High Compression 650 Pistons?

Short of the JS piston/long rod combination, I can't seem to find much in the way of 650 high comp pistons.

One can reportedly fit domed 600 pistons, but finding NOS domed 600 pistons seems equally impossible.

There are some old Hap Jones domed pistons floating around, but they've got quad valve pockets resulting in some very thin aluminum on the tops, and what I can only imagine is very bad flow and a recipe for detonation.

Anyone have much experience with the Hap Jones pistons, the 600 domed pistons (or where one might aquire some), or anything else out there? Not looking to blow the head off, but somewhere above 9:1 would be interesting.

I ran 40 over, 10.5:1 CR in my Atlas with no problems kicking over. When octane in fuel at the pump fell off, I had pinging unless I was careful with the throttle.
I would expect you would have similar findings if you were to use similar pistons in a 650.

Good luck in finding a pair of pistons.

Slick

Yea, I figured that may be a possibility (probability), but I'm hoping dropping compression with thicker head gaskets to where it needs to be is a better bet than shaving the head to gain compression with std pistons.

The pistons in my 650ss are new from GPM. They are made for 8.9 to one in standard size, but .060" oversize was used. That bumped the compression a little. I believe it's around 9.2 or 9.3 to one. It doesn't ping and it goes very well. This was done just before I purchased the bike, so I haven't seen them. Pretty sure they are flat tops.
Glen

I fitted GPM 060 to my 650SS, Due to further engine problems I pulled the Barrels to find the pistons were touching the heads, a further skim aloud the piston to protrude correctly into the hemisphere.

With the scarcety of barrels and the possibility of breakage at the flange , I would not want to load up the old girl more than necessary.

norton bob said:

With the scarcety of barrels and the possibility of breakage at the flange , I would not want to load up the old girl more than necessary.

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I've got a couple barrels still at standard bore and cranks at standard grind, so I'm not worried about breakage too much. Not looking to make a drag bike, just warm things up a bit.

possm said:

I fitted GPM 060 to my 650SS, Due to further engine problems I pulled the Barrels to find the pistons were touching the heads, a further skim aloud the piston to protrude correctly into the hemisphere.

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Whoa! That's good to know that even the flat tops are that close! I don't think my head has been skimmed and looks to have the stock depth of gasket face.

worntorn said:

The pistons in my 650ss are new from GPM. They are made for 8.9 to one in standard size, but .060" oversize was used. That bumped the compression a little. I believe it's around 9.2 or 9.3 to one. It doesn't ping and it goes very well. This was done just before I purchased the bike, so I haven't seen them. Pretty sure they are flat tops.
Glen

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It appears GPM has two versions. A 9 / 10:1 or 7.4 / 8.25:1, probably with the two listed CRs depending on the head used. Thanks for the tip! That might just be the way to go.