oily bores

[Reckless Rat]

I'm sure one of the motor cycle engines that I have tinkered with, a two stroke, had a pin in the ring grooves, with the ends of the piston rings ground to a semi-circle to match it, thus preventing the rings from rotating. Can't for the like of me remember which is was. Probably a Lambretta, or a Bultaco or perhaps even a Yamaha. It was a long time ago.Like this:

[John Mason]

Reckers. I maybe wrong. Time has proved that many times. I always thought all two stroke engines had dowel pins in the rings to prevent them moving. Thus preventing them moving in line with the ports. Four stroke engines do not have this problem so no need to dowel the rings.

John Mason.

[Ilmoro]

I think most, if not all2 stroke engines have a pin to stop ring rotation, thus preventing the ring ends from traversing an inlet/exhaust port, snagging and trashing the engine. Tuned 2 strokes in particular had wide port openings.

[Reckless Rat]

That makes sense. Thanks.

[JonE]

ok, well an update. Found this on 3 which is also a solid skirt piston and had one bad top ring and a broken second. Also very stiff gudgeon on 4. So that is positive to find but does beg the question of whether a fault on just one can result in oil in 1 and 4 as well as the global blowby issue. I'm doing ring and bore clearance checks on the rest.
Question - do new pistons come at exactly the stated dimension, or are they minus the few thou in expectation that the bore has been done to stated dimensions? If anyone has a split skirt +040 in decent nick, do shout!
The excavations also unearthed a single, shot pair of shell bearings on journal 2. All the shells are not drilled for the upper rod. But that is another story for another thread!
   

[Parazine]

Question - do new pistons come at exactly the stated dimension, or are they minus the few thou in expectation that the bore has been done to stated dimensions? If anyone has a split skirt +040 in decent nick, do shout!

Generally, the bores are made to exactly the oversize stated so a +0.040" bore will measure 2.240" exactly. However, most responsible rebore shops will ask for the pistons so that they can check the piston size for themselves to make sure that the clearances have been allowed for correctly in the process. Split skirt and solid skirt pistons have different clearance requirements I believe.

[JonE]

so any views on new pistons sizing generally in relation to Parazine's comment? I also have come to realise the enormous chasm in my knowledge of measurement and calibration. I've now (from a 10mm and 3mm drill base which I assumed might be correct-ish) calibrated my Japanese boxed caliper to about 99.2% accuracy. So as long as I keep using that for comparisons its ok, but there are all sort of places where its use becomes difficult.

[Bob Culver]

Seven pistons came in a myriad patterns. That one seems to have 3 comp rings. And the relief below the 3rd is unnecessarily generous. Drillings need only break into the relief. The part below oil ring should be of reduced dia. Times have changed but piston types can be mixed, provided all weigh the same. Solid skirt pistons should be of reduced dia across the gudgeon dia as Austin originals, not fully circular. Tight gudgeons are a recipe for failed little end screws.
Vernier callipers and use of offer scope for many errors. Digital readouts give a false sense of super accuracy. Can be used locked with feelers as comparators.
On a vehicle which does not ping broken lands are unusual, and often done during assembly. I dunno if anything can be deduced from the angle of the breaks

[Chris KC]

Split skirt and solid skirt pistons have different clearance requirements I believe.
1.5 thou for split skirts, 4 thou for solid, as a guide. Measured at bottom of bore.

Vernier calipers are jolly handy but not really up to the job of measuring journals and bores. It's well worth looking out for good used micrometers at autojumbles or even on ebay. Even with the right tools there's a definite art to precision measurement. 

[Chris Garner]

Excellent cylinder bore gauges can be picked up ( new ) on eBay for around £30.  Invaluable tool and always being bored by tight fisted friends!!