Forum

[ChrisD]

Hi Guys. The piston in my 1996 Classic 500 cracked and I need a new one (it was a JP from Australia). I have our hosts alloy barrel (checked) and maybe the current need for4 a new piston is a good excuse to bore it out to 535cc and fit one of our hosts 8.5:1 pistons. Any thoughts on the likely success ofthe boreout? I have heard it said the liners can sometimes have blowholes? I know I will be fitting compression plates so the over-compression aspect is in hand.
Any thoughts or advice?
Cheers, ChrisD

[Midge]

Do you know why the JP failed? I thought they were supposed to be better. By cracked, do you mean horizontally at the oil ring.

[Norm]

Midge, I'm tipping a vertical crack, I have seen one before but that piston had been removed several times for various reasons apart from the piston. Owner claimed the gudgeon was too tight but I doubt that was the reason and from memory I think the crack was at the rear anyway

[ChrisD]

It was cracked from the expansion slot through the skirt. And it made a lot of noise - I thought the bigend had gone. Yes, the gudgeon pin was indeed a bit tight.
For those of you who remember old stories, I'd had a hitchcocks 8.5:1 piston which seized because the bore (iron barrel then) was honed to 5.5thou and it continued to seize. So I filed it down leaving a space of ~20thou. Still worked though and quite well but it slapped like a chinese whip.
So I'm going back to the 8.5:1 but now with a alloy barrel.
Cheers
ChrisD