Which pistons are available for my ariel

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

Post by Simon.Gardiner »

Didn't realise the 7.5 was causing nightmares!

A couple of places in the UK have lower compression VH JPs available (eg 10793)
Thorntons
http://vintagemotorcyclepistons.co.uk/e ... /Pistons/3
Red Lizard (who are new to me)
http://www.classic-motorcycles.co.uk/cg ... &Start=300
(Thorntons also seem to have Gandini in - to me - an unusual 7:1 ratio, but they don't go bigger than +30)

(Oversize cylinder bores also raise the original static compression ratio and of couse fuel characteristics will have changed quite a lot - but I must say that with a manual advance retard, and the very crude static full advance ignition setting that we use on these bikes, I'd think that some amount of 'limit' could be avoided with a bit of pragmatic lever adjustment.)

SG
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cmfalco
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Re: Which pistons are available for my ariel

Post by cmfalco »

Simon.Gardiner wrote: Didn't realise the 7.5 was causing nightmares!
Maybe this means the rest of my life is so tranquil and problem-free that I concentrate all my worries on Ariel pistons...
Simon.Gardiner wrote:A couple of places in the UK have lower compression VH JPs available (eg 10793)
Thorntons
Thanks very much for this link. Thorntons lists it for £228.66, which isn't exactly cheap. But, Draganfly also shows this 6.3:1 piston in +20 and +40 (but not +60), however as no longer unavailable, so I wonder if Thorntons really does have them in stock or if this is an old listing from when JP was still producing them.
Simon.Gardiner wrote:(Oversize cylinder bores also raise the original static compression ratio...
That's a very good point well worth mentioning. Although 0.060" doesn't sound like much it increases the swept volume to 518 cc, in turn increasing the compression ratio to 7.78:1.

I haven't yet measured the volume of the combustion chamber, but if we take 7.5:1 at face value as being accurate for the stock piston it means the volume of the head is 66.57 cc.

Since compression plates were mentioned by someone else, to decrease the compression of a stock engine to, say, 6.5:1 with a "7.5:1" piston would require a combustion chamber of 76.81 cc, an increase of 10.24 cc. This could be achieved with a compression plate of thickness 0.077" (1.95 mm). After re-examining the rocker box it's possible there could be this much adjustment possible in the screws, in which case the geometry at the tip of the valves would remain unchanged. So, I shouldn't have been so quick to dismiss that suggestion. However, I'll have to do a test re-fitting of the cylinder, head, and rocker to see if there actually is that much available adjustment or, if not, if longer screws could be fitted without some unintended consequence.
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Re: Which pistons are available for my ariel

Post by Simon.Gardiner »

As there's been some discussion of piston weights, to go with Greg's comprehensive details of the Hepolite 9:1 I've weighed some of the ones I've accumulated.
My scales aren't massively accurate and I used imperial (oz) which it only attempts to 1 decimal place, but I weighed each item several times to check I got the same reading each time. As Greg used grams I've done a (rounded) calculation to for the equivalents of my imperial units.

Set of Hepolite +020 piston rings (2 compression, 1 oil) 1oz (28g)
Gudgeon pin 2.8oz ((79g)

6.2 Hepolite 10793 (number cast inside) standard bore split-skirt 12.5oz (354g) (with rings, no pin)
6.2-hepolite-10793.JPG
6.2-hepolite-10793-inv.JPG
6.2 Brico ('Ariel Brico' cast inside) standard bore, oval ground 10.3oz (293g) (no rings, no pin)
ariel-brico1.JPG
ariel-brico-inv.JPG
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Re: Which pistons are available for my ariel

Post by Simon.Gardiner »

6.2 non-Hepolite (unknown make, has radially deeper rings than Hepolite, may or may not be GPM), +020, oval ground 12.4oz (352g) (with rings, and carbon! - no pin)
6.2-non-hepo.JPG
6.2-non-hepo-inv.JPG
7.5 Hepolite 10503, +020, oval ground 12.9oz (366g) (with rings, no pin)
7.5-hepolite-10503.JPG
7.5-hepolite-10503-inv.JPG
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Re: Which pistons are available for my ariel

Post by Simon.Gardiner »

7.5 Hepolite 3-compression ring +020 oval ground 11.7oz (333g) (no rings, no pin)
7.5-3ring-10503.JPG
7.5-3ring-10503-inv1.JPG
When I looked at the decimal point ounce differences I thought they were big, looking at the gram equivalents I'm not so sure!
Depending on how many decimal places you want to work to these could all seem quite close, except maybe that strangely light Brico.

HTH

SG
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Re: Which pistons are available for my ariel

Post by cmfalco »

Simon.Gardiner wrote:As there's been some discussion of piston weights,
Since some of your weights are without pins or rings I weighed the ones from my +0.040" Heplex 10793 using a scale for which I have calibration weights and is accurate to +/-0.01 g.

pin only 96.12 g
rings 27.13 g
(2) circlips 1.90 g

On a different calibrated scale (with a higher range than used for the above) that is accurate to +/-0.5 g the complete piston (with pin, rings and circlips) is 467.0 g
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Re: Which pistons are available for my ariel

Post by Simon.Gardiner »

I gave some weights for rings and pin at the start of the posts - your +40 rings are pretty close to my +020 but your pin is some 17g heavier - I wonder if that's production tolerances?
The +040 10793 comes out slightly heavier than my standard 10793, which would be expected. (And assuming they're both the same split-skirt design.)
SG
Last edited by Simon.Gardiner on Fri Aug 11, 2017 9:08 pm, edited 1 time in total.
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Re: Which pistons are available for my ariel

Post by cmfalco »

Simon.Gardiner wrote: your pin is some 17g heavier - I wonder if that's production tolerances?
Given the density of steel (~8 gm/cc) that difference is quite a bit. It represents your pin "missing" the equivalent of an additional cube of steel ~1.3 cm on a side.

The OD (0.810") and length (2.684") of the pin are pretty much fixed (in principle, my pin could be 0.040" longer than a stock one) so there are only two possible sources for the difference: the bore of the pin, or a measurement error. In my case I've confirmed the weight I quoted previously by repeating the measurement today on the 0.01 g scale as well as, to less accuracy, on the 0.5 g scale.

My pin has an ID of 0.614" at either end, slowly tapering to 0.498" in the center. Call that ~0.55" ID on average (a more accurate average could be calculated based on the weight and other dimensions, but I won't take the time to do that). Converting to metric, and rounding:

OD 2.06 cm
ID 1.40 cm
length 6.82 cm
(volume 12.23 cc)

To remove 2 cc from the volume would require the ID to be increased to 1.54 cm (0.606") on average. That increase of average dia. by 0.056" certainly seems possible, but you'll have to check your pin to see if that, or a measurement error, is responsible for the 17 g difference. If the ID of your pin is larger, the difference is large enough that it would seem to be more of a "design difference" than a "production error."
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Re: Which pistons are available for my ariel

Post by Simon.Gardiner »

Selection of 4 VH gudgeon pins, 2 used, 2 NOS (in a box of NOS pins, not from pistons), weights (oz) 2.5, 2.8, 2.9, 3.0.
ODs not very accurate, lets say 0.811 with half a thou each way (sometimes each way in the same pin - in my experience what went out as 'spares' is sometimes a little off-standard). Lengths - two at 2.685 and two at 2.690. They're all taper-bored, I can't get accurate measurements for the IDs but three are 0.7 at the outside edge and one is 0.685. The last one is the heaviest one.....

SG
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Re: Which pistons are available for my ariel

Post by cmfalco »

Simon.Gardiner wrote: weights (oz) 2.5, 2.8, 2.9, 3.0. ... three are 0.7 at the outside edge and one is 0.685. The last one is the heaviest one.....
Very interesting. Thanks for posting those figures. That's a big range, but mine is heavier still at 3.4 oz. The additional weight of mine comes from it having a smaller ID of ~0.61", hence more steel and thus heavier weight.

Makes the idea of setting the balance factor to the nearest gram interesting when the uncertainty in total weight could be an ounce...
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