Piston Weight

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Danny.Lee
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Piston Weight

Postby Danny.Lee » Sun Aug 10, 2014 8:57 am

Hi all

My Ariel VB has a new JP piston from Drags.

I weighed it complete with pin and it is 2.5 ounces heavier than a Hep original I have. 1 ounce of that is in the gudgeon pin.
The Aussie pin is much thicker than the Hep original.

Questions for the experts,

Will it cause bad vibration?
Should I bore out the pin to reduce the weight?
Is there any acceptable way of reducing the weight of the piston?
Does it matter anyway on a slow revving SV single with a piston as big as a bucket?

As ever, thanks in advance for any contributions

Dan

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Postby ian.scott » Sun Aug 10, 2014 10:37 am

Hi
Yes it will cause a lot of vibration. It made my VH unrideable.
Use the original gudgeon pin ,its unlikely to be worn to any extent.
You can have the inside of the skirt skimmed out. I tried shortening the skirt but it cracked after a few thousand miles.

Ian

Danny.Lee
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Postby Danny.Lee » Sun Aug 10, 2014 4:57 pm

I did think about that, but I dont think I can use the original pin. Its slightly smaller than the new one and so is a sloppy fit in the new piston.

Thanks for telling me about the vibration though, now I know I have to find a solution. Think I will bore the new pin, and skim out the skirt.

Where did you shorten the skirt and where did it crack? I did think about shortening the sides after the manner of a slipper piston leaving
the thrust faces full length.

Dan

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Re: Piston Weight

Postby Geoffbrown » Sun Aug 10, 2014 6:27 pm

Yes you should drop the piston weight back to the weight at which the bike was originally balanced to, if the bottom end is original, back to the weight of an original VB piston is good. Having said that you piston is not much over weight so won't make a massive difference to the vibration. You can lighten the piston skirt and if you wish remove metal from below the gudgeon pin, I dropped the weight of my AP piston from 502grams to 438grams and in reality it didn't make a lot of difference, in the end I balanced the crank to make the real difference. If I knew how to post photo I would show you what I did to the piston - it was quite extensive......

Danny.Lee
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Postby Danny.Lee » Sun Aug 10, 2014 10:04 pm

I would love to see that photo
Dan

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brenton.roy
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Postby brenton.roy » Sun Aug 10, 2014 10:08 pm

Hi Danny,
I understand that 62% might be a good place to start with a VB.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

Danny.Lee
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Postby Danny.Lee » Sun Aug 10, 2014 10:18 pm

I should have thought of that before I cased up the crank...
Dan

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Postby Danny.Lee » Mon Aug 11, 2014 1:50 pm

I have been comparing a +30 Heplex with a +40 JP. Perhaps surprisingly, the JP is slightly thinner in the skirt than the Heplex so not much scope for removing weight there unless I shorten the whole skirt.

The Gudgeon bosses are shorter, but much thicker than the Heplex as much as 5mm thicker I would say. So I did think about milling 2mm each off the opposing faces, and possibly running an endmill along the four sides to remove say 1.5mm on each pass, that would still leave the bosses 3mm thicker than the Heplex.

The piston deck/ top/ crown is about 2mm thicker in the JP and the reinforcing rib is also much thicker than in the Heplex, but I dont think I have a milling cutter that would reach that deep.
I could take it off the top of the piston crown, but the top ring in the JP is about 2-3mm closer to the top than in the Heplex and I am worried about weakening the top land. I suppose I could leave a band full height around the piston perimeter just take the center down a bit, at the cost of losing a bit of CR. ( its a VB, would I even notice??)

Can anyone say why I shouldnt do any of this, or suggest another approach before I do something horribly expensive.

Regards
Dan

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charles.bulloch
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Re: Piston Weight

Postby charles.bulloch » Mon Aug 11, 2014 6:23 pm

Hi, Danny,

since the bore wore out on my iron cylinder, which had a +60 GPE piston, I obtained a brand new alloy cylinder and club +20 piston which is the correct weight compared to the old GPE one. There is little difference in vibration, if anything it is harsher. This could be due to the increased compression ratio. The machine is now 10 to 15mph faster.

Charlie

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Postby Danny.Lee » Wed Aug 13, 2014 11:43 pm

In the end I decided to put the piston in as it is. See what happens

Dan


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