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Old 08-14-2006, 05:49 PM
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SBC 350 balancing question

Hey guys, ford guy here for 20+ years, got a quick inquiry about a 355 inch that has serious vibration throughout the rpm range. What is the deal with the 350 balance throughout the years. The shortblock was of the early gen, 2 piece rear main, supposedly an 80-83 year 4 bolt block. What size balancer, 8" ? Also using a flywheel, not flexplate. Are all these small blocks internal or 0 balance?

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Old 08-14-2006, 06:07 PM
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except your 400 SBC, which is external balance
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Old 08-14-2006, 08:22 PM
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The 350 of that vintage is internally balanced.

You can use either an 8" or a 6 3/4" harmonic balancer, but make sure you know the TDC timing position. Some are set up for TDC with the timing pointer at the 2:00 o'clock position (upper driver's side as you look down from the front of the vehicle), while other balancers use a timing pointer at 12:00 o'clock. You need to make sure your timing pointer matches the balancer marks.

Bruce
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Old 08-14-2006, 08:37 PM
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well bruce, I got a mr gasket timing cover with the pointer built on, its for an 8 inch balancer whihc I am using. The balancer just has a single line on it, nothing else. The real issue here is a nasty vibration, I couldnt break in the cam due to this, really pissing me off. Yes now that I think of it, the TDC is at 12 oclock. I got a bone stock reman shortblock then put my heads and cam etc... into it. The vibration is the problem here. Is it possible I got the wrong flywheel? This is in a boat so it uses a manual flywheel. Im just searching for anything before I gotta yank this again.
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Old 08-14-2006, 08:47 PM
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few pics of the engine in question
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Old 08-14-2006, 08:59 PM
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My 350 was used with a TH350 automatic, so I have a flex plate instead of a fywheel. My flex plate sure looks a lot simpler than the picture you have of your flywheel.

There are special flywheels that are used with externally balanced versions of the GM small block. The most common are the 400 and the 383 stroker. Maybe you got one of these externally balanced flywheels by mistake.

Bruce
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Old 08-14-2006, 09:15 PM
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Im wondering how to tell. I have the old flywheel here which was off the original 87' 305. Its alot of work yanking this thing back out.
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Old 08-14-2006, 11:44 PM
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Not all 350 are internally balanced. The newer style block with the one piece rear main seal are kind of a half breed. They are internally balance in the front and externally balanced in the rear is as good of a way as I can think to explain it. If memory serves me correctly, '87 is the year they went to the once piece rear main, so if your new engine is the old style with the two piece rear, and you bought a new flywheel for a one piece rear, you are way out of balance.
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Old 08-15-2006, 05:36 AM
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Its an old style block 80-83, with 2 piece rear main and I specified an old style flywheel, and if I rememeber isnt the back of the crank on the early 350's have a different bolt pattern or something making it impossible to bolt a late model flywheel on?
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Old 08-15-2006, 05:46 AM
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Quote:
Originally Posted by StrokedSBC
Not all 350 are internally balanced. The newer style block with the one piece rear main seal are kind of a half breed. They are internally balance in the front and externally balanced in the rear is as good of a way as I can think to explain it. If memory serves me correctly, '87 is the year they went to the once piece rear main, so if your new engine is the old style with the two piece rear, and you bought a new flywheel for a one piece rear, you are way out of balance.
had a 1 piece rotating assy come back from the balancer a week or so ago... internal in the front, external in the rear... this is also a boat application. uses the SMALL flywheel. and there is a distinct raised area on the block side (so you have to remove the flywheel to see it) where the offset weight is cast in.. you cannot use a 1 piece flywheel on a 2 piece crank and vice versa...

early 265, 283 and 327's used the 12 O'clock timing position (actually i believe it is 2* off vertical).. anything after i believe 67 - 68 uses the standard 10* off vertical position. if you have an 8 inch balancer make sure it is NOT from a 400 which has an inbalance in it.

btw, if this motor is going to put out some serious HP... ditch that alpha coupler and invest the 200 in a bravo coupler...
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Old 08-15-2006, 06:35 AM
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Quote:
Originally Posted by 1bad18
had a 1 piece rotating assy come back from the balancer a week or so ago... internal in the front, external in the rear... this is also a boat application. uses the SMALL flywheel. and there is a distinct raised area on the block side (so you have to remove the flywheel to see it) where the offset weight is cast in.. you cannot use a 1 piece flywheel on a 2 piece crank and vice versa...

early 265, 283 and 327's used the 12 O'clock timing position (actually i believe it is 2* off vertical).. anything after i believe 67 - 68 uses the standard 10* off vertical position. if you have an 8 inch balancer make sure it is NOT from a 400 which has an inbalance in it.

btw, if this motor is going to put out some serious HP... ditch that alpha coupler and invest the 200 in a bravo coupler...
I just looked at my 6/34 inch from the 87' 305 and it looks just like the 8 inch I have on there now. No counter weight inside the inner ring. I pulled the drive hoping for a bad u-joint. Will start it today for 5 seconds and know if the vibration is still there. As for HP, nah its a mild build 350horse or so. No holeshots and I keep the prop in the water.
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Old 08-15-2006, 10:59 AM
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Update, yanked the outdrive still vibrates like a bastard. Only thing I can think of is the flywheel is for an external balance and slipped past me. Time to yank the motor
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Old 08-15-2006, 03:01 PM
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another update. Checked flywheel, no casted in counterweight, back of flywheel is smooth where the cast in would be. Was hoping to get lucky and have an external balaance flywheel on it.
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Old 08-15-2006, 04:06 PM
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A few questions are in order.

Was the rotating assembly balanced?

If not, was the crank original or has it been replaced?
What rods are in the engine?
What pistons?

tom
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Old 08-15-2006, 04:24 PM
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Tom, keep in mind this is a low dollar reman shortblock. All they do is cut crank, new bearings, rings, pistons etc. No computer balance. Now the shop owner I know fairly well. He says they havent had an issue with not balancing these short blocks in the 6 years hes been selling them. I tend to believe him for one reason only.

We have torn apart stock 302 ford engines that are sometimes 40-50 grams off and they run like glass still. Heres a few pics of the short.
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