Help identifying pistons

Hi all,

I have a pair of pistons in my 750 Commando engine that are marked on the underside GPM and the number 739


What i don't get is the markings on the top - no sign of STD or the usual marking for oversizing.

Just a number 5 stamped underneath 73

My guess is that this 5 signifies that the top of the piston raises .050" above the deck height at TDC (it's a combat engine)

But i'd be really grateful to hear your take on it all.

- the bores measure 73.4mm
- the pistons measure 73.02mm

Thanks for your thoughts!

Is this out of a recently running engine, or something that is apart for some reason ?

Without doing the conversion back to inches, and a stock bore of 2.8750", that seems like a big clearance.
Stock sized pistons in worn or oversize bores ?
Not being too familiar with GPM pistons though, so expect more comments.

Are both pistons and both bores the same sizes ?.

The engine was built up a few years ago by a norton 'specialist' - the previous owner clearly spent a lot of money on it. It's been drilled and had super lend bearings etc...

The engine was run up a few times, tuned, then packed up and hidden in the corner of a workshop until I got it.

Yes, both pistons and both cylinders measure exactly the same.

bore = 73.4mm = 2.89"
piston = 73.02mm = 2.875"

If the bore is 73.4 mm at the bottom of the bore, and the pistons are 73.02 mm at the bottom of the skirts, you have .38 mm, or .014" clearance, which is way too much. It would run, but I wouldn't say much for the longevity, and it would probably smoke like a chimney, use oil, and have a lot of breather blow by. You would expect the 73 and 5 to mean a .020" oversize, or 73.5 mm, but that doesn't match your measurements. Not sure what's going on here.

Ken

Yes, this is what i don't understand.

I measured the cylinder with a bore gauge in several places (excluding the top and bottom inch or so) on both cylinders
It was surprising consistent all the way through +/- 0.02mm but my 73.4mm was the average of all readings.

The pistons were measured with two sets of vernier calipers just above and just below the rings.
I measured at several points around the circumference, and took the average - actually all measurements were well within +/- 0.02mm anyway.
I appreciate that verniers not as accurate, but i think using this method I got a pretty good measurement.

Have you tried the pistons in the bores ?
The manual would suggest that clearance was excessive, and time for new pistons/and (maybe) a rebore.

its almost as if those were a previous set of pistons, and the new ones have not been supplied to you ??

what is the measurement of the piston halfway down the skirt on the thrust side ( fore and aft)?

Tried a google search of GPM and not much good re quality came of it. Hope this helps.
Ta.
PS: perhaps this page helps re the 739 numbers... http://www.gandini-industria.it/www/pis ... rticoli=PM
or this one for 'Old Timers' - http://www.gandini-industria.it/www/pis ... ticoli=GPM

I have 3 sets of that series of pistons. Mine are std bore like yours, yet all are +0.165" domed above std deck height. The bare casting is 738 but how are yours finished??? Are yours "flat" top?
Mine are 265gms bare.
Stock commando heps are typically 250 gms.
GPM are probably OK. My 68 dunstall used GPM, though a different GPM # since they were made 5 years earlier.

gtiller said:

The pistons were measured ................just above and just below the rings.
I measured at several points around the circumference, and took the average - actually all measurements were well within +/- 0.02mm anyway.

Click to expand...

I trust that you did not use the measurements above the rings as part of your averaging.

Take one (or multiple) reading(s) 90 degrees to the wrist pin at the level of the wrist pin on the skirt thrust surface. Better yet, for this discussion, slide the piston into the bore with a thin feeler gauge between bore and bottom of piston skirt. Start with say 0.003" and repeat with thicker gauges until until you find the gauge thickness that begins to bind. I would not go beyond 8 or 9 thousandths as that is getting too loose.

Down dirty way is stick in a new set of rings and see if their gap in within spec but otherwise take barrel and piston to engine shop with real bore guage & experience to see if bores oval worn enough to matter or not and if time for a rebore next size up. Worthwhile to check old rings gap to for wear factors too.

Thanks all for your help - I have this afternoon spoken to the guy that sold me the engine - he bought the entire bike from the Previous Owner, and broke it for parts.

PO sent away bottom half and head to Mick Hemmings and Norman White for machine work.
Previous Owner then reassembled - that would explain why the pistons were installed the wrong way round when we opened it up (left and right were correct, but exhaust and intake were the wrong way round, despite them being marked)

The pistons themselves do indeed seem overly loose in the cylinder - the slack has been taken up with the rings, which seem to be gapped about right. But I have a very uneasy feeling about them.
Also the fact they are flat tops is a cause for concern - as DynoDave points out, the expectation would be for a dome on top, and that is certainly what I was looking for when we took it apart.
I am wondering that given the seemingly non-standard top markings, whether someone has taken a skim for some reason.

I took some of my measurements at around the 70-110 degrees to the gudgeon pin - as the skirts of most pistons are of course slightly oval.

Everything else about this engine has been very well put together, so I am currently thinking that a new set of pistons is the way to go. I think that if i don't go that route, then i will always have a nagging doubt in the back of my mind!

Thanks all for the comments!

As Dances pointed out, the only piston measurement that should be used for clearance is the diameter measured near the bottom of the skirts and at 90 degrees to the pin. If you averaged that with the much smaller diameters in the ring area, you'd get a value that is way too small. What was the measurement at the bottom of the skirts?

Ken

You should not measure pistons next to the rings, this area has a smaller diameter, you measure as advised above.

I think that if i don't go that route, then i will always have a nagging doubt in the back of my mind!

Click to expand...

Thats what all of this stuff is really about peace of mind throttling away from home regardless whether could get away as is or not so just buy new pistons to match present bore or next step up if bore time to change. Could even be the next fella that gets most the life out of your detailing. I did 3 ring jobs so far and each time dry install that stopped smoke in about 50 seconds done then hit road as normal w/o lugging yet enjoyable throttle zips here/there more than usual for 20 miles just in case.

Ah yes, the critical piston clearance measurement is low down on the piston skirt on the thrust faces, at 90 degrees to the pin.
Averaging it with other measurements has clouded the issue, so ignore previous comments until taking this measurement.
All is probably well in piston-land.....

and that measurement is??

I will take some further measurements at the weekend - I'm not with my bike at the moment

...standby!

Back with the bike today, and have had the opportunity to take some proper measurements with some calibrated Mitutoyo kit

Piston measurement at 90 degrees to the gudgeon pin is 73.350mm (2.8880 inch)
Bore measurement (measured fore-aft, about halfway up) is 73.56947mm (2.89305 inch)

gtiller said:

Back with the bike today, and have had the opportunity to take some proper measurements with some calibrated Mitutoyo kit

Piston measurement at 90 degrees to the gudgeon pin is 73.350mm (2.8880 inch)
Bore measurement (measured fore-aft, about halfway up) is 73.56947mm (2.89305 inch)

Click to expand...

How does that match with the manufacturer's website link I posted earlier in this thread i.e. 739 73 5 ?
Ta.