Lancia Motor Club

Model Technical and Interest => Fulvia => Topic started by: ncundy on 08 December, 2008, 10:14:22 AM

Title: Engine component balancing
Post by: ncundy on 08 December, 2008, 10:14:22 AM
With my Fanalone engine in bits now, I have started weighing the reciprocating components. This has thrown up a few questions that I hope someone may be able to shed some light on. The weights are:

                          1       2        3       4
Rod                  696.5   702    702.5   696
Big End             490.5   494.5 495.5   489.5
Small End           206    207.5   207    206.5
Piston               384.5  383.5  383.5    383
Pin                   100.5   118.5  100      119
Rings and clips     37.5   37.5    37.5    37.5

What is puzzling me is the difference in weights of the pins. They are also different designs. The lighter set have a tapered hole (ie the thickness of the pin gradually increases to its maximum at the centre as a taper), the heavier has a step change of section in the centre third of the pin (where maximum bending occurs). The corresponding pistons also have different marks on: the two pistons on the light pins are marked "H" on the top and those on the heavier pins are marked "B" - does this have any relevance ?

My dilemma is if I equalise the weights of the pins (or the reciprocating assembly) do I destroy the balance of the engine. I am aware of the fact the crank is of the "unbalanced" design common to V engines, but I do not know the balance factor applied to the big end pins during the balance process (the Aurelia engine 60^ V6 is 100% rotating + 50% reciprocating weight for example), so cannot calculate the effect of removing the weight and, if I do equalise the weights, should I rebalance the crank.

I assume Lancia did not individually balance the engines, and that they were assembled from components that were matched assuming a nominal balance figure based on achievable manufacturing tolerances. If this is correct, then do I have an out of balance engine or an in balance engine (by which I mean has the engine been assembled incorrectly to start with) ? What have others found when they have weighed these items ?

Any thoughts or advice welcome !


Title: Re: Engine component balancing
Post by: davidwheeler on 08 December, 2008, 10:35:03 AM
Did you run the engine before stripping it and was it smooth?  in which case leave well alone.
There is an article posted in the Lambda section describing the mathematics of balancing narrow V4 engines.  If you have a math. degree you can theoretically calculate the required bob weights.
If all else fails, balance up the components by aquiring a set of 4 similar pins and send the engine to Vibration Free who will, for a fee, return to you a perfectly balanced engine - see my post in the Aprilia section.
Lambda engines were balanced at the factory but I've no idea about Fulvias.  I don't think that having heavier pins on one bank than on the other is a deliberate ploy so someone has miss-assembled your engine.

Title: Re: Engine component balancing
Post by: ncundy on 08 December, 2008, 12:47:43 PM
Hi David,
I did run the engine, but it was 3 years ago and I never ran it long enough or hard enough to draw any conclusions. I have a copy of the article you mention. I tried to do the calculations a year or so ago but got lost in my sines and cosines ! May be time to try again. Vibration Free would be an avenue (I imagine there are others as well) if they know the correct bob weights to fit, but before I make any decisions I thought it would be a good idea to see if anyone had seen similar when taking a Fulvia engine to bits, and if they had what did they do about it and did it make any difference. I agree with you on the pins, I have never seen variances of this magnitude, but I am worried I should be struck by lightening if I thought Lancia made a mistake  ;D

Title: Re: Engine component balancing
Post by: St Volumex on 08 December, 2008, 01:26:19 PM
Hi Neil,

Some of the critical components for balancing an engine that you didn't mention are the flywheel, clutch & pressure plate, and any other crankshaft pulleys?

Balancing engines is so cheap here in SA I just get a professional to do it anyway whenever I do a rebuild.  It's well worth it.

Kindest regards,

Title: Re: Engine component balancing
Post by: ncundy on 08 December, 2008, 01:56:22 PM
Agreed ! They had slipped my mind as I intended to get them balanced anyway (the flywheel is having a new starter ring fitted and I am told by "those that know" that modern clutches are a bit hit and miss on balance)- but they are a bit easier as they can be done seperately from the crank - rods - piston assembly and don't really require any specialist knowledge. You can do it at home with a drill and a piece of rod if your drill revs high enough ! The only reason they tend to be done together is that the balance shop does not need to make up an adaptor for the machine, but on a V engine you have to balance the crank - rod - piston as an assembly before adding the flywheel / clutch etc otherwise it can all go a bit Pete Tong.

I have no idea what it costs in the UK, unfortunately even with the generous reduction in VAT I guess it is still pretty expensive !


Title: Re: Engine component balancing
Post by: Philm on 08 December, 2008, 08:39:01 PM
Are you planning on balancing rotating masses only or reciprocating as well?

Title: Re: Engine component balancing
Post by: ncundy on 08 December, 2008, 09:33:06 PM
Probably both although I was hoping I wouldn't have to balance anything, but the varience is very large in my experience. If I do balance the reciprocating mass (pins etc) I wouldn't normally worry about the rotating because the effect of balancing process would be in reality 2/3 of nothing (I am not trying to lighten them). But 20 odd grammes is a lot to take off without considering the effects on the rotating mass. So I am trying to understand what I have got, either:

an out of balance engine that should be balanced
an in balance engine (despite the large varience) that I should leave alone

A starting point would be has anyone else seen this type of varience. Ultimately I think I will take it to someone who knows the correct bob weights to add to test, but there is a bit of thinking to be done before then.

Title: Re: Engine component balancing
Post by: Philm on 08 December, 2008, 10:24:15 PM
Usually it is the other way around, balance out the rotating mass first (crank pin, big end, shells & bolts, portion of the rod) followed by the reciprocating (piston & rings, pin, remaining portion of rod). That is why you probably have a smooth running engine with large variance in pin masses. The pin creates a far smaller out of balance force as it is purely reciprocating mass as opposed to an out of balance crank pin. If you are interested I have a fairly straight forward method for balancing rod assemblies. Drop me pm if you are interested.

Title: Re: Engine component balancing
Post by: ncundy on 08 December, 2008, 11:00:09 PM
My understanding is that a straight engine it is balanced as you describe because once the rotating mass is balanced, as long as the reciprocating masses are all equally balanced they do not effect the rotating balance. However as a V angle is opened out, the weights you have to add (or take away) to balance the rotating mass are a function of both the rotating mass and the reciprocating mass, therefore to balance the rotating mass on a V engine you have to start with balancing your reciprocating mass (as this value needs to be known to derive the bob weights required to balance the rotating mass). Thus in theory if you "lighten and balance" your pistons and rods on a V engine you should then have the crank re-balanced. At what point this begins to matter I don't know. I'm learning on this so feel free to correct me.

PM sent.

Title: Re: Engine component balancing
Post by: Mark Webb on 09 December, 2008, 09:12:09 PM
In think you will find that the tapered pins are genuine HF lightened items and the heavier ones are standard 1300 pins (or maybe supplied with an aftermarket set of pistons).
Like the valve spring caps Lancia took the trouble to go that extra mile to lighten them. Probably did the same for the 1300HF engine, who knows.
The best 1600 engine I have seen was at Harrys for a head gasket replacement, a light blue S1 ex competition car with polished conrods, rockers etc, a work of art.
Even Harry was impressed (but I will not say what he said ;D)

Title: Re: Engine component balancing
Post by: ncundy on 10 December, 2008, 09:59:21 PM
I think you are probably correct. Does anyone have a set of standard pins out of the car they could have a look at to see what the internal design is (or even weigh one)?

Title: Re: Engine component balancing
Post by: ncundy on 12 December, 2008, 02:56:36 PM
I have got to the bottom of the pistons.

During assembly there are two different manufacturing bore sizes, and with them corresponding different piston diameters. These are shown as "Class A" and "Class B" in the Data Book. Having now measured the piston accurately I have two Class A pistons (those on 1 & 3), and two Class B pistons (2 & 4)....I guess the H is actually an incomplete A stamp. So the block hasn't been re-bored.

Still need to confirm Mark's suggestion on the pins then I can decide what to do.


Title: Re: Engine component balancing
Post by: davidwheeler on 15 December, 2008, 05:11:46 PM
To clarify re. Vibration Free.  They take your assembled short engine and clutch, minus rings but with weights added to the tops of the pistons to compensate for the rings and then balance the whole thing on a rig.  No messing about with bob weights and calculations and the result is a perfectly balanced engine - provided that you have marked everything and put it back together (with the rings on) in exactly the same way.  Job done!  Ideal for a bitza like my Aprilia engine.  I presume they want the rings out so as to reduce friction.  I have been for a fast run in my Aprilia today and it is smoooooth.

Title: Re: Engine component balancing
Post by: ncundy on 15 December, 2008, 06:51:12 PM
Thanks David, thats interesting and glad to see it has done the job for your Aprilia. Presumably as there is no lubrication it is done at low speed ? I will investigate.

I am still pursuing the calculation route in the first instance and can now see light at the end of the tunnel. Hopefully I will have it sorted shortly after I get my block and crankcase back to take some dimensions off.


Title: Re: Engine component balancing
Post by: JohnMillham on 15 December, 2008, 08:45:35 PM
There's no reason why you don't have the oil pump connected and working. I did with both my Lambda and Augusta engines, so the bottom end was lubricated while it was being balanced. I made simple gadgets to squirt the oil back down into the sump. Oil is occasionally squirted down the bores while the engine is run at about 250 rpm, which cuts down the piston slap. It was interesting to note that on the Lambda engine, which had really bad piston slap, the pistons most noisy depended on the direction in which the crank was turning. The Augusta had new pistons, so didn't make much noise at all. I am very happy with the results from Vibration Free.
Regards, John

Title: Re: Engine component balancing
Post by: ncundy on 15 December, 2008, 08:57:36 PM
Thanks John,
That makes sense - do they run it for long, and do they mark and drill ( /add weight) the crank in situ or mark it and remove it ?
I guess the thrust face on the Lambda pistons may have been slightly worn in profile ?
Thanks for the info gentlemen - it's all adding to the mix and hopefully getting me to the right place.

Title: Re: Engine component balancing
Post by: JohnMillham on 16 December, 2008, 01:24:04 PM
With both of my motors, I asked that, if possible, they would add weight, rather than remove it from the crankshafts. The Lambda ended up with weights added to the flywheel and fanbelt pulley, but the Augusta had weight added to the pulley, but a small amount removed from the periphery of the flywheel. With more modern, presumably stronger crankshafts, it would probably be OK to remove some weight from the crankshaft itself, if necessary. It's important to have the oil pump working, by the way, to enable the crankshaft to be kept full of oil, the weight of which would have an effect on the engine's balance. My Lambda pistons are a bit too much of a sloppy fit in the bores! Unfortunatley, not just "slightly worn".
 Regards, John