G80S Bore Measurements

Max Autobrake · Post by **Max Autobrake** » Sat Apr 02, 2011 11:00 pm

Please would somebody tell me the allowable piston to bore clearances and max ring gaps on standard bore? Also, what's the crown and skirt diameter on a standard piston?

I ask because I can't find any marking on the piston to indicate what size I have. There's some blow-by evident so I'm having the bore honed and will fit new rings, the piston seems in okay order but I have no idea whether it's standard or oversize.

Many thanks.

Don Madden · Post by **Don Madden** » Sun Apr 03, 2011 2:30 am

First you have to find out who the piston maker was as the clearance will vary. If not standard, the oversize is usually stamped into the top & if AMC the part no. & the maker will be, too. The crown will have to cleaned of all carbon first but that should always be done when the head is off. Other makers cast identifying codes on the inside. Oversize pistons are made in increments, usually +0.010 or 0.020, so easy to measure with simple calipers & compare with standard. Cheers, Don.

wilko · Post by **wilko** » Sun Apr 03, 2011 5:25 am

Standard bore is 82.5 so the piston will be around 4or5 thou under that. You could probably get away with 8 thou.

Max Autobrake · Post by **Max Autobrake** » Fri Apr 08, 2011 8:53 pm

Thanks for the info' chaps. I've cleaned the crown and have found the following markings:

10197

I D 16
Made in England

There is no maker's name and the piston appears to be forged rather than cast.

Any clues to identification? Typically the piston is just too big for my mic' and the battery has run out on the vernier but will find out size tomorrow.

I'm very lucky, the bike only had one owner before me and has never been restored, but I can't imagine for a moment that it's still on a standard bore. Look forward to hearing your thoughts.

Roland

Eric · Post by **Eric** » Fri Apr 08, 2011 9:25 pm

I am not really very knowledgeable upon pistons, but I do know quite a lot about forging, particularly upset forging, as for several years I worked in a small company that did quite a lot of it and I designed all of the forging tools.

I would not have thought it was possible to produce a piston by forging but would welcome expert knowledge.

Max Autobrake · Post by **Max Autobrake** » Fri Apr 08, 2011 9:39 pm

Hello Eric, almost all good quality pistons are forged. Most OE pistons on Italian bikes are forged and the only people to use cast ones on road bikes are the Japs. This isn't too much of a problem for normal use as the pistons on a multi-cylinder are small.

I used to race Ducatis years ago and one of my competitors used high-comp cast pistons with disastrous and expensive results!

Anyway, any idea what my piston is from the numbers above?

Roland

Lance · Post by **Lance** » Sat Apr 09, 2011 4:02 am

Hi Roland,

10197 is a Hepolite number with compression ratio of 7.2 to 1.

82.548mm

I have two of these pistons as follows:

1 - Hepolite 10197, ID 16, +.060 oversize (stamped on top).

2 - Hepolite 10197, ID 10, +.020 oversize (stamped on top).

Not sure if there is a correlation between ID # and oversize amount.

Does yours have any markings relating to an oversize?

Some piston info available here in the archives:

http://archives.jampot.dk/Technical/Pis ... gs_Valves/

Regards,
LanceEdited by - Lance on 09 Apr 2011 03:03:07 AM

Max Autobrake · Post by **Max Autobrake** » Sat Apr 09, 2011 10:52 am

Thanks again for all the input. My eyes are not what they were it seems as I've found a faint marking at the centre of the crown. It is stamped STD which I assume (unless the bike has syphillis) means it is on a standard bore. The piston is not wire-bound below the oil ring.

So, next question is whether I must use Hepolite rings or whether they're generic to all manufacturers.

Thanks again, I'm making progess!

Roland

iandusud · Post by **iandusud** » Sat Apr 09, 2011 3:09 pm

Hi Roland. I measure piston to bore clearance with feeler gauges at the bottom of the skirt. Once you've established the piston ring groove depth and width you should be able to fit piston ring from any manufacturer that correspond. I generally work on approx 1.5 thou per inch of bore for skirt to bore clearance up to 2.5 thou / inch max. For ring gaps 3-4 thou / inch of bore.

The piston to bore clearance assume a non wire-bound piston. These use much smaller clearances but I have no experience of them so would rather not comment.

Ian

Max Autobrake · Post by **Max Autobrake** » Fri Apr 15, 2011 10:36 pm

I have an exciting week-end planned in my shed! The ring set has arrived and so the bike can go back together again.......

Do the rings have any marking on them to indicate which face should be upwards, or does it not matter? I know that normally the scraper and compression rings have to face in a certain direction.

Thanks again for the input.

Roland