Which pistons are available for my ariel

Singles, twins and fours.
nevhunter
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Re: Which pistons are available for my ariel

Postby nevhunter » Sat Aug 12, 2017 12:09 am

You measure each total result and alter the components as desired. Usually drill holes on the inside of the flywheels as original practice. Doing it to ONE gramme is not really justified, when the factor is an "intelligent " guess. Variations in frames and handlebars would cause more differences in effect. If you have your con rod installed use light oil in it to reduce friction when checking where it stops with various weights on the knife edges and have them level. I let the conrod swing to help the assembly settle in the best indicating position when checking. Nev

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Re: Which pistons are available for my ariel

Postby cmfalco » Sat Aug 12, 2017 12:14 am

nevhunter wrote: Doing it to ONE gramme is not really justified, when the factor is an "intelligent " guess.
I was being facetious when I wrote that. However, if someone has a 1928 piston with it's 1" gudgeon pin gathering dust it would be very helpful to know its weight since that would let me do better than guess at the original factor balance factor.

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Re: Which pistons are available for my ariel

Postby nevhunter » Sat Aug 12, 2017 3:39 am

I often used to measure the original ones from the factory (not necessarily ARIEL) and with some, it's all over the place. I have at least one Ariel 1" pin and it's a very lightweight thing. I'd have to find it. They are surprisingly thin walled and distort when the end pads are pressed in requiring lapping to correct it. I have made them new in the past. Any gudgeon not fitted to a diesel should be made of something good and taper bored and be as light as possible,(commensurate with required strength) A lot of common car pistons are only case hardened mild steel, which I wouldn't use in an bike motor unless I had nothing else or was in a big hurry and had to get the job done for an event. I wouldn't balance a motor to a temporary heavy piston (or Pin) Use the figures for the proper one and when you fit it later the motor will be balanced and you don't have to strip it, again. Nev

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Re: Which pistons are available for my ariel

Postby cmfalco » Sat Aug 12, 2017 4:27 am

nevhunter wrote: I wouldn't balance a motor to a temporary heavy piston (or Pin) Use the figures for the proper one and when you fit it later the motor will be balanced and you don't have to strip it, again.
"Use the figures for the proper one" is the problem. I haven't yet found what those figures are. That's why it would help a lot to know what a '28 piston & pin weigh, even given that there is some amount of variability to the result.

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Re: Which pistons are available for my ariel

Postby cmfalco » Sun Aug 13, 2017 1:27 am

More information will follow in a few weeks, but for now the following are the weights of the complete assemblies (piston + rings + pin + circlips) of three pistons for a post-1935 500 cc Ariel (or, in my case, a 1928 Ariel with connecting rod bushed for a smaller gudgeon pin):

Heplex (used) +40 6.2:1
10793 cast inside
467.0 g

Omega +60 7.5:1
6425.0.2 etched on crown
435.0 g

Gardini +60 7.5:1
P810D-152 on box (only the bore in mm is etched on crown)
515.5 g

All were measured on a calibrated scale accurate to +/-0.5 g.

Again, if anyone has access to a piston with 1" gudgeon pin as originally used in a Black Ariel, please weigh it and post the results. Even if it's missing the rings I can make a good estimate of their weight (especially so if you also provide the height and depth of the grooves, but even if not...).

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Re: Which pistons are available for my ariel

Postby nevhunter » Sun Aug 13, 2017 4:17 am

The only one I have outside a motor is semifinished and they are usually about 2 mm oversize. so not much help. Every one I've had has a fairly large dome so I would estimate the comp at around 7:1. Having a 1" pin wouldn't add a lot to the piston weight, but you want exact figures. Up till about 1947/9? the factory pistons didn't have an oil ring and most of them weren't heavily ribbed and would be lighter than say the 1955 Hepolite with t slot but not much.. Nev

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Re: Which pistons are available for my ariel

Postby cmfalco » Mon Aug 14, 2017 2:24 am

At present I have no idea how much the factory-original piston assembly with its 1" gudgeon pin weighed, nor the optimum balance factor to use. Knowing the former would allow me to calculate the balance factor used by the factory (assuming when I disassemble the crankcases I don't find "modern" machining marks indicating it has been altered). The reason this is very important can be seen from the following calculation.

As can be seen from my earlier post, there is a difference of 80.5 g (2.8 oz.) between the Omega and Gardini. If I pull a number out of the air for the weight of the small end of the rod (say, ~100 g, since it's very roughly the size of the gudgeon pin) in order to estimate the effect of this on the balance factor, it means the total weight to be balanced if the Omega were used would be:

435 g + ~100 g = 535 g.

If the balance factor with the Omega happened to be, say, 66% that means the weight balanced by the flywheel would be:

.66 x 535 = 353 g

Given this estimate, this means if instead the Gardini were substituted the balance factor would change to

353 g / (515.5 + ~100) g = 57%

Because this is a crude estimate the exact numbers aren't significant. What is significant is that this calculation shows the choice of these two aftermarket pistons would affect the balance factor by a very large amount. Clearly, such differences would have a significant effect on the vibration I will be subjected to for 4000 miles, which is why I'm spending the time required to try to get the balance factor "correct" rather than using some value pulled out of the air.

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Re: Which pistons are available for my ariel

Postby david.anderson » Mon Aug 14, 2017 10:54 am

Even if you can obtain an original piston and gudgeon there is no guarantee that you will be able to correctly calculate the exact design balance factor, although you may get close. There will be differences in weight between individual original pistons and gudgeons, and how accurately were the original flywheels balanced.
We do not know how critical Ariel considered the balance factor to be. We are talking about a time when rider comfort did not matter and rigid frames with poor front suspension were state of the art. These suspensionless bikes were ridden on roads that would today be subject to court proceedings by injured parties. There does not appear to be any factory information on balance factor, which suggests Ariel may not have considered vibration from heavier oversize pistons a problem. (rider comfort and fillings falling out may not have mattered). The only balance information that I have found is a statement by Val Page during an interview, that Ariel balanced all Red Hunter singles to 65%. We do know that Ariel balanced by boring holes in the inner rim of the flywheel. It is unlikely that the flywheels were assembled and taken apart to be balanced, so it seems to me that the initial balancing may not have been that accurate, but rather a ball park figure. (It is well known about the Commando, that Norton sent the crankshafts to a contractor to be balanced. It was later revealed the contractor never balanced any crankshaft but bored random holes to make it look as if the crankshaft had been balanced.)
The Red Hunter and Deluxe Ariel singles initially mounted the motor in a single down tube rigid frame, and then plunger frame and subsequently the motor was fitted in a twin down tube frame. The engine plate assembly of the twin down tube SA frame were quite different. But Ariel maintained the same engine balance factor. My own 49 rigid and 57 SA framed bikes are both balanced to the same balance factor and are virtually vibration free. (Although my wife claims the 57VH has a little vibration through the pillion footpegs above 60mph that does not exist on the 49VH, or the rider footpegs for that matter.)
I have ridden a 350 red hunter that vibrated very badly. When I checked the balance factor it came out at 60%. After I rebalanced the vibration disappeared, so that small amount is significant.
Good luck with the hunt for the piston weight.
David

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Re: Which pistons are available for my ariel

Postby Simon.Gardiner » Mon Aug 14, 2017 1:52 pm

cmfalco wrote:Gardini +60 7.5:1
P810D-152 on box (only the bore in mm is etched on crown)
515.5 g


A previous thread also has some posts on piston weights and it includes this info for a standard bore Gandini.
viewtopic.php?f=4&t=1833

Gandini piston details: VH500 1950/51, std piston 81.8mm, 7.5:1 comp.
Weight inc pin & clips = 485g.
Expansion @ 250deg = 0.012" / 0.29mm (bottom of skirt, 90deg to pin).

That would seem to make 30.5g (more than 1oz) weight gain for the full oversize on the same piston.

SG
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

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cmfalco
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Re: Which pistons are available for my ariel

Postby cmfalco » Mon Aug 14, 2017 4:33 pm

david.anderson wrote:Even if you can obtain an original piston and gudgeon there is no guarantee that you will be able to correctly calculate the exact design balance factor, although you may get close. There will be differences in weight between individual original pistons and gudgeons, and how accurately were the original flywheels balanced...

Good luck with the hunt for the piston weight.
Yes, what you wrote is true, but especially important is the "you may get close." I fully realize the uncertainties involved, but I'll get a lot closer -- and, very likely, close enough -- once I know the weight of an original piston. As my previous calculation showed, if I do nothing the range of the resultant balance factor will be a very large ~10% with that range not necessarily even centered on the "correct" value. However, if I know the weight of the original piston assembly to within, say, 20 g this drops the range to ~2% as well as centers it.

Of course, I could get unlucky and my flywheels could happen to be at the upper end of the distribution in the amount of material the factory machined from them, and when I do -- fingers crossed -- learn the weight of an original piston it could be at the heavy end of the distribution Hepolite supplied at the time. If the tolerances conspire against me to add in this way the balance factor I calculate would be skewed to be lower than optimum. Still, it would be way closer to use the results of this calculation than to just install either of the aftermarket pistons and cross my fingers, or even to use some number from a later engine and frame.

Simon.Gardiner wrote: That would seem to make 30.5g (more than 1oz) weight gain for the full oversize on the same piston.
It should be twice that based only on the increase in annular cross section due to a 0.060" larger OD. However, the ID of the bottom of the skirt of the Gardini is machined so if they kept the lower skirt the same thickness on the oversize piston (i.e. if they also increased the ID of that portion by 0.060") the weight gain only would be due to the extra thickness of the upper part of the piston, reducing the weight gain to the above noted ~30 g.

Again, this illustrates the importance of actually measuring whatever replacement piston that will be used and then re-balancing the flywheels to that measured weight and to the, er, um, as-yet unknown balance factor.


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