OT - IC engine piston ring questions

SLG · Jun 12, 2014

I ordered a master rebuild kit for a diesel engine and it arrived with a set of Mahle pistons with piston rings already installed. This seems absurd to me, as there is no way to measure ring end gap without removing the rings from the pistons. I'm assuming Mahle installed the rings, but I haven't found out for sure - yet.

If you were building an engine, would you trust Mahle to have ground the rings to the proper end gap or would you remove them and measure? (I think I know your answer)

Are piston rings apt to be damaged by installing/removing/re-installing? IOW - should I demand a replacement set of rings?

Thanks

tdmidget · Jun 12, 2014

Call 'em and ask. I would bet that Mahle knows their shit. I never understood this ring gap thing anyway. They only fit one size bore so why wouldn't they be manufactured with the proper gap?

Bobw · Jun 12, 2014

They probably got sick of people wanting to do it all themselves and screwing it all up and whining about it...

Probably cheaper for them to gap and install the rings themselves... Think about how many people have screwed up a ring
trying to install them with a pair of vise grips, a screwdriver and a hammer....

I'd bet they are dead nuts where they are supposed to be.

Like TDmidget said, its not that hard to make a ring with the right gap from the get go... This isn't the '50's.

The last rebuild kit I bought, which was over 20 years ago, and DIRT DIRT DIRT cheap from PAW, the rings where dead on...
In the cylinder bores that were the right size.

emcmike · Jun 12, 2014

Measure the bore , compress the ring and measure = ring gap +.002 or.003. Being a diesel the bore may be out of round at the oil skirt line. Of coarse good builders bore first

howieranger · Jun 12, 2014

My experience with MAHLE piston and ring sets for VW diesels and Porsche gas engines was that the rings were right on always.

Vancbiker · Jun 12, 2014

My experience is the same as howieranger's, I put a set of Mahle pistons in my turbo toy car and checked the gap on every ring. They were spot on.

omrc7771 · Jun 12, 2014

Really depends on the overbore of an individual engine. Having built many racing engines, my experience is that you must be able to accurately measure the bores and use mfg ring gap specs. Some grinding may be required, if the gap is improper, you can have a mess of a cylinder wall or alot of blowby.

moonlight machine · Jun 12, 2014

I have never seen a Mahle ring gap be incorrect if the bore is the proper size. All the Mahle pistons and rings I have ever used over the years, (100s) have been super quality, dead on size.

CWB · Jun 12, 2014

Considering Mahle's the piston supplier for a major percentage of the recip engines produced today, Id have no qualms about trusting them to have matched the piston and rings from the factory....its what they do. Beyond that, Ive always had great experiences working directly with them.

CarbideBob · Jun 12, 2014

I've seen a lot of their parts.
If you did find one that checked off the first thing I'd do is recalibrate my measuring equipment.
Bob

KB3AHE · Jun 12, 2014

General "rule of thumb" is .003-.004" of end gap per inch of cylinder bore. IE: an engine with a 4" bore should have .012-.016" of end gap. Usually if the engine has been honed (to establish a crosshatch pattern) but not rebored, the end gaps will usually be a bit more due to cylinder wear.

If the end gaps are a little wide it won't hurt anything, but.......... If the gaps are too tight (close) you could have a problem when the engine gets hot and the gap closes up (ring expands) and the ring binds up and breaks causing sometimes serious cylinder and piston damage. Especially if the engine overheats for whatever reason. I have assembled hundreds of engines of all shapes and sizes throughout my life and have always preferred to "sin" in the direction of a wider end gap.

Just my $.02 worth....

Frank

manualmachinist · Jun 12, 2014

If you are dead set on checking the gaps before assembly couldn't you bore a sleeve to cylinder size and drill a hole through it that you could check the gap with a pin?

tdmidget · Jun 12, 2014

omrc7771 said:
Really depends on the overbore of an individual engine. Having built many racing engines, my experience is that you must be able to accurately measure the bores and use mfg ring gap specs. Some grinding may be required, if the gap is improper, you can have a mess of a cylinder wall or alot of blowby.

And, if the cylinder bore is off enough to affect the gap, they won't seal anyway.

KB3AHE · Jun 13, 2014

tdmidget said:
And, if the cylinder bore is off enough to affect the gap, they won't seal anyway.

Nah....... don't kid yourself, the gap has to be extremely wide and the bore has to be sloppy enough that there is little or no tension on the walls.

I have a little tractor with a 7Hp Petter diesel engine on it. I put a set of rings in it well over 30 years ago. I still use it to haul a load of kids around on a landscaping trailer every year for the "neighborhood hayride". It was in pretty bad shape when I got it and should have been bored and fitted with a new piston, but I was broke at the time, so it only got a quick deglazing and a set of standard bore rings. It definitely had a way wide end gap, but it fired right off. It now has many years and hundreds of hours on it. It starts as soon as you hit the starter and the oil stays clean between oil changes. Even though that thing really "rolls the coal" when pulling the trailer load of kids up a long hill.

If a one-cylinder diesel seals and runs well with a funky bore, and starts easily. The width of the end gap and how perfect the bore is is not as critical as one would think. (within reason)

If I was doing the job for a customer, it would have been a way different story, but this was for me and I was broke at the time.......
Sometimes a little experimentation can teach one many things........

Frank

Nick Mueller · Jun 13, 2014

If you were building an engine, would you trust Mahle to have ground the rings to the proper end gap or would you remove them and measure?

Mahle is making pistons and piston rings since the spring of 2014.
Their assembled sets get delivered to VW (not BMW, they buy Chinese). At the factory, they disassemble the rings and check each and every one. The ones with a too narrow gap get a short touchup with an angle grinder, the ones with a gap too big, get a blob with a welding stick and slight smoothing with a bastard file.
Then they go on onto the piston again.

The reason why they install the rings is to save a bit on packaging.

Nick

tdmidget · Jun 13, 2014

I have a great respect for simplicity but that Petter is not a highly stressed sophisticated engine. Per your post it is not running that well if is working that hard to pull kids on a wagon.
A set of rings is only round at one diameter. They may wear to match the bore wear but they need to start out as a reasonable match.

awake · Jun 13, 2014

Nick Mueller said:
... the ones with a gap too big, get a blob with a welding stick and slight smoothing with a bastard file.

... The reason why they install the rings is to save a bit on packaging.

Nick, or anyone -- I am surprised by two things here, and would like more info. First is the idea of extending a ring with weldment. No doubt I am revealing my ignorance here, but I thought rings were made of cast iron, or perhaps a high-alloy steel? Are they instead made of a mild, weldable steel? (I am assuming even a weldable, high-alloy steel would come out too hard to smooth with a file ... but again, may just be my ignorance ...)

The other is the packaging issue. I wouldn't think rings require much space; is the issue the care needed to protect them? I would have thought there might be as much cost in installing the rings to ship as there would be in packaging them.

kustomizingkid · Jun 13, 2014

As long as your engine builder hones the bores proper I bet they will be bang on. What engine are you rebuilding? I am planning to order a similar kit for a 1.6d.

Nick Mueller · Jun 13, 2014

Are they instead made of a mild, weldable steel?

Mahle makes them out of coat hangers. Or if they can get them, out of old sloppy bed springs.

I have heard, that Mahle already sold 300 piston sets. Only 279 had to be returned. Customer broke when they were hammered into the cylinder sleeve. VW claims, their bores were right, as they got a new drill bit just last year. Maybe Mahle's folding measure got a bit wet, their roof is leaking.

Here, you can read more about the facts: Irony - Wikipedia, the free encyclopedia (a special iron alloy pistons and rings are made of)

Nick

RayG · Jun 13, 2014

Good point, lot's of quality automotive parts made from Irony...

OT - IC engine piston ring questions

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