oily bores

[Ruairidh Dunford]

The tapered rings I have seen and used in Sevens did not have a step (two dots on the uppermost surface each side of the gap) - this certainly would make them easier to install correctly.

[steve davidson]

This description of scraper rings seems relevant:
http://korihandbook.federalmogul.com/en/section_19.htm

Thanks Colin, that's an excellent reference

[steve davidson]

Hi all,

Happy to say that I reversed the ring orienation and solved the problem. The following points summaries my learnings;

Rings that have dots must have the dots facing the top of the piston. If there are not dots, then,
Rings with a bevel or lip on the INSIDE diameter are installed with the bevel towards the top of the piston.
Rings with a groove in the OUTSIDE diameter are installed with the groove toward the bottom of the piston.
Rings having no dots, bevels, or grooves.....suit yourself and put them in any orienation.

[JonE]

bumping this as it's an excellent thread - and thank to OP for the final summary... really good practice and extremely helpful for notetaking.
Might it be that OP would consider also adding 'and piston ring orientation' into the title on the very first post for future searchers? There are so many threads which are only easily found as a result of a particular reader remembering what it was.

[Nick Turley]

I am not sure if this will help you but for what it is worth here are my suggestions.

Your pistons look fine, I think the issue is totally to do with the way you have the gaps positioned.
The top oil ring gap being lined up with the piston gap gives a hole for the oil to come straight through !
In my view it renders the bottom oil rig useless.

Personally I would reposition the rings using the 1/3 positioning and avoid any gaps being near the piston split.

I believe that will cure the problem.

On oil may I suggest initially using a cheap straight mineral oil - your rings will then bed in quicker, then after two hundred miles or so put in your oil of choice.

Good luck.

[Bob Culver]

Reading all this through again it is not over clear that rings with a slight external taper should be fitted so the largest external dia is down. The ring then planes over the oil film on the upstroke, scrapes on the downstroke..
A very slight shallow bevel can be filed on all ring top edges. Ideally stop before the gap.The lower edges of oil rings should have a relieved area below to scrape oil into. ie the dia should be less then skirt dia.
Seven pistons are prone to tilt due th very short bearing area. With patience relieved zones can be discontinued on the thrust faces to preserve bearing length. Any relief below the top oil ring should have drain holes.

Interested in the shims. If filed flush on outside as shown can be positioned with certainty. A legitimate way to correct over filed big ends.

[JonE]

It might be handy if someone summarised the general ring advice for the most commonly used new pistons. i.e. 7 Workshop-type standard pistons... and slipper ones from a number of suppliers. Do they all come disassembled - and will they always need gapping/checking as a matter of course?

[Tony Griffiths]

The piston ring gaps should all be staggered and they need to be correctly gapped...

Many years ago I knew one Maurice Patey, a long-time Scott aficionado and an ex-Rolls Royce engineer who, among many other things, had worked on the development of tank engines. He told me that, curious to find out if it made any difference, in 1941 they checked to see if anyone in the department had experimented with lining up ring gaps. As nobody had, they assembled an engine in this way, ran it on the testbed for a couple of hours and carefully measured the power output, fuel and oil consumption, etc. The engine was dismantled with the intention of staggering the ring gaps to see if it would make any difference. There was no need, the rings had had already done it themselves.
On Ruairidh's point about the rings being the right way up, in the 1970s a friend bought a new Yamaha TZ350 racing motorcycle. Being of the rather thorough kind - and having a fully-equipped toolroom to hand - stripped the bike to its components parts to check tolerances and the quality of assembly. He could find only one thing wrong - the piston rings were upside down.

[John Cornforth]

Hi

The need to stagger ring gaps is one of those obvious things that has been repeated for decades. It's also a myth, in my opinion. It might help you assemble the piston into the bore more easily, but as soon as you start up the engine the rings will all move about at their own different rate and lose their alignment.

[Bob Culver]

Service manual of the 1980s often included ring gap spacing details. I have a selection which cover nearly every combination!

I read somewhere that huge ship engines had some bore device which monitored oil film thickness or somesuch and this detected when the the ring gap was slowly rotating by.
For anyone who has followed all this, comparison with the pistons in the current ring ompressor post may be of interest.

Possibly on two strokes the rings were reversed to reduce the chance of catching in ports.