Tight new rings
 

I have installed a new set of Goetz rings on my Mahle pistons. They were measured up by my machinist and given the green light to rebuild (as were the cylinders). Anyway, the top rings all fit just loose enough that they can be rotated easily, however some of the 2nd and 3rd rings are quite snug. Should I be concerned?

Thanks,

Brian

Got feeler gauges?

Take a 0.004" gauge and check the side clearance. Shouldn't be more than that.

Sherwood

Its no side clearance on certain 2nd and 3rd (oil) rings. The top I'll check with feelers.

Brian

Hmm. Was able to measure as wide as .007". THe tight rings are what really baffles me. Do I need to replace these based on that ring clearance for the top rings alone? Crap.

Don't dump the pistons based on a stranger's (my) post on a public forum. Locate the specs, then remeasure. However, based on the .007" side clearance, I'd say the pistons have exceeded factory specs. You can run loosey-goosey rings in your engine, but don't expect any decent life out of the rings. If they don't break, the edges will wear to a round profile instead of remaining square and blowby will soon follow.

Same with the tight 2nd ring. The 2nd ring should be able to rotate with the similar side clearance. Same with all pistons? Or get a second opinion. It takes too much work to return to this point in a rebuild.

Hope this helps,
Sherwood

.004 inches is the limit specified in my books. Are you checking with the rings all the way into the groove? According to the 911 performance handbook you could have problems with the new rings if the clearance is too high. I'd consider using the old rings if the end gaps are in spec. Otherwise you need a different set of pistons.

-Andy

So the gap is measured at the back of the groove? I have to say what really puzzles me is the the lower ringsbeing so tight. Very strange, especially for used pistons. It is not all of them, just a few.

I recently had one piston that had a tight spot. In this case it was on the oil ring. I located the tight spot on the piston by rolling the ring around the outside and seeing when it gets stuck. I then put some magic marker on the ring and slid it in the tight spot to mark the high points. After I talking to my machinist, I took a small jeweler's file and gently relieved the two high spots and followed up with some red scotchbrite to polish the area. Measure to make sure you are in spec.

The side clearance is measured between the side of the ring and the groove. If the side clearance exceeds wear limits, it doesn't matter what's happening in the lower ring grooves. The piston is dead. Sorry.

BTW, find another machinist to advise you or just tell him the correct clearance to measure. It doesn't seem like he's paying attention to the required specs.

Sherwood

Tight rings in a used piston sounds odd.
There are many ways to massage a ring to fit a piston. What might work is to take an old ring, snap it in half and use it like a deberring tool to keen the ring groove out.
If the ring groove is in spec and the ring is a little thick you can massage the ring with wet/ dry 400 on a surface plate.

If the pistons are really what you want to run and the top ring show excessive clearance you can have a machinist resize the groove and run a large ring.
Companies like Total Seal, JE, Deeves and Hastings all offer different ring thickness.
Of coarse a nice new set of my JE pistons in QSC cylinders is another way to fix the problem.
http://forums.pelicanparts.com/uploa...1138391634.jpg

THe top ring groove is what kills most 911 pistons I see, I'm afraid.

They do not last very well..

Like Henry says, if you really must reusre the piston, lots of places can machine the groove for thick rings, or for normal rings plus a spacer shim..

I'd look at Hentry's new pistons..

Kind regards
David

I spoke to the machinist for clarification. He did look at ring gap. He said his assessment was they were "in spec" but at the far end of it, so they wont last too much longer one way or the other. I guess the question is, is it okay to run them (based on his measurements, which are probably better than mine) for this year and then replace next winter. My overall plan was to go back in next winter and beef up the engine for racing with a bunch of new thing, including pistons. I'd prefer not to do that now because 1) the cost, of course, and 2) I don't want to have to decide what pistons to run now and chance that they wont fit into my engine plan next year, which is far from final.

Henry, thanks for the advice on the tight grooves, I'll try it this weekend. I don't want to go through the expense of remachining the pistons only to replace then in a year anyway. I will likely put less than 10k miles on the car this year.

Hello again.

There is confusion here.
You say he checked ring gap, but how about the groove side clearance?

That is much more important IMHO..

If that is 7, its not worth the labour of assembly..and disassembly..

Kind regards
David

I misspoke - groove side clearance is what I'm referring to. Sorry.

It's worth the labor if it will last until winter! If not, you are certainly right, David!

This particular measurement is not at all difficult..

I do not push the feeler into the depth of the groove, since the base of the groove often has a lip in it (I'm often looking at cylinders with lwrge running clearances.)

If a New ring has 7 clearance in the groove, it is NOT in spec..

Kind regards
David

why are you guys talking inches when the specs are metric?
it's just wrong. rocket ships have been self destructed over these kind of things...

metric is right... obi wan kenobi said so. :D

Lazy I guess;

0.004" = 0.1016 mm
0.007" = 0.1778 mm

I still can't get used to a size 46 shoe.

Sherwood

Henry, with the 1124 posts and all the advice you've given, when do you have time for the shop? I assume there is a family too. Oh ya, and SLEEP. How do you find the time?

i was just yanking the chain Henri... :D
i'm pretty sure Obi One Kenoby couldn't assemble a flat six like you do, he'de probably try some jedi mind trick on an mfi pump that's out of wack: "all is fine, i am in spec", then the pump goes "**?*"