALLY balanced iirc.


01-22-2009, 07:59:41 PM
I think we are talking all of the older, L-48 type 350 and smaller cubed engines here, not the 1-piece crank seal jobbers.


01-24-2009, 12:55:38 PM
PDQ, the 400 also came with a standard trans in a few pickup applications, but I couldn't find a correct counterweighted stock 400 flywheel on the shelf here locally for any of those. All I found were a ton of mis-packaged 350-type flywheels without the counterweight in the 400 flywheel boxes.

It was remarkably difficult to track a pure SBC 400 flywheel down, that's why I finally went with the counterweight plate, and it works very slick. In hindsight I should have gone with that suggestion from the start.

01-24-2009, 01:17:56 PM
I think this is it??


And my old 2004 PAW cat lists GM part number 3986394 for an 11" 400 SB flywheel too.