Lancia Motor Club

Model Technical and Interest => Aprilia, Ardennes and Ardea => Topic started by: davidwheeler on 23 October, 2007, 09:10:16 AM

Title: Balancing an Aprilia engine
Post by: davidwheeler on 23 October, 2007, 09:10:16 AM
I am in the process of rebuilding a 1500cc engine, something of a mix and match job.  Does anybody know the correct percentage of reciprocating total to use in the calculation of the bobweight total?  50% has been suggested (the normal factor for a vee4) but the Aprilia is more of a wiggly four so may need a slightly different factor.

Title: Re: Balancing an Aprilia engine
Post by: Scarpia on 23 October, 2007, 05:15:56 PM
I thought the answer was a little more complex but now you started me thinking.Is it not the rotating weight +half the reciprocating? Now you've made me curious I'll dig through my books and see what they say.

Title: Re: Balancing an Aprilia engine
Post by: DavidLaver on 24 October, 2007, 05:52:26 PM

There was a write up in Viva - I think is was a Lambda engine.  Have heard nothing but good about them.   John Cundy went to town on the balance of his Aurelia engine and am not sure if it was these guys or somewhere else he ended up so he's worth a call.


Title: Re: Balancing an Aprilia engine
Post by: Dilambdaman on 24 October, 2007, 09:33:14 PM
John Millham had his Lambda engine balanced by Vibration Free and they did a great job on the Dilambda prop shaft (I gave them the mention in Viva Lancia!). Can't recommend them highly enough.

Robin Lacey.

Title: Re: Balancing an Aprilia engine
Post by: johnturner on 26 October, 2007, 02:40:09 PM
Bill Smith, one of the heroes of the Australian 3 litre Lambda engine project, did some work on balancing offset V4 engines and wrote this up in an issue of the Australian Register Newsletter and Bill Stebbins in California wrote a piece on 'Balance Theory and the Lancia Vee-Four' in the Winter 2005 edition of Lanciana.  Both of these are essential reading for anyone interested in Lancia V engines but large parts of both go whizzing over my head (trigonometric identities anyone?) and they wont provide the balance factors you need to take to your machine shop.  If you can provide the weight of little and big ends and pistons Bill will, for a small fee, calculate the balance factors for you and you can reach him at  There is no knowing whether Bill's theories are right but the performance of 30 or so Lancia engines balanced this way is impressive and I have used Bill's factors to balance the engine I am about to install in the Weymann, but I wouldn't hold your breath until that is on the road. Vibration Free take an altogether more pragmatic approach.  As I understand it they assemble the engine without piston rings and whirl the crank round, bolting on bits until it stops jumping about, much as you would balance a wheel. It sounds alarming but John Milham speaks highly of them and his engine is impressively smooth. 

I had planned to attach Bills paper but cannot get it into the right this space.


Title: Re: Balancing an Aprilia engine
Post by: Scarpia on 26 October, 2007, 04:23:30 PM
I would be very interested to read the paper John.  I have been unable to upload files for the last week due to the upload folder being full. Rodders is looking into it.

From first principles, if David wishes to weigh components then the nitty gritty is contained in the following paragraph borrowed from an american website.
"To determine the correct weight for the bobweights, the full weight of a pair of rod bearings and the big end of the connecting rod, plus half the weight of the little end of the rod, piston, rings, wrist pin (and locks if full floating) plus a little oil are added together"
which roughly translates to my guestimate of 100 percent of the rotating weight plus 50 percent of the reciprocating weight. The problem being to weigh half your conrod without a specialist balancing support and judge how much is "a little oil" . Lots of ingenuity required. I'd go to Vibration free!

Title: Upload Folder
Post by: fay66 on 26 October, 2007, 11:36:10 PM
.  I have been unable to upload files for the last week due to the upload folder being full. Rodders is looking into it.

Apologies for cutting the quote about but,
Thank God for that, I thought it was me , I tried about half a dozen times to upload something only to be met with that answer which might just as well said, "No Frying Tonight":o
I do wish wish all the eggheads who put out meaningless messages (to the average person) such as this, would also supply an explanation of what it means :(

Brian Hilton
8227 8)

Title: Re: Balancing an Aprilia engine
Post by: ncundy on 29 October, 2007, 08:51:55 AM
I think you will find that the Aprillia crank is not a bob-weight design. It is probably balanced as a straight four would be, ie with a balanced crank, not as the later Lancia engines V which are designed around an out of balance crank that, when installed brings the whole into balance. Lancia didn't start using this technique until the Aurelia, the approach being developed by de Virgilio. There was an IMechE paper on the subject around this time from Lancia. Whatever, the balance weights will not be 100% rw +50% rec w as that is for a V engine with the crank on centre line of the piston bores, which the Lancia V4's are not, they have a raised crank center line relative to the bore intersection.
Given the small V angle, I would suspect that the residual  "out of balance" using this method is very small.

Title: Re: Balancing an Aprilia engine
Post by: Scarpia on 29 October, 2007, 05:14:49 PM
Neil, having struggled through the articles by Bill Smith and Bill Stebbins which were kindly provided by John Turner, I'm a bit loath to make any profound statements on the subject as the maths and engineering theory is extremely demanding (to a point beyond the practical application of most mere mortals).Nevertheless, I had checked the parts book and thought also that the original crank had no apparent provision for bobweights but bobweights do seem to be required during the balancing itself otherwise the crank would rotate out of the machine, precisely because of the lack of inherent balance.This then allows possible machining of the crank to achieve balance when rebuilt.A V4 crank is not inherently balanced making it a somewhat different proposition to balance than a straight 4.

I know your right about the raised relative position of the crank to the intersection of the narrow v but I thought this was much earlier than the aurelia?

Title: Re: Balancing an Aprilia engine
Post by: ncundy on 29 October, 2007, 06:37:32 PM
Maybe I explain myself badly:
V engines in general are now balanced using the "out of balance" method - but not in the days of the Aprillia. In those days they were most probably balanced exactly the same as straight engines (and the roughness put up with). You can see this by looking at the crank - they are almost always symetrical. Look at an Aurelia crank or anything post Aurelia and you will see weights around the webs that are not symetrical as the crank is designed to be out of balance, and to achieve the correct out of balance, bob wights are used during the balancing process to identify the correct position and weights.
I think an Aprillia crank is designed to be in balance, and balanced by spinning without bob weights as you would a straight engine - so it wouldn't fly out of any machine. Thus in theory it is a little bit rougher than it could be, but because of the narrow V angle and the short crank length this is probably negligable, and I have never seen any complaints of a rough Aprillia (or Lambda / Augusta either).
It may be possible to use the bob weight method if
a) you can calculate the required bob wieghts (how's your vector maths !)
b) the crank has enough metal in neccessary places to place (or remove) any metal.

All Lancia V4's had vertically off set cranks I think, the V6 didn't. The 100%/50% does not apply to any V4 configuration with the raised crank - it is a different calculation (has both Sin and Cos components).

Incidently I wonder how many Aurelia owners fit lighter pistons but don't have the crank rebalanced ?

Title: Re: Balancing an Aprilia engine
Post by: ColinMarr on 30 October, 2007, 11:56:47 AM
Not that itís going to help get a V engine balanced now, but you might be interested in the attached photo and my experience with an Aurelia engine. When I rebuilt my B20 engine almost 20 years ago it was part of a rebuild of a 6th Series car I had bought in pieces from Holland.

For the engine it was a case of assembling parts that had come from a number of engines, including a crank and a flywheel that had been dramatically lightened on the basis that all it needed to do was support the starter ring! The whole lot needed to be balanced and this was entrusted to Hillthorne Engineering at Hanwell, which was then the London expert company for this sort of thing. The photo shows some of the parts on the kitchen worktop, about to be taken to Hillthorne. I donít know how they did it, but they didnít have the block and simply worked with the crank and related bits. Some weight was added to some parts of the crank webs by welding metal on, and some was taken off by drilling.

The end result was fantastic, with one of the smoothest B20 engines ever. I was lucky in that the prop-shaft didnít need any adjustment. I am not sure that the lightening of the flywheel made much difference, because there is in effect another flywheel in the clutch, but it did rev very freely. Sadly, Hillthorne seemed to disappear in the mid Ď90s, but hopefully their expertise lives on somewhere.


Title: Re: Balancing an Aprilia engine
Post by: johnturner on 30 October, 2007, 11:40:37 PM
Aurelia owners might be fortunate enough to find lighter pistons but currently available pistons that fit the Lambda are likely to be significantly heavier. I am using Fulvia 1.6HF pistons in the torpedo but even these are heavier than the original, and quite apart from having to take variations in the weight into account in balancing the crank as Neil says, Bill Smith's paper makes the point that the 'lambda factor' (no relation) twice engine speed vibration cannot be balanced out at the crank and can only be reduced by reducing the weight of the pistons and/or reducing the stroke. So if you are replacing the pistons in the Aprilia you may want to take care that the new ones are no heavier, and preferable lighter, than the original.   

Title: Re: Balancing an Aprilia engine
Post by: davidwheeler on 03 November, 2007, 10:00:46 AM
Oh dear, I seem to have started something now.  My real problem is that I have new conrods which weigh 600g as opposed to the original 400g.  I also had my VIIIth Lambda crank and flywheel balanced using the 50% ratio and it is reasonably smooth up to 2700 rpm but increasingly vibrational above that as opposed to the VIIth which came from Oz and it notably smoother than the VIIIth ever was.  I think the thing to do probably is to balance as a straight 4 and see how it feels.  If that fails then I think I take the engine for dynamic balancing.  It will need to be looked at in some degree because the flywheel does not belong to the crank and, though crank and flywheel are machined symmetrically all over, there are some small eccentric holes drilled in the flywheel which may be for balance.

Title: Re: Balancing an Aprilia engine
Post by: davidwheeler on 08 April, 2008, 09:16:32 AM
I went to Vibration Free and left the engine with them.  They told me that it was 142G out of balance and returned it with a number of new holes in the flywheel and a weight attached to the crankshaft pulley.  It took them ten hours to balance it for which they only charged me half rate!  They took it up to 400 rpm rather than their standard 200 rpm and it should now be smooth.  The only reason I do not know yet is that I am having new piston rings made in Maryport )by the Clupet Piston Ring company - Alan will make any ring at all).

Title: Re: Balancing an Aprilia engine
Post by: davidwheeler on 05 August, 2008, 05:46:37 PM
I have at last put the engine back in the car and today towed it down the road in top gear at about 25mph and SO FAR it is smooth.  I hope to fire it up in the next few days!

Title: Re: Balancing an Aprilia engine
Post by: davidwheeler on 08 September, 2008, 08:36:13 AM
It took a little longer than anticipated to get her running but now she does and she is very smooth indeed (up to running in speeds).  I can therefore thoroughly recommend Vibration Free!