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swp1983 08-10-2011 11:11 AM

383 engine vibrations????
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New engine installed and still chasing a vibration. It is present in and out of gear and starts at 2900 rpm and smooths out at 3200 rpm. I have double checked all drivetrain bolts, timing, ignition, rocker adjustment, new plugs ( non foul), disconnected belts and pulleys, put on the old balancer, and checked the compression. Now im wondering if the flexplate / torque converter was installed correctly. Any pointers? Here are the specs:
1969 C10: 383 with scat rotating parts externally balanced, isky 280 cam, dart heads, turbo 350 with 3000 stall.

lmsport 08-10-2011 11:32 AM

Was it a balanced assembly?
Scat rough balances all their cranks but the assembly must be balanced to the rod-piston-flywheel at some point.

68NovaSS 08-10-2011 12:49 PM


Originally Posted by lmsport
Was it a balanced assembly?
Scat rough balances all their cranks but the assembly must be balanced to the rod-piston-flywheel at some point.

X2 ^^

Are you sure it's externally balanced? Assuming the crank, rods and pistons weren't balanced as an assembly? A 350 would be internally balanced OEM. You say you put on the old balancer, which should have been non-weighted for an internally balanced motor if it was stock, for an external balance you would have used a 400 balancer with the balancing weight adjustment "lump" cast into it. I'm assuming the flexplate or flywheel is externally balanced, has weights? Just curious if the sum of all the parts are compatible, and properly externally balanced. Odd that it's only vibrating within a 300 rpm range.

swp1983 08-10-2011 04:40 PM

The Scat assembly was externally balanced and included the 9000 crank, rods, pistons, rings, 400 weighted flexplate and weighted harmon balancer. It included a balance sheet with weights of all parts. I tried my old balancer which was a 400 weighted piece from my old 383 to rule out a bad balancer. I put the new one back on though. If it was a balance issue, shouldnt the vibration get worse as rpm's increase?

swp1983 09-04-2011 10:59 AM

well i figured it out. After hours of research i finally dropped my trans to check for correct flexplate install. Sure enough, the flexplate was a couple holes off. The scat crank did not have a dowel pin and i just slapped on the flexplate. So, I learned, on a externally balanced 383 the counter weights on the flexplate and harmonic balancer need to be aligned properly. If not, you WILL get a vibration.
Live and learn, my next engine will be evan better. This is the first engine I have built, still cant believe it runs lol

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