I recently finished a complete rebuild and engine swap on a 2 bolt main 400. The engine is now installed and fired right up with no issues at all. The engine is outfitted with the original harmonic balancer specific to the 400 and a new externally balanced flywheel (the motor originally had a flex plate for an automatic transmission). Despite running perfectly the engine has a massive vibration from just above idle and up. The vibration is so strong I'm concerned I may crack the bell housing or other. Any thoughts on what may be causing the problem... old balancer faulty flywheel???
sbc 400 vibration problem
- Thread starter smeds76
- Start date
It must be the pistons then. You will have to pull apart and balance. Are you sure on the firing order? Is it firing on all cylinders? Check everything before disassembly. What other changes were made? Distributor, wires, coil, etc.... Are the lifters adjusted too tight? Lots of things to inspect.
I would put the flex plate back on and see if the vibration goes away.
Fill us in , did you run this engine before you rebuild it and did it vibrate then, or did not run and you rebuilt and that's when vibration occurred. I would have suggest switching back to flex plate also , pulling tran's and removing flywheel and installing flex plate ,start it see what it does. A pain that way , but if that would be the problem it would be better than tearing motor back down . I would only do this only if you are positive it is a vibration and not a miss as suggested earlier . I bought a engine running test stand year's back $ 125.00. Radiator and oil pressure and temp guage, I have run at least 3 engine's. on that stand. It sure does save a lot of swearing, i can set timing, carb mixture , adjust valve's running , check oil pressure, and over all engine running issue's and i had a few fuel leak issue's on one motor. So when i install a engine i know i won't have any oh sh@t 
And it is just fun to start motor every now and then on the floor before installing!!!!lol
And it is just fun to start motor every now and then on the floor before installing!!!!lolThe stand is a great idea. I purchased the engine as a core from a salvage yard. Stripped it down and started over. New or refurbished everything except rods and balancer. So I've never ran the engine before. The engine is in a mid 80s 4x4 truck now... so pulling the transmission and transfer case is a heavy venture. Engine pull may be easier with my tools and working space. The engine appears to be running perfect... no miss... no knocks... firing order is correct... valves and rockers are running as expected ( set at zero lash right now). It COULD be pistons but it just seems that it has to be a failed balancer (even though its aligned and looks fine).
Yeah , pulling tran's not simple on that one , a pain no matter what!!!!
These engines have been remanufactured by the "mass market" rebuilder types for years using stock replacement cast pistons for years without rebalancing,or problems is the reason I suspect a harmonic balancer or flywheel issue here.Will they make any power,or turn any rpm,no,but they are pretty smooth.and do make fair low end torque.
The ring on the balancer can rotate on the center and look fine. Id put a new one on and if that didnt work id take out torque conv bolts and back it off and try again to be sure its in the eng before I pulled it. 400s can make real good power and pull like crazy