[Aubryl]

Hi,

Doing things the wrong way, I ordered lightweight pistons and rods and I realize now that it might require a crankshaft re-balancing (Piston and Rog balancing won't be an issue, but the crank may...). I mean, it's not (probably) out of balance now, but how lighter rods and will affect the general balance of it? did anybody install lightweight rods and how did you do it?

Thank you

Ludo

 

[Fwkwallis]

Hi

What I accepted early on in my rebuilds was that 'probably' wasn't really in the dictionary of the engineer at the shop I used, so this is the way I looked at my rebuild;

Find a machine shop with the right machinery and abilities to check the balancing of the entire assembly.
If nothing is amiss then regard it as a safety net for the build (this will be the less costly outcome)
If remedial action is necessary then you've saved yourself endless issues down the line (more costly outlay)

I weighed the projected cost of these actions against the risk (cost) to my new components and it was an easier choice. Plus I ensured the shop detail all work so it can be attested to once buried in the engine.
I don't have cash to burn, but obviously not everything of value is bright and shiny.

So my V8 build cost far more than I counted on to get to better than factory state; due to components and complexity.
I understand that in comparison the 4 pot will have less sensitivity for reciprocal and rotational imbalance, but if you are already investing in lighter components that suggests your intention to push the unit harder. Seems a false economy not to have peace of mind.

best

Jim

 

[Aubryl]

Hi

What I accepted early on in my rebuilds was that 'probably' wasn't really in the dictionary of the engineer at the shop I used, so this is the way I looked at my rebuild;

Find a machine shop with the right machinery and abilities to check the balancing of the entire assembly.
If nothing is amiss then regard it as a safety net for the build (this will be the less costly outcome)
If remedial action is necessary then you've saved yourself endless issues down the line (more costly outlay)

I weighed the projected cost of these actions against the risk (cost) to my new components and it was an easier choice. Plus I ensured the shop detail all work so it can be attested to once buried in the engine.
I don't have cash to burn, but obviously not everything of value is bright and shiny.

So my V8 build cost far more than I counted on to get to better than factory state; due to components and complexity.
I understand that in comparison the 4 pot will have less sensitivity for reciprocal and rotational imbalance, but if you are already investing in lighter components that suggests your intention to push the unit harder. Seems a false economy not to have peace of mind.

best

Jim

Thanks Jim,

That makes sense... I was hoping that a 4 in line engine would not be affected by lighter rods, but as you mentioned peace of mind is probably better than guess work...

Best

Ludo

 

[Fwkwallis]

If the crank was already in balance then I'd agree with the logic of that assuming rods/pistons were likewise, and as I said I was given to understand that on the 4 any minor issue was for 'normal' purposes less problematic.
However on the V8 I do know that the counterweights came into play with any assembly change in mass. Chances are to one degree or another that would be true on the 4.
Doubtless someone knows far better the specifics.

Great thing about this site is the things you can learn. Problem is what you KNOW, and for which there is no real substitute for having done it!

Jim

 

[lenul]

Hi,

It's a four cylinder in line, to have a balanced engine, the crankshaft, the pulley, the flywheel and the clutch system (without the disk) must be balanced together without counterweights, and you have to check the rods and the pistons have the same weight, it's all.
It's different for a v-engine...

 

[Aubryl]

Hi,

It's a four cylinder in line, to have a balanced engine, the crankshaft, the pulley, the flywheel and the clutch system (without the disk) must be balanced together without counterweights, and you have to check the rods and the pistons have the same weight, it's all.
It's different for a v-engine...

Hi,

I contacted Alfaholics in the UK, and they advised the same procedure. Pistons and rods, I can do it at home, but I now need to find a shop that can do the pulley/shaft/flywheel/clutch assembly.

Ludo

 

[Fwkwallis]

On the similar theme of 'invisible' money well spent I'd suggest getting the oil pump professionally assessed by someone like Gordon Raymond.
£60 shipping round trip to the USA for me (and his costs) exchanged for 50 years of hands on experience, and you get a blow by blow record of your improved pump build compared to a one liner on invoice.
He'll advise on what you have before sending anything.
One of the better calls I've made......

 

[genericwood]

If your rotating mass was balanced properly before, going to lighter rods and pistons isn’t going to change that. It does, however, change your reciprocating mass. That is what your crankshaft counter weights are for. I’ve been told by someone I highly respect, that Alfa 2 L crankshaft are under counter weighted anyway. I’ve built plenty of smooth, reliable Alfa race engines with very light pistons and rods and no worry about the crankshaft counter weights.

 

[Alfar7]

It's a four cylinder in line, to have a balanced engine, the crankshaft, the pulley, the flywheel and the clutch system (without the disk) must be balanced together

No the flywheel should be balanced separately, or if you change flywheels the engine has to be apart. The front pulley is really to small to be a real factor.

If the crank is balanced and the big end of the rods balanced with total weight of the piston/rod assembly within a gram or two that is good to go.

 

[Aubryl]

Thank you all for your replies. I’ll find a workshop that will be able to balance my crank/pulley/clutch assembly, and do pistons and rods myslef. BUT, I have a bigger issue now. When I removed the shaft, I noticed the second main bearing was worn (I could see the pinkish layer) on one side. I didn’t see any mark on the crank, so I didn’t worry much. Today I removed the bearing and cleaned the engine block, and this is what I found (See picture). That’s bloody ugly, and I wonder if my block is good for scrap now. I’ll see what can be done, but the guy who did that job deserves to go to hell!!!

 

[BaselJ]

Richard
I’d like to make sure I understand you correctly. Is this what you are recommending:
1. The weights of each piston and rod set should be equal within 1-2 grams.
2. The crank should be balanced with nothing attached.
3. The flywheel should be balanced with nothing attached.
Questions:
1. Is there one more step regarding matching the rod big ends? Could you explain how that is done?
2. Is there any value in matching pistons separately from the rods?
Thanks
John Basel

 

[Aubryl]

Richard
I’d like to make sure I understand you correctly. Is this what you are recommending:
1. The weights of each piston and rod set should be equal within 1-2 grams.
2. The crank should be balanced with nothing attached.
3. The flywheel should be balanced with nothing attached.
Questions:
1. Is there one more step regarding matching the rod big ends? Could you explain how that is done?
2. Is there any value in matching pistons separately from the rods?
Thanks
John Basel

Pistons and rods don’t have the same dynamic. Pistons are only moving up and down, while rods small end is moving up and down, but big end is spinning. So you can’t balance it as a whole: you need to make sure that pistons weight the same (factory specs is 2gramms), and you also need to balance rod big and small ends separately. The best tutorial I found is from HP academy:

 

[calibrator]

Here's my $.02 after balancing over 1000 lower ends in the last 30 years.

Inline and opposed engines do NOT need bobweights, only V types. I prefer to balance rods end for end, you need a special fixture to do this correctly. If the lower end (inline) was balanced properly, changing rods/pistons will NOT affect it as long as they (rods/pistons) are matched to each other. Balance the crank alone first, then add the pulley(s), balance the pulley, then the flywheel, and finally add the pressure plate. That way if a component needs replacing it can be balanced on an arbor. Grinding a flywheel will likely alter it's balance. Re-balance it on an arbor after grinding and then mount pressure plate and balance it finally. I've found pressure plates to often be the biggest offender when balancing. The disc is NOT a part of the equation.

 

[BaselJ]

Aubryl and calibrator,
Thank you both for the clear explanations and the link to HP Academy video.
Now I understand.

Do we believe that Alfa balanced individual components this way (probably not to +/- 0.1 gram like in the video) when they assembled engines so, for example, a flywheel or pressure plate could be replaced without causing an imbalance?

 

[calibrator]

I didn't watch the video. While achieving .1 gram tolerance is admirable, it is tedious. If time/money is no object, ... ??? Anymore, when I get under a gram I feel I'm doing 100% better than the factory did. When you set up bobweights for a V motor, you always add 4 grams for "Oil Cling". There is SO much oil wrapping around the assembly while running that it has an effect on the balance to some degree, however it's not something I would obsess about.

Most quality replacement flywheels can likely be installed out of the box with no notable seat-of-the-pants effect, but it's the rare one that will be less than 3 grams from perfect. Same with the pressure plate, if the pressure plate is under 3 gms, I usually don't chase it. Grinding an overheated flywheel, you will see that the grinder starts grinding one side sooner than the whole, because the heat warped it where it mounts to the crankshaft. This wheel is no longer in balance because you've taken more metal from one side than the other.