Standard or High compression piston?

[carl.horrocks]

Hello All,
I am doing an overhaul on my 1957 VH500. I have just set the timing using the 5/8" BTDC. BUT, I have just noted that it is 1/2" for a 'Standard compression' piston.
I have read the various items from AOMCC website and my piston does not fit any of the decriptions. It has the marking "JP0797" on the crown. It is not domed or totally flat. It has a flat top but there is bevelling around the edge and there are two cut-outs for the valve clearance. Can anyone identify whther this is a standard or high compression piston?
If not, which timing measurement would be best if in doubt?

Regards,
Carl Horrocks

[john.nash]

Using my skill and judgement, then I think this is a JP piston:

http://www.jp.com.au/MCA.html

VG, VH, Red Hunter, High Comp
1935-46
JP 0797
497cc, OHV, CR 7.5:1

As a newbie at this, as well, then some AOMCC members tell me to use 5/8" and some to use 1/2".
I've run the bike on both, or as near you can anyway with a bit of paper and a lump of metal down the plug hole, and I'd be hard pushed to say if there was any difference at "A" road speeds

I'd too be interested in the opinions of those with more experience ....

[carl.horrocks]

Thanks John. Where did you get the info on the JP site?

[john.nash]

I got the information from that link I posted

http://www.jp.com.au/MCA.html

I found it on JP's main site.

[Wali.Badger.Taylor]

Never put metal down the plug hole, it can do through the piston. Use dowel or a pencil, a few years ago my mate Marc phoned me in a panic as he had broken a pencil off in his 350. We just fired her up and watched the sparks come out of the can as it burnt up!

[john.nash]

Here is my "lump of metal".
Graduated scale (in mm though - but easily converted) just add a little fudge as it's going in at an angle, as opposed to straight down.

Does this make the grade, Wali ?

I actually have a magneto timing light, but using some paper proved as good