I built this engine, a 355 SBC w/ 700r4, internal balance about 2 years ago. I had the rotating assembly balanced from the harmonic balancer to the flex plate. The only thing stock in the rotating assembly is the crank and the flexplate. Motor runs good. I had a trans problem and took it to my buddy's trans shop. In the process of fixing the trans (It had lost lockup), they replaced the torque converter. When I got it home I noticed that it has a vibration starting at about 1800 rpm. At 2000 rpm there is a very noticeable vibration, with the trans in park. I never noticed this vibration before the TQ replacement. I suppose it could have been there before the TQ replacement, but I am skeptical that I would have missed that. I was convinced that it was the TQ out of balance. I talked to my trans guy. He suggested that I unbolt the TQ from the flexplate, push it back, and run it without the TQ attached. I was positive that it was going to smooth out without the TQ. I was wrong. It still vibrated. This makes me wonder if that vibration has been there all along and I just missed it. It is not missing to make this vibration. I can only think of a couple of things that would make a balanced engine start vibrating. Either one of the pistons has lost a big chunk of skirt, or a big chunk out of the flywheel, or a slipped outer ring on the HB. (The HB is brand new). Or if the assembly was not balanced correctly and that vibration has been there all along, and I missed it, (I'm skeptical of this). When I was a broke youngster, I built a lot of SBC engines and never had any of them balanced. Don't remember any one vibrating like this one does. I'm about ready to pull the motor for a deep inspection. Oh boy, just what I wanted to do! I guess if I don't find any obvious problem the rotating assembly will have to go back for another balance job (to a different shop). Anybody got any ideas or had similar problems?
See if you can find someone with a vibration analyzer. They give a readout of the intensity and frequency of vibrations. You take those reading plus the RPM and go thru a chart that will pinpoint what the possible source of the vibration is without tearing into it. I used them at the dealers I worked at to isolate vibrations and it is real accurate in its findings.
Drop the pan. The “HEAVY METAL “ that was used to balance your crankshaft, maybe laying in the bottom. It’s worth taking a look.
I'll vote for " not noticing" before . 36 years ago I bought a 2 year old 1ton crew dually ,454 , th400 ,it has a minor harmonic vibration ,had the driveshaft rebalanced after I'd had it for a few months , decided to live with it , I'm still living with it after 150k miles of towing a camper & boat piggy back , 62' long &18k pounds. drive it till it breaks , then fix it !
I chased a similar vibration after I had a flywheel resurfaced and clutch and pressure plate replaced. I ended up taking the trans out a half dozen times chasing it, couple different pressure plate clutch combos etc.. Turned out the problem was at the other end of the engine. As part of the "upgrades", I had also installed a deep grooved "hipo" pulley on the alternator. It was machined wrong and center hole was about .020" off center. With the belt tight, the symptom was exactly as you described. Suggest removing belts off the front of your motor and run it up. That will eliminate anything you may have going on there.
If you look at my thread this summer on chasing a vibration similar to what you have you will see that I chased down every little rabbit hole I could think of. The actual problem was the rubber biscuit motor mount had turned into a hockey puck so essentially a solid mount. I replaced the biscuits and the vibration is gone. Perhaps in your case the trans mount is causing a problem as it would have been moved/removed during the work you had done…good luck and hope this helps!
My buddy has a 34 Ford with a 400 SBC and had a very noticeable vibration at about 2000 rpm. The engine had the correct harmonic balancer and flywheel. The vibration was there with the vehicle stopped and revved up in neutral. He spent a lot of time chasing the vibration. I borrowed a Kent-Moore vibration analyzer from work and it confirmed what we already knew that it was engine related. What the analyzer did allow us to do was to move the sensor around to find the place with the highest amplitude. That was the bottom of the transmission and/or cross member. My friend had replaced all the engine and trans mounts and said the trans mount was very hard to get in place. What we found was a lip on the cross member touching the upper area of the transmission mount. Jacked up the trans, cut the lip off and the vibration is gone. Bottom line, take a real good look at all your mounting areas for metal to metal contact. Also, closely check the exhaust for any place it might be touching metal. You might even loosen and re-tighten any flanges between the manifolds and exhaust pipes. I really think that most SBC's have an internal vibration at around 2000 rpm. As long as all the mounts are good and nothing is grounded out or bound up, you just don't feel it.
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