Piston for 600 VB

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IanR
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Piston for 600 VB

Postby IanR » Fri May 11, 2018 12:11 pm

Hi All, I posted a query about piston weights in Dec. 2017 but the thread got lost . I rebuilt a 1954 VB engine using a +60 JP piston and was a bit worried about its weight (740gm with pin and rings) compared to the old +30 Hepolite( 605gm with pin and rings) , and its effect on vibration. I hoped it would be ok as the VB is a low revving engine. Now everything is working I find vibration higher than I expected, such that I am limiting its speed to about 45mph (roughly 3000rpm). The last reply I got to my question, before I had the engine running, from Leejm recommended rebalancing to the new piston. I assume that involves adding weight to the crank, which sounds expensive, and may make the horizontal out of balance worse(?). Another possible would be to resleeve back to std. size(86.4mm) and find a NOS hepolite. But I now have a +60 cylinder.
Has anyone tried to reduce the weight of a piston and if so what happened? Cutting a "window" about 70 x 40 mm in the piston front and back, leaving about 12mm of skirt would save about 45 gms ( maybe more if the JP is thicker than the Hepolite) but still well above the original std. piston weight, and would mess about with heat transfer and no doubt other things.
If anyone has a Hepolite 11327 +60 I would be interested in its weight, and buying it if it was lighter than the JP.
Any help appreciated

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adrie.degraaff
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Re: Piston for 600 VB

Postby adrie.degraaff » Sat May 12, 2018 6:04 am

I would make the piston lighter, much better than distroying the crank, on the outside round the pin (redusing the chance of seizure) or the inside roof.

david.anderson
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Re: Piston for 600 VB

Postby david.anderson » Sat May 12, 2018 9:02 am

I would never cut slots in the piston. It is however ok to reduce the piston wall height under the gudgeon as per the photo below. You will only get a minimal weight reduction however you need a lot. In reality, any piston that is .060” oversize is going to be much heavier and the crank will need rebalancing. Rebalancing is normally done by drilling the flywheel rim and a couple of 3/8 holes of appropriate depth is probably all it will take. However fixing mallory metal in existing holes to increase weight can also be done. The balance factor should be 65%.
David
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