thrust side of piston & clocking sf

[Evan]

I talked w/ Ross Racing Pistons on piston installation and I am getting ready to put them in. I mentioned clocking and then he brought up thrust side. They do mostly cars and he does not know Laverda. Is this something I should worry about?  He said on cars it is the direction of the engine rotation. Ok when riding Laverda engines turn forward. Would the thrust side be inside or spark plug side or forward side or backward side??? Then I was going to clock in quadrants...Green bk doesn't say anything.

 

[GregT]

In an engine which rotates "forward" the thrust face of the pistons is the back side.

By "clocking" I assume you're referring to the orientation of the ring gaps. Follow the Ross instructions as detailed in another thread.

But of course where you put the gaps isn't where they stay, lol. Except for pinned 2 stroke rings and some pinned BMW's, they do move around in the grooves.

 

[Rusty Spokes]

Clocking and the thrust faces..you do not want those end gaps of the ring directly on the thrust sides of the cylinders/pistons..that is 90 degrees from the piston pin. You want those ring gaps staggered 180 degrees, but never with those ring gaps ending there.

https://www.rosspistons.com/wp-content/uploads/2016/08/Ring-Installation.pdf

 

[sfcpiet]

Quite irrelevant where the gaps of 1-piece rings are located upon assembly, they move around randomly in use.  3-piece rings should be assembled to manufacturers specs, the individual rings dont't move much relative to the expander springs. 

Only issue I see is to avoid having the ring ends opening in the gap of your favourite ring compressor (if it has a gap) and snagging the liner upon assembly. 

Staggering the gaps is solely for peace of mind, the tiny amount of blow-by that may occur with gaps inline immediately after intial start up is quite negligable, there's loads more blowing between liner and ring contact surfaces until the rings bed in.

There's a nice piece somewhere in the www written by a builder of Ford and Lotus Formula 2(?) racing engines on this subject.  Apparently performed numerous experiments with quite surprising findings, blowing several myths out the window.  

piet

 

[GregT]

Clocking and the thrust faces..you do not want those end gaps of the ring directly on the thrust sides of the cylinders/pistons..that is 90 degrees from the piston pin. You want those ring gaps staggered 180 degrees, but never with those ring gaps ending there.
[/quote

I'd point out that current best practise in two-stroke race engines is to have the ring pin - and hence the gap - dead center on the back of the piston.
And yes, this is the thrust face, and yes, it is often over a rear boost port too.
300 hp/litre - and the ring is the least of the problems, LOL.