Originally Posted by
Trident
And I've been resisting the urge to start a urinating contest. As for a gap, you won't see one... but you will create areas of differing surface pressure on the bore.
See below:
Ring Gaps: Mercury 44, 40 and 30 cu. in. fours.
I absolutely do not agree with oversize rings with the gap filed to fit… Yes, I know that was Harry’s mantra but I'll say it again, I absolutely do not agree with oversize rings with the gap filed to fit.
With oversized rings, I feel that in an effort to solve one problem, you create another…
First off, rule of thumb is gap should be 2 ½ times bore dimension, in thousandths. Then, remember our Mercury pistons with their heavy section deflectors hold a lot of heat, so these pistons may need more ring gap clearance than other designs... I think .004” that some recommend is too tight under heat… I’d love .008”. No less than .007”
And an oversize ring squooshed (technical term) into a tighter bore distorts, putting three pressure points on the distorted ring and creates two areas of lesser ring tension pressure on the bore. You are smart enough to figure out where those light and tight spots are, I’m betting. Any reason you’d prefer less sealing at the port areas?
Mercury stock rings are approx. .012” gap. That’s looser than I’d like, so I pick the tightest for top rings. And for the ‘leakdown’, I don’t worry about it, as I get 140 # at roping speed which is what, 3 rpm?
What’s the leakdown at 7200 rpm?
Lets do a thought experiment here…
That too big ring gap sticks out what, say .007” piston clearance (cold), so say 3 ½ thou on a side, so the 'extra' gap is 3 ½ X 4 thou (.012” - .008” that I’d like). That’s a tiny, tiny gap. At 7200 rpm, we fool ourselves using RPM, should be RPS to be more relevant, that’s 120 times UP AND DOWN per second, and only roughly half the travel up and down is under compression or combustion pressure, when ports are closed. And that tiny, tiny leak has a second and third line of defense, the #2 ring and #3 ring… How much leakdown can that be? A gnat fart?
And that gap is smaller yet under heat, running, and with lots of oil (I run 8 : 1, BTW)
I’d rather have a round perfect ring fit to the bore, than .004” less gap with ring distortion. And, with possible ring gap binding from being too tight... I do see a lot of guys running really tight ring gaps trashing pistons regularly...
Never had that in my motors.
All my UIM record motors and National Championship motors ran 3 Mercury OEM correct bore size rings. The one thing I did, early on, was bevel the bottom only of the bottom ring(s). Then I quit doing that. Now I select the tightest rings for the top and install all 12 right out of the box. On 44’s with 2 Keystone rings, I take ‘em right out of the box and make sure to put them right side up.
Hope this helps.
Jerry
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