Two of the best Ford tech's I know have given me opinions (and they agree) on removing and replacing a piston from the bore without replacing the rings. I'd like to know what everyone else thinks or knows about this subject.

 

A piston will not wear equally (the bore wont be round). Reinstalling rings that have shaped themselves to an "egg shaped" bore, doesn't strike me as bright.

 

I have put many pistons with the rings that were "wore in" back into the same cylinder they came out of (after changing rod(s) for example), and even swapped a few piston/ring into another engine MOSTLY without issue. Heck, I've pulled pistons and had them knurled and re-installed them with the used rings (Admittedly a flaky repair). This was my own stuff though and the budget was tight, also this was on gassers with compression ratios of 10 to 1 or usually less.

So...what if it's one slug on a v8? The remaining rings have the same amount of wear, what's the benefit on one cylinder having new rings? Well, some considerations:
1.What was the ridge like, could you hang a fingernail on it? If so it needs ridge reamed for disassembly or the rings will likely be damaged.
2. Did you have to ream it? If so the upper ring will bump your transition (the bottom of where the reamer stopped cutting) to the original bore surface. The transition needs to be honed.
3.did you hone it? If so, wear will be increased significantly until the parts "wear in".
4. Check side play in the ring land. If it was significant on the old piston and much tighter on the new one the ring will be a little farther up the bore at TDC and the ridge becomes a consideration again. Also the ring now has a rounded surface (facing the bore) and it's going to take longer for it to wear in or it may not wear in at all.
5. Ring tension good? If the engine has been overheated the rings will relax and the reduced spring tension will also reduce you chances of it wearing in properly.

No matter how slick you are the old ring(s) on the new piston are not going to be in exactly the same place when you reassemble it. So if you can see the wear marks from the ring grooves it need to be honed. The accelerated wear of the honed cylinder wall and the already worn ring will make this repair the likely first failer down the road.

So, I would say it could be ok IF you don't have to ridge ream or hone it, the old piston/ring didn't have excessive side clearance, and the ring tension is good.

So what do you save? The cost of rings on one cylinder and a few minutes to deglaze/hone one cylinder. Seems to me it's well worth the time and a few $$ to properly prep the cylinder and put new rings in it. Now your repair won't be the first cylinder to fail.

 

It's not one hole it was all 8 and they would have only come out to clean and check. Now I'm having to replace one piston because of a freak accident so it's obviously getting new rings (I had a piston), but I won't pull the rest.

This block only has 40K miles on it and had a lifter failure which required replacement of the cam. I had the crank polished while it was out because of a funky looking main bearing. It didn't require turning at all.

 

vloney i understand your statement. i have you had any charge backs for excessive parts latly?
ive pulled enugh pistons on fords dime already got a charge back that was a piston and set of rings on a 6.4 for over repair was told i should have replace only the rod and honed the cyclinder.
any thoughts?

 

Document, Document, Document!

 

I ive been down this road too. part was called back and inspected. no amount of document will save you from that.
sadly you know were im comming from. I dont miss it ... yet

 

I've got one under review now. They want to know why I replaced the converter on it and what was wrong with it (eco-boost f150), It was their tsb that I followed!

 

f-me. this says wonders and tim sorry for

 

As far as the pistons are concerned, I've done it on race engines and had no issues.

Brian

 

Piston rings rotate as the piston is moving up and down in the bore.

Josh

 

Man, I've heard some sparky discussion on that subject!


(Valves rotate too)

 

Sealed-Power of Federal Mogul have done numerous tests on piston ring rotation. Didn't think people still thought otherwise since the early 80s on that subject.

Guys still debating it?

As Barry Rabotnick, formerly of Sealed-Power and currently of Survival Motorsports fame explains:

"they rotate slowly - maybe 10-12 RPM on average, in a near random fashion. At Sealed Power they used a radioactive pellet embedded into the rings to track that in the lab. Found that the rings did move, sometimes even changed direction. If they rotate fast something is wrong with the cylinder finish and they'll wear out quickly."


And high speed cameras show the valve rotation, tons of Youtube videos of that.

Josh

 

This also brings into question the necessity of installing ring gaps at various intervals on the pistons although I'll continue to do it "just because".

 

It helps at start-up I suppose, and I still clock the gaps accordingly knowing they rotate.

Watch the high speed camera stuff on valves, amazing how spaghetti like they are at 9000+ rpm.

Josh