13 replies to this topic

[rozzer1275]

A friend of mine is embarking on a new engine, and we would like to know how critical balancing of the rod piston assemby is?

the plan is to have the crank/pully/flywheel clutch balanced and posibly the piston and rod assembely

we may equal the piston and rods and also balance end to end with each other

would the pistons and rods be dangerously out if not balanced?

my thoughts are its only realy needed to equalise the pistons and rods for high reving engines, we mignt just weigh them to see if there is major differnces then just fit them to a balanced crank assembly.

what are your thoughts people?

[Ethel]

That's how a machine shop would do it. All 4 cylinder inline engines are inherently imbalanced as far as the pistons and rods go, so you'd only ever be making the best of a bad job anyway.

[DaveRob]

In my opinion balanced rods, (end over end), are more like the icing on the cake..... yes they will make for a 'smoother' less vibe rebuild at the top end of revs but its far more benificial to work on the crank/ pully/ clutch balance.... in my strip and rebuild I measured the rods gross weight and they were all within 4 grams of each other...... I then checked them end over end by machining some bosses and making a fixture. I have a scale accurate to 0.1 gram and end over end variation was never greater than 1.9 grams per rod......... weighing the pistons I got an even tighter spread with each piston within a total of 2 grams of each other....... In my opinion Id take the total rod weight and simply match the heaviest rod to the lightest piston / heaviest piston lightest rod etc etc . I do realise that this is NOT an accurate way of doing it, but in terms of bang for buck id spend money elsewhere.... lightened rods is worth doing if you have time and ability to do them yourself.... or you want the ultimate engine..... or you have loads of cash ..... or you like the look on the spec sheet..... take your pick..... nice to have..... not really needed........

Just my opinion and findings....... by the way my engine had never been stripped so rods were ex factory... pistons were new from Minispares

Hope this helps

Rob

[Guess-Works.com]

my thoughts are its only realy needed to equalise the pistons and rods for high reving engines, we mignt just weigh them to see if there is major differnces then just fit them to a balanced crank assembly.

Spot on....

You may not believe this, but I've never had a crank, rods or infact anything balanced other than how they came out of the factory

My engine is never treated with any respect what-so-ever...

So unless you're building something which is designed to rev over 7k... don't bother IMO.

The next engine I will be building for myself will be balanced, but as it'll have an expected red line of something between 9 and 10k it's one of those things which MUST be done.

[rozzer1275]

many thanks guys.

I was suprised with what Dave rob said about weights only 2 grams between them on an engine designed in the 50s, hardly seems worth balancing.

adds up with what guesswork said about not balancing anything and engine running fine.

Will prob just weigh them to check there are no hidden demons.

[Rosslin Racing]

what size engine are you talking about?
I balance all my engines its the diffrence between tractor and Rolls.
An 1100 is a must for balancing it will live longer.
I have had weight diffrences from up to 40 grams on rods and pistons. what you may find is that the pairs 1& 4 are about the same weight and 3&2 another.

[dklawson]

I take the stand that any time you're doing an engine rebuild, the added expense of balancing is justified. Balancing the pistons/rods typically does involve weighing the assemblies and grinding or drilling off a bit of material from the bottom cap of the rods to reduce each to the weight of the lowest. I suspect you'll find that balancing piston/rods will cost less than the cost of balancing the crank assembly.

I just find the engine runs smoother all across the rev range when balanced. I think it's justified. However, if I were paying someone else to do the engine assembly and labor costs started eating into the budget I would certainly consider saving money by not balancing the parts.

[rozzer1275]

what size engine are you talking about?
I balance all my engines its the diffrence between tractor and Rolls.
An 1100 is a must for balancing it will live longer.
I have had weight diffrences from up to 40 grams on rods and pistons. what you may find is that the pairs 1& 4 are about the same weight and 3&2 another.

Its a 3.5L V6 mitsubishi, get more informed answers on this site.

[NunoGT]

Greetings.

Hope you don´t mind that I borrow this thread for a doubt of mine.

I´m rebuilding a 1275cc preA+ engine from my 1275gt.
It's now reground to 1293 but so what the crankshaft reground to +0.2.
The shop already assembled the crankshaft with rods and pistons. Everything onto the block and also added the center main strap.
Cannot really tell you at this stage if everything was balanced. Though if it was, my stock flywheel wasn´t even there.

The question is if I happen to want a lighter flywheel, (say Minispares's C-AEG421 ) should I need to strip everything apart again and have the whole set balanced?

The engine will be running with a Piper BP285 but should do no more that 6500rpm, and live 90% of it´s life below 6000rpm.

With this in mind, do you think it´s a must dobalance job or could it run like it is?

Thanks in advance.

Nuno

[Southy]

There will be no need to take all your engine back apart to have your lightened flywheel balanced.
If the company who built your engine has done a proper job, all the internals of the engine will of been balanced seperatly.
The crank will of been done on its own as would each of the rods and pitson to get them to match.

All you will have to do is get a machine shop to balance your new flywheel on a pre balanced crank they have in there shop (like we do) and then your flywheel will be balanced and any material will be taken directly off that.

If your going for a lightened flywheel, it might be worth getting the full clutch assembly balanced, ie clutch plate, pressure plate and diaphragm.

[dklawson]

I agree with Southy.

However, if you want the C-AEG421 flywheel... and you find that Mini Spares DOES have them in stock, please PM me. They've been out of stock for at least 6 weeks and I've been contacting them frequently to see when they will be available.

[NunoGT]

Thank you for your valuable information, It will help me decide what to do.

Be sure I´ll warn you if I have any news on that flywheel from MiniSpares.

Happy New Year to you all.

Nuno

[dklawson]

I received word from Mini Spares today...
"not yet... but we're confident they'll be here next week".
Sadly, I've received a similar reply once a week for about 6 weeks now. This is a purchase I've put off for years (literally) as the exchange rate has been so unfavorable. Finally when the dollar/pound ratio is good... no stock!!!

[morley]

I've had my new engine balenced, my whole clutch/flywheel assembly and pistons and rods were balenced, it'll help to make it rev more be alot smoother. my crank was out by 13 grams! which my engine builder was pretty shocked at.
just imagine it this way, when your wheels aren't balenced properly it can cause all sorts of problems.. now imagine that happening inside your engine