Gandini pistons

[stu.sharratt]

Evening all.
My new piston arrived yesterday from Draganfly, it's a Gandini job, made in Italy.
I was expecting one of the JP ones, but the Gandini one looks fine.
Having read that the JP ones are relatively heavy with a heavy pin, does anyone have any experience of Gandini ?

[robin.parker]

Hi Stu,
I have no personal experience with Gandini pistons, but have heard that getting replacement rings for them can be a bit tricky, so be careful!
Before you fit your new piston I would be most grateful if you had time to weigh it (complete)---most digital kitchen scales are accurate enough.
Thanks, Robin.

[robin.parker]

And---Stu, I should have asked what cc your engine is?
Robin.

[stu.sharratt]

Hi Robin
The piston in question is a std 81.8mm 7.5:1 comp ratio for my '50/'51 VH500, I'm about to have the cylinder relined & was looking to get some more specific info on clearances for the machine shop to work to. I'll weigh it & measure the hot expansion.

..

[stu.sharratt]

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).

My old piston was Hepolite +0.040", but I stripped the engine down 20 yrs ago and lost it somewhere along the line!
The bore was heavily worn anyway, so going for re-line back to std. Anyone have any idea how the weight compares to Hepolite ? Is it lighter than the JP pistons that Drags have supplied ?

[nevhunter]

Looks OK but the valve reliefs are STRANGE. You might have to do something about them. Check your expansion compared to a hepolite (or such). If it is a solid skirt piston it will require more clearance. The JP is usually heavier than most.. I have a hepolite I can weigh if it helps. Nev

[stu.sharratt]

Nev
It is a solid skirt, so was going for a generous clearance.Draganfly suggest 0.005"-0.007", I was thinking in the region of 0.008"-0.010". If you could weigh a Hepolite to give me an idea how the Gandini might affect balance that would be useful, also could you post a pic of the crown so that I can see where you're coming from with the cut outs,its so long since I pulled it apart I don't recall what the old one looked like!
Cheers
Stu

[nevhunter]

The split skirt hepolite for the VH is 355 grams with rings (no gudgeon). This is one of the lightest pistons likely to be available . the face of the cutout should reflect the angle of the valve to the vertical. You will have to put the piston positioned at the top extent of it's travel and see if either valve contact it at the amount of lift likely to occur. 2/3rd of the weight difference is what the correction would be. You would be assuming that the original one was 355 Grams. That may not be correct. Nev

[brenton.roy]

Hi Stu, I reckon you could get away with 7 or 8 thou., even if you aren't the best runner inner (like me...).
I have a second hand one of these pistons (same cutouts) and it looks quite well made.
If it's heavier than the original piston, it's likely to vibrate more.
While my VH still vibrates plenty, it had a high frequency vibration at 60mph that was very uncomfortable - which went away after the crank was balanced.

If you haven't had the sleeve fitted yet, it's worth making sure the engineer is aware of how critical it is to have the bore square to the cylinder base. I had a sleeve that I eventually measured as being bored / pressed in out of square. Subsequent rebores kept repeating the error. It's hard to spot and it caused me huge problems. It would have been ok if it was out "fore and aft", but it was toward the pushrod tubes.
Also there are a number of posts on having the sleeve pegged to stop it slipping down.

[robin.parker]

Hi Stu,
Thanks for the data on your Gandini piston.
My NOS "Heplex" std VH piston , complete with rings and gudgeon pin weighs 437gm.
I'm requested some bore clearance info on Gandini pistons, will pass it on if and when it arrives.
Robin.