Guys,

I am trying to understand from my balance card, what kind of a flywheel i need. Here are the details.

'87 350 SBC, roller cam, 1pc RMS. Engine is rebuilt.

Balanced to the following weights:
Piston: 700 gr
Piston Pin: -
Piston Rings: 58.3 gr
Pin Locks: -
Total Rod: 548 gr
Insert: 37.5 gr
Rotary: 887 gr
Reciprocating: 904% factor
Bob Weight 1791 gr

Balanced on a Hines Microcomputer Balancer.

My harmonic balancer is an ATI zero balanced unit.

Can you help me determine whether I need a zero balance or LT1 style flywheel. I am installing a T56 with a SPEC flywheel which has a bolt on weight. Do I need this weight or not?

Thanks,

John

 

"Zero balance" and "neutral balance" can be confusing terms when discussing 1-piece RMS cranks. Even though "zero" or "neutral" balanced, there will be a weight on the flexplate/flywheel. But, I'm not familiar with the Spec flywheel specifically; the weight may be for a stroker crank. Hopefully the instructions, or a call to Spec, will answer that question.

 

Yes I agree it is confusing. But I am learning. The weight on the SPEC flywheel IS the counterweight for a stock LT1 or 1pc RMS SBC. My question is what MY motor needs since it has aftermarket replacement internals. I was hoping someone could look at my balance card and help. I understand it may not be possible to know just from balance card.

 

Well, you answered it yourself:

It needs exactly what a stock 1-piece RMS 350 or LT1 needs. If you use the weight on a stock 1-piece RMS 350 or LT1, then you need to use it on your engine. (Although I can't answer why Spec made it bolt-on.)

 

'57

I understand some replacement rotating assemblies can actually convert a 1pc rms block to totally internal balancing requiring a neutral flywheel. Granted I suspect these are typically light weight pistons and rods where bob weight can be reduced to the point where the flywheel weight is unnecessary. Not likely to my setup, I SUPPOSE.

I left out a few details as I didn't want to create suspicion. But for the sake of argument here are the rest of the details.

I recently completed a T56 swap and used the SPEC flywheel with bolt in weight and a Competition Clutch clutch and pressure plate.

I am now getting drivetrain vibration at 3500rpm and above. It is even noticeable with the trans in neutral and the clutch disengaged. It is slightly more noticeable with the trans in neutral and the clutch engaged. With the 700r4 and stock flexplate, I had no vibration.

Since I was not totally certain about what balance was required for my modified engine, I took my new SPEC flywheel and CC pressure plate, old flexplate, and balancing card to a local machine shop. I believe they decided to balance the new stuff based on the balance of the old flexplate rather than using the data on the balance card. This makes sense since I did not have vibration with the 700r4. They ended up adding weight to the SPEC bolt on weight.

So now I am trying to figure out what got screwed up. I can only suspect the balance is off since I can't make any other explanation make sense in my head.

 

I also wanted to add a tidbit of info that I found interesting about my stock flexplate.

I have never noticed this before, but there are several holes drilled in my stock flexplate that must have been drilled when the engine was balanced as it was assembled many years ago. These are clearly not GM holes. I would not think this would matter as the machine shop supposedly balanced the new flywheel/pressure plate the same as the old flexplate. But I figured it is worth mentioning.

 

Here are some pictures. In the third picture you see the markings left by the machine shop that balanced the new flywheel/pp. I'm guessing this means that the counterweight needs to be 121gr on a 6" radius and 8 degrees off the circled bolt hole. Interesting, but I can't find any info on the net as to how heavy the stock weight is.

In this picture you can see the stock counter weight at 6 o'clock and the added holes at 9 and 1 o'clock. 3 new holes at each.


Holes up close. They are not deburred and cannot be stock.


Balance markings.

 

So at this point unless I get some good ideas here, I think I need to remove the weight that the machine shop added and try it. Thank god for SPEC's bolt in weight. I think i can get it off by just removing the inspection cover of the trans.

 

It's too late now but the engine should have been balanced with the flexplate/flywheel you intend to run. Even a zero balance flexplate is not perfectly balanced.

 

Agreed. Except who is going to tear apart an engine at every clutch change? The reasons you mentioned are exactly why I had the new assembly balanced to match the old assembly, instead of just bolting parts on out of the box. You would think a machine shop could match balance without issue.

 

I have only heard of that with regard to stroker cranks not requiring 400 SBC type counterweights. You still need the weight like that which was on your original flexplate.

If you are familiar with 2-piece rear main seal cranks, you will recall that they had a counterweight built into them outside of the RMS. The weight on your 1-piece RMS flexplate takes the place of what used to be built into the crank. All 2-piece RMS cranks had that "external" counterweight on the crank, including the 400 cranks.

 

That very well could be the case. Now consider the holes in the stock flexplate shown in the pictures. These must have been drilled by the machine shop that balanced the rotating assembly years ago. The engine has been running with these holes without vibration for years. Any ideas? I was to young and dumb at the time to even notice the holes.

 

I went through the exact same question when I put my combo together. My neutral-balanced 1 pc RMS 350 was balanced at the machine shop (without a flywheel or flexplate) before it was assembled, but then i decided to do the T56 swap. Bought a regular 1 pc RMS LT1 flywheel and took off that counterweight. The engine shook like crazy. I then bolted the weight back on, and it runs as smooth as a car with a medium sized cam can be expected to run.

Definitely leave the weight on it. I have an L98 crank but all the rest of my internal parts are aftermarket and it was balanced by a shop. the only reason you'd need the weight removed is if your rotating assembly is balanced differently with the flexplate taken into account - which is why I had my machine shop balance my stuff without the flexplate. If the flexplate is counterweighted more or less than stock to balance the rotating assembly as a whole then it's anyone's guess.

 

Unfortunatley it would seem that is exactly the case. However the new one should still be well balanced since the machine shop used the old flexplate as reference.

I'm going to pull that weight off and see what happens. If that doesn't fix it, I'll pull the weight off that the machine shop added to the SPEC weight and try the flywheel basically as if I had pulled it out of the box and installed it with no mods. So much for trying to do things the right way.

 

When I had the ZZ3 rotating assy balanced, the machinist asked for the flexplate. I gave it to him, but with strict instructions that the flexplate was not to be modified in the balancing process. He complied, and didn't have any trouble with the balancing.

To me, that's the only way to do it (other than telling the machinist to use a standardized counterweight instead of the flexplate/flywheel). Seems to be a tall order all to often.

 

I bought a forged rotating assembly for my '91 with the 1 piece RMS. It has a 3.75" stroke and uses 6" rods. It balances internally without the counterweight on the flexplate/flywheel. This is a PITA with a flexplate as nobody makes a 1 pc RMS flexplate without the weight and they are usually welded on. I'm now using a Mcleod Street Twin with my T56 swap with the bolt-on weight removed from the flywheel.

Can you call the shop that balanced your engine and just ask them which way they did it? Running it the wrong way can wreak havoc on your main bearings!

 

Yes sometimes it surprises me how dumb the so called professionals can be. I was 18 when the short block was assembled, I figured they would know more about it than me.

 

It has been 7 years since it was balanced, but that is a good idea. Maybe they have records. I was hoping we could tell from the balance card, but I guess not.

What is interesting is the vibration is pretty subtle and comes on right around 3500rpm. If it was there at all rpms, i probably wouldn't even notice it.

 

After rereading your posts I would say you will need the weight on the flywheel. Those extra holes on your flexplate are from the balance job. It looks like they took the weight off the flexplate to balance your new parts. This sucks because you cannot even replace that flexplate without rebalancing the entire engine again. I'm guessing your engine will now vibrate with or without the weight on the flywheel as either way will not match the balance of that flexplate. You may be able to find a good/smart machine shop that can put your flexplate on a balancer to measure the imbalance and then transfer this imbalance to your new flywheel.

 

Agreed completely. And that is what I tried to do. I took everything to the machine shop and asked them balance the new flywheel and pressure plate the same as the old flywheel. Now if you notice from the pictures, on the old flexplate, the drilled holes are generally on the opposite side of the flexplate than the stock counterweight. I think this would effectively make the stock counterweight "heavier". The machine shop that balanced my new flywheel added weight to the SPEC counterweight. So I tend to think they basically did the right thing. Maybe adding weight to one side is different than removing weight from the other side though. I'm not sure.

 

You are right about making the holes making the counterweight "heavier". They also "moved" the counterweight a little. It sounds like the machine shop did the right thing too. Now I'm really stumped! Did you add a pilot bearing/bushing to the crank with the T56 swap? Is the clutch fully disengaging?

 

Yes I have a new pilot bushing installed. That was actually the first thing I thought of. Since I still notice the vibration with the clutch engaged I don't believe it could be the pilot bushing. I checked for proper clutch disengagement before i started driving the car.

 

Ok guys. I was able to unbolt the weight from the back of the flywheel without to much trouble, thank god. All I had to do was remove the inspection cover and the starter.

The weight was 141g without the bolts and 166g with the bolts. Recall from the picture above, the machine shop wrote "121g sample" on the old flexplate. Maybe the 121 was for the weight without the extra they welded on. I have no idea. Maybe they were trying to hit 121g and missed badly. I think my next step is to cut off the weight that the machine shop added and see if its any better.

 

Ok guys, I did some testing.

I took off the weight that the machine shop added and then I could no longer notice the vibration revving the engine in neutral. However once I drove the car I could still notice the vibration at 3500rpm. So it seemed maybe very slightly better but not much. With the additional weight removed the SPEC weight is 123g vs 141g with the weight added.

Next I removed the SPEC installed weight as well. This really threw off the balance. It was really obvious the balance is way off even at idle.

So now I'm not really sure what to do next. Any ideas?

 

I forgot to update this thread.

The issue was the flywheel balance. I took it off and had it rebalanced by a different shop. Much better now.