oversize rings on standard pistons?

[fitterski]

I'm looking at two maybe three little used engines that measure bore diameters above new but still far short of any rebore OS threshold. One piece of advice I got from a Harley guru is to hone them out and then use oversize rings making sure that the gaps are near max tolerance. I can relate to this idea given the cost of 16* new pistons except for one detail: the OS rings to use in such a case should be OS rings MADE FOR standard pistons i.e. the inner diameter should be standard, in other words the ring width should be above standard or leaks would be a risk not to mention piston groove damage.

Anyone done this already, horror stories, success, theory?

* two 6-holers and one 4-holer

 

[sd455dan]

if the bore is still under the overbore necessary spec, I would look for (file to fit) std. size rings if available- that way the ring is correct for the piston and the end gap can be tailored to get within the standard end gap spec.

 

[Walt 2002]

I have done dozens of engines over many years with your description by using ten thousandths oversize rings and filing them to min. end gap. I seldom use std. rings. I always look for ring sets with a conventional oil ring as opposed to the segment ones though I have used those, just takes more time. I have had excellent results.

Walt Conner

 

[TractorGuy]

I have also done this with .010" over rings. The thickness of the rings will be the same. Just the circumference is different to make up for a larger bore. Funny this is mentioned in reference to Harleys. I worked at a Harley dealer in 1975 and ALL the std sportster rings were defective. Seems they overlooked the tempering on them and they were coming back using oil and smoking under the meager 3000 mile warranty. I spent most of that summer doing ring jobs on AMF Sportsters. We were filing the ring gaps on .010" over rings for every one. .010" over rings will have a little more outward squeeze and should seal or seat better.

 

[jaydee325]

I've done this too. But realize this is a bandaid.

Couple things to consider. The cylinder bore needs to be in spec for both out of round and taper.

If there is taper, then you want to set your ring end gaps at the smallest diameter of the cylinder. This is usually at the bottom of ring travel.

There is also piston skirt to cylinder clearance that is important. I've even had a knurl put on piston skirts and then gently filed the knurled area to get proper clearance.

Carefully inspect the ring lands on the piston. If they are not square, the new rings won't seat.

 

[Walt 2002]

I am equipped to and have re-sized cylinders when needed with professional equipment. I do not consider my use of plus ten rings a bandaid. I get much better end gap specs. and piston clearance is not an issue in these cases. Thankfully perhaps, B&S engines are very forgiving anyway. Note that I am mostly retired now and have serious qualms about doing this with the current B&S very short skirt pistons though I did an Intek V twin but I think I got the last two sets of plus ten rings for it in the USA. Last I checked, they no longer offer them for V twin Inteks

Walt Conner

 

[fitterski]

The Harley guru that advised me IS a real Harley guru. I don't ride no more at my age, my son now has the Harley. The engines I'm working on (the first one a Deutz F4L912 because I have to get that one on line in a little less than two months) are all very young in the 1000hrs or 100,000kms range so while I do want to give them new life (the Deutz is down a bit on compression) I don't wanna begin by overboring and spending the highway-robbery price of $100+ per new OS piston. I'm not sure if I undestood his lecture on the gap tolerance, I'll get back to him on that. OS rings do NOT have the same ID as standards so that when you do get them snug against the piston they are actually a bit ovalized around it and this can lead to pinching and upsets. He too DID say however that he'd been using OS rings on bores that don't need a rebore to OS. Myself I'm not knowlegable enough about this topic to have an opinion. I've asked around for Deutz standard "file to fit" rings but no one seems to know what I'm talking about.

 

[buickanddeere]

I'm going with what jaydee325 said.

 

[Walt 2002]

I do not agree that plus ten rings, "ovalize" and that they get, "snug" around the piston. Some claim that they distort and do not seal against the cylinder wall. None of this is true. There is less chance of deforming with these than re-ringing with std. rings which are made to have ~.008" end gap but now have ~.018" which is typical when re-ringing with std. rings, think about it.

Walt Conner

 

[LD1]

Splitting hairs with the rings. All the rings, be it std, file fit, +10, +20, whatever are going to be the SAME width and thickness.

The larger overbore are just longer around the circumference.

If a std ring is giving you a 0.018 gap and you want a 0.008....take Tha 0.010 and dive by pi. That's roughly 0.003 difference in diameter.

Your worried about the inside diameter and ring to land contact area.....you are worrying about 0.0015 of contact area per side. (Half the thickness of a hair).

For small engines....you are worrying about something that is totally irrelevant