Calculating Balance Factors a simple example

[john.whiting]

The crank must be on precision ground ,hardened knife edges,levelled with a precision level,to .003" in 12".....Im afraid two lengths of handy angle on saw horses dont cut it.......consequently,there is no point worrying about the finer details of what can only be approximate.

[nevhunter]

Yes Alan, and if we want to be a little more precise we should use the centre of the conrod small end bush. The easiest practical way to do this is to hang it by a small thread of cotton though the bush. Flush the thick oil from the big end assembly, too. I take a few measurements and average them. The variation is rarely more than a couple of grammes.
If the flywheels are mismatched or have been previously drilled erroneously you might have to do each wheel separately on a mandrel with a tapered arbor and get an even offset effect (not an even weight for the entire wheel). Some makes have the drive side flywheel much heavier than the timing side. When you are drilling, be careful to not create a weakness in any area. The lighter the "Up and down" parts are the smoother the result even with the correct factor being used.. Nev

[Brian_Walker]

From what I have read on this forum and notes on Drags it appears that values obtained from this exercise don't need to be taken as exact when numbers from 57 to 67% have quoted as suitable.
If you get much heavier pistons (oversize with thick walls it is well worth checking to see how they look) my crank was originally done on knife edges on a special level bench after a crankshaft grind, I had it balanced to 67% and at 6000-8000 rpm it was very smooth which is fine for racing but probably not the best for cruising at 50 mph.
If I now fitted a Hepolite standard compression piston with a new liner my balance factor would be 74% which is probably way too high, a 0.060 oversize 9:1 JP piston with a standard crank would be about 50% which would probably be too low for smooth vibration free touring.
I would think that the values I obtained would be within 1 or 2 % of the correct values obtained in a good engine re-builders shop and if the values were a long way out on the home shop method I described then it would be off to said shop for drilling or filling and values measured at a higher precision and accuracy.
(In my Ariel box of bits I also located a nearly new used but unmarked Poulson NG, I remember them and they appear very well made I wonder if there are still available as NOS.)

[david.anderson]

65% is the figure quoted by Val Page for all singles.
I use 2/3 or 66.6%.
I tried a 350 at 60% and it shook bad enough to shake the fillings out of your teeth.
Polson pistons (Australian made) have not been available for over 50 years. All Polson pistons that I have seen are 7.5 to 1. It would be very rare now to find NOS.
If you cramp a pair of steel rulers to an angle iron frame, you have a pair of precision knife edges that can be set level to the required accuracy.
David

[nevhunter]

Brian, if you have the time and patience I would lighten the JP piston as much as you reasonably can. They are miles too thick and would probably be ok in a diesel.. for strength. They need a bit of clearance, despite them saying they don't. They DO have silicon in them. but. Use the taper bored gudgeon or lighten yours .You can JUST machine the case hardened metal with the right tool.. Nev

[john.whiting]

Mentioning,JP,..I believe they started out with Polsons equipment and old stock,so its likely they are still using Polson cast iron molds.....as these things wouldnt be cheap to make ,and uneconomic for low volume sales.

[nevhunter]

That may be but the choice to have a thick wall is probably deliberately conservative. The gudgeon boss is often too thick, unless a large diameter pin is used. They use the usual metal 3 piece die almost universal for pistons of that style..They aren't porous and they often have a lot of ovality, unless they are for two strokes which have only taper. They list a piston for the British Seagul outboard. I fitted one to mine. so IF you have one you may be in luck. Nev

[Brian_Walker]

If I can work out how to mount the piston on my rotary table I should be able to mill the skirt down quite a bit with a slot mill cutter however it means I may have to make a fixture to mount through the gudgeon pin hole unless I can use "V" blocks to hold it securely.
Has any one seen a reference where lighter gudgeon pins are available?

[john.whiting]

The simplest way to reduce the pin is to bore it taper from each end,to about where the gap between rod and boss is......that way the pin isnt weakened over the load bearing area.Ive found ordinary TiN coated inserts on an ordinary boring bar are OK.......It often helps a lot with case hardened steel if you grind off the case where you are going to start the cut....Some also bore out the entire pin ,but that always reminds me of Alan Chance,who bored out a Bedford pin for his Manx Norton........The blowup was spectacular,the mag must have gone 50 yards......Lucky Alan had plenty of Manx stuff for spares........when he died ,some con artist "collectors" stole all the Manx stuff from his widow,on the pretext of "sorting out the mess"

[Mike Nash]

Inspired by Brian Walker's original post I thought I'd have a go. The method seems crude (so skilled and exacting engineers such as Messrs Whiting and Falco should perhaps look away now) but with care you can get a good idea of where you are - and just as importantly http://forum.arielownersmcc.com/downl ... w&id=17620 - a good idea of where you aren't. Here's my kit below.

The angle irons are clamped to a simple work bench and its feet and the end of the irons are packed with card etc to get the irons as level as I can using normal DIY spirit levels. (No hope here of getting only 3thou runout in 12inch run that Mr Whiting recommends.) The balance weights were measured either side of the big end to try and allow for lack of level in the irons.

The split skirt +20thou piston inc pin and circlips weighs 596g, the conrod small end is 207g and the balance weight is 563g therefore giving a Balance Factor of 58%.
Regards from MikeN.
PS The scales were ordinary kitchen scales bought from the John Lewis store and I'm pleased to say that checked against a certificated scale its accurate to the gram.