What an excellent question Biba. I'll begin by stating that I have no idea.......
But I'm willing to float a few things to add to the confusion/discussion: 1. Is the groove in the piston square, round or vee-shaped in X-section? >From a stress point of view I'd say a round groove produces less crack propagation points in the piston, and this would mate up best with a wire type snap ring. 2. Wire rings are likely cheaper to manufacture, and easier to accurately replicate as they would be cut from a continuous length of wire stock. 3. Given #2 above, I'm guessing the metallic structure of a wire clip is better than a snap ring, and more likely to remain consistent at higher temps. 4. The 'fit' of a wire ring, to my eye, just seems better. Less prone to movement and therefore wear and ultimately failure. 5. The groove for a snap ring must be wide enough to accept the ring, but no wider. This makes the groove dimension critical. A groove for a wire ring has much more tolerance as the wider the groove, the deeper the ring will sit, but still exhibit no side movement (within reason of course). So this makes the groove dimension less critical, and you can also source wire rings with much wider size tolerances (relatively speaking). It's a cost Vs risk exercise. 6. The snap ring has sharp edges and so much more likely to crack, and it will also 'cut' into the soft metal of the piston. 7. Wire rings are usually much shinier than snap rings, which has to be better....... ;-) 8. You've mentioned the mass of the ring, and while it's miniscule under normal gravity, it does have an effect at the top and bottom of the stroke at 6500rpm (one of you engineers should be able to estimate the numbers). 9. I've tried to make it to ten, but struggling to get to nine. So lastly, I've been imagining Biba swearing soon after a wayward wire ring, no doubt the last one in his shop, has exited a piston at high velocity, opened up a cut in an eyebrow, and disappeared somewhere across the floor and possibly down a drain. From that I reckon the real reason Alfa uses wire rings is purely for the comical relief of the Italian design and assembly engineers ;-) Cheers and Beers Biba! Beatle Alfaless in Oz ---Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Tuesday, 6 March 2012 7:30 AM To: AD Subject: [alfa] Snap rings vs retaining rings for pistons? This place has been pretty quiet of late so I'm bringing up an old thread...are the PIA wire snap rings being used simply to save a penny or so, or are they supposed to be superior to the considerably more civilized retaining rings? I'm tired of chasing that/those snap ring/rings that got away and am about to order a bunch of 20 and 22 mm snap rings - unless there is a really good reason that they're no longer being used on pistons/wrist pins. Yes, more brittle and probably weigh a few hundredths of a gram more...but otherwise? By-the-by, I'm Not asking as to how to install snap rings the correct way. Biba Irwindale, CA USA -- to be removed from alfa, see http://www.digest.net/bin/digest-subs.cgi or email "unsubscribe alfa" to [email protected] -- to be removed from alfa, see http://www.digest.net/bin/digest-subs.cgi or email "unsubscribe alfa" to [email protected]
