Piston ring groove prep.

[4v6]

Hello Guy i hope youre well.

Reading through your book yet again, i noticed that re-ringing is an operation you'd generally frown upon.
I find myself in the situation of having to do so due to the cost of a replacement set of pistons for an Audi 5 cylinder, basically the budget simply wont stretch that far.
I have measured the ring side clearances in their respective grooves and end gaps and they are all within manufacturers specs, however, i found some damage to the facing on one of them which renders them useless and neccessitating replacement.
My question is can you describe the procedure i would need to follow in order to properly dress the ring grooves for new rings and give them a sporting chance of performing correctly?
Obviously im keen to ensure i do this right so i dont want to just install the new rings and hope for the best, hence the question.
Ive included a few photos of the damage which is located at the ends of one top compression ring, im wondering if poor installation technique has created this?
As always any advice or pointers appreciated.
Tony.

Undamaged section of magnified ring.

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Damaged section magnified by 30X.

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[Guy Croft]

You'll need to use MS Paint to annotate those pics Tony I am not sure what I am looking at.

There may be a wear ridge in the bores too?

G

[4v6]

Hello Guy, sorry about that i thought id explained it with the captions, not to worry though i only included them to show the ring facing damage in closeup.
This rather wider shot should be a bit more helpful i hope and shows the damaged facing on the top compression ring, only one of which was actually damaged, but i'd like to replace them all for good measure.

Damage to ring facing.

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The bore wear is very very slight with the "ridge" being barely felt.
A run down with the flex hone has polished it even finer and bore gauging it confirms the bore is almost unworn to any degree.
You can just barely see it on this photo so i would think/hope that new rings would not be impacted (no pun) by it.

Bore condition, very light wear "ridge".

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I hope theres enough info there, if not please let me know and ill endeavour to supply it.

Tony.

[Guy Croft]

At first I thought you were talking about damage in the ring grooves in the pistons. Are you asking about damage in the groove of the rings themselves?

Sorry, confused.com

G

[4v6]

Ahh i see, sometimes i get ahead of myself and expect people to instinctively know what im waffling about, apologies i wasnt clear.

No Guy, the ring grooves in the pistons are as far as i can determine in good condition, the one top compression ring is simply a damaged item with the surface in contact with the bore wall being the damaged part, hence id like to replace the full set of rings in the engine.
My worry after reading your book is simply that the new rings will not bed in to the pistons unless the piston ring grooves are correctly prepared, something to which you alluded to.

Basically the question is, how do i correctly or adequately prepare the ring grooves in the piston to accept new std rings so that they bed in to the pistons without problems, do you have a recommended procedure i could follow?
Hope thats clarifies it , Tony.

[Guy Croft]

The ring grooves are not particularly easy to clean. I don't recommend scraping with any sharp implement or tool except that an old broken ring is good for getting carbon out of the groove.

The sealing region is the lower face of the groove and that is what needs refreshing; if you don't restore the surface the new rings will just not seal properly under compression & firing.

Although there is a small risk of embedding of abrasive the only way to clean the top and second ring grooves effectively is to take some 500 grade (with oil) wrapped onto say, a thick feeler gauge and give that face a gentle scrub. The 3rd (oil control) ring groove is wider and you may be able to do the same but with fine Scotchbrite*.

Give the piston skirts and lands a good scrub with the same * and finish the job with a good clean in brake cleaner.


G

[4v6]

Guy many thanks sir, i will follow your instructions to the letter.

I now have 15 piston grooves to detail and may be gone some time.
Many thanks for your help on this, ill let you know how i get on.

Best wishes, Tony.

[4v6]

Just to say a "Thank you" for your advice Re: preparing the piston rings grooves Guy.

I spent many hours as expected doing this particular job but im absolutely confident the new rings will now seat as they should.
I thoroughly cleaned the pistons also which again took a long day to remove all the fouling, i removed it with a toothbrush and traffic film remover that dissolves the gum, varnish and carbon, hence no other mechanical methods were necessary, however its slooow!
Removing the many burrs and sharp edges i found on both the pistons and rods took up at least another day and attending to the lack of a chamfer under the seating area for the heads of the conrod bolts added to the length of it all.
The necessity of resetting the ring gaps on the top compression rings to that for a new item took even more time.
I can say with some conviction that i will never again re-ring used pistons, its simply not worth the time and effort and i can fully understand why itd not be recommended.

I finally got the rotating and reciprocating assembly back together the other day after carrying out the innumerable checks, piston ring side clearances, end gaps, and so on but im very satisfied with all the effort so far.

New ring set required top rings to be trimmed.
Took hours and burrs had to be carefully removed each time the rings were fitted.

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A slightly better situation.
Gap here is at 0.10mm and too tight still.

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Checking the side clearances, all were within standard specifications.

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New rod bolts have a better underhead radius than stock oem items.

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Rods have variable chamfers formed, some of the them displayed absolutely none at all.
Bolt underhead radius can foul on the corner meaning the bolt head dosent contact the seating surface.

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The same rod from the other side has a much larger chamfer on it.

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Finally back in one piece and still much to do.
Nice radius applied to the tops of the bores meant no issues with the new rings on refitment.

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