8 replies to this topic

[087dave]

Hi all
I'm rebuilding my mg metro engine I have just put the rings on the Pistons but the oil rings are 3 piece so,

Do u line up the bottom and top part of this type of ring or do you move them
Thanks

[carbon]

For the 3-part oil ring I line up the two ring grooves, but have the gap on the expander ring at 180 degrees to these grooves.

 

Not sure how much difference this makes once the engine is running, as I think the rings all tend to rotate slowly.

[Spider]

For the 3-part oil ring I line up the two ring grooves, but have the gap on the expander ring at 180 degrees to these grooves.

 

Not sure how much difference this makes once the engine is running, as I think the rings all tend to rotate slowly.

 

As Carbon has said, but don't worry too much about where the ends of the expander (the wavy bit that you fit first) ends up, that won't make any difference, just make sure that it's not near the ends of the scraper rings.

 

Just to correct the rings moving, they shouldn't rotate when the engine is running and they will usually only do so from poor honing or a mis-aligned (bent / twisted) con rod.

Edited by Moke Spider, 24 April 2015 - 09:32 PM.

[TimDaly]

I know this is an old post but....  Is there any gapping to be done with the central expander piece?  I'm assuming the construction of it makes it stay tight to the bore without binding.

 

Thanks,

Tim.

[Spider]

The Expander is essentially a Spring. You may wish to fit it and be sure there is no gap and some tension on it, though, that's self evident when fitted to the piston with the Scrapers.

[TimDaly]

That's what I thought.  Thanks Spider.  They fit 'sprung' with no gap.

 

Tim.

[GraemeC]

Just to correct the rings moving, they shouldn't rotate when the engine is running and they will usually only do so from poor honing or a mis-aligned (bent / twisted) con rod.

 

I don’t agree here - the rings do rotate. They are not necessarily designed to do so, but it does happen.

It is mentioned in this piece by Wiseco (as a ‘big name’ piston manufacturer) which goes into having the right ring clearances allowing the rotation:

http://blog.wiseco.com/ring-grooves

 

this is also an interesting article saying that the rotation is hard to model:

https://www.highpowe...of-piston-rings

Edited by GraemeC, 04 August 2021 - 06:22 AM.

[Spider]

Graeme,

 

This is one of those debates that's never likely ever settled !

 

The Rings can rotate in operation, but it is highly undesirable in the utmost. As you could appreciate, there's next to no oil in the Ring Lads of the Piston, so there's no lubrication and also because it's Iron or Steel on Aluminum (of one type or another) the lighter material tends to 'grip' the harder material.

Ring Rotation when it occurs is usually from poor Bore Honing, Bent Rod, Misaligned Bores and probably a zillion other causes.

Here's an extract from Repco's info, who (for what ever it's worth) were an OEM Supplier to BMC, GM, Ford,Toyota and others;-

[GraemeC]

I think 'ring spinning due to corkscrew condition' is a very extreme mode and is different to the slow rotation seen in a well built engine.

 

That described would indeed cause bad wear as the ring would be constantly forced to rotate back and forth with every engine rotation.  Whereas the usual ring rotation of (around) 1rpm per 1000rpm of engine is not as extreme and is usually due the ring floating on the gasses that get behind and under it - the ring gap to ring thickness being critical here (indeed some piston use a drilling to allow the gas into, and out of, the space behind the ring.

 

And of course - piston and ring material, bore honing technique and of course research and development has (hopefully) improved since Repco wrote that.

I'd agree it is not ideal, but still happens and is accepted as OK.  If not then surely they would adopt the approach used in 2-stroke engines where it is actively prevented.