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Old 07-03-2020, 11:16 PM   #1
modela4shane
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Default 255 merc excessive ring end-gap?

Has anyone ever encounterd a situation with too much piston ring end gap? Im in the process of re-ringing a 255 Merc that has a.030 overbore. The cylinder has been honed and now 3.217. I am attempting to use a NOS set of Sealed Power iron rings that are .030 over and i am getting .032 on the ring end gaps. My guestion is it OK to get a .040 or even .050 ring set and file fit the ring to the appropriate gaps. Any help or suggestions or thoughts would be appreciated. Thank You.
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Old 07-03-2020, 11:36 PM   #2
Aarongriffey
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Default Re: 255 merc excessive ring end-gap?

What you suggest is an approved method of obtain a better ring gap.
030” is almost ok so I would just get a +.040 set of rings.
If the gaps are still a little over the recommended space they will be just fine.
There are ad lot of engines out the running just fine with excessive ring gap.
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Old 07-04-2020, 06:54 AM   #3
cadillac512
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Default Re: 255 merc excessive ring end-gap?

A .040" set of rings is .010" larger in diameter than your .030" set. Multiply the .010" difference by Pi and you get approx .031" more circumference. Should be more than enough (with some filing) to solve your end gap problem.
However, 3.217 is correct for a .030" overbore on a stock 3.187 bore...so maybe it's actually worn a bit more than that or the rings are not what they should be?



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Old 07-04-2020, 09:04 AM   #4
Ol' Ron
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Default Re: 255 merc excessive ring end-gap?

Using the old formula of .004" per inch of cylinder bore, a 3.312 would have aprox .14". I've always used .015/.016 for the top ring and .016/.018 for the second. So.030 sounds quite large, to be considered OK. Unfortunately, I've never given the ring clearance much thought. I was always todl, to littel can damage the piston, so error on the big size. I've followed this rule for many,many engines. so far , so good.
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Old 07-04-2020, 11:09 AM   #5
rotorwrench
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Default Re: 255 merc excessive ring end-gap?

I've always gravitated towards Hasting rings myself but it's interesting that Sealed Power would have such gaps. They generally have pretty good quality stuff but times are a changing.
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Old 07-04-2020, 10:31 PM   #6
Fordestes
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Default Re: 255 merc excessive ring end-gap?

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Be sure to check piston to cylinder wall clearance to avoid piston slap if you are going back together with used pistons, In rare cases I have been known to knurl the pistons that are close but not quite in spec. rather than install pistons too loose .
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Old 07-06-2020, 02:16 AM   #7
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Default Re: 255 merc excessive ring end-gap?

The reason for end gap is expansion of the ring, per inch, per degree.

The top ring runs hotter and needs a little more gap than the second and oil ring.

I don't know the alloy of cast iron rings but, the expansion ratio of gray iron is, .0000058. The circumference of a 3.217 bore is, 10.1035 - .018 = 10.0885

10.0885 x .0000058 x 300 deg = .01755. Not quite touching but, if the ring gets to 350 deg (say a severely over heated engine), it will touch and need a .020+ gap or it will butt together and possibly break.

The 46/48 spec is .014 and I think .016 would be great in a street car that gets reved a lot, on the street, .018 for race and .020 would be fine but, .032 is to much.

The .040 rings should be fine after file fitting.
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Old 07-06-2020, 01:27 PM   #8
modela4shane
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Default Re: 255 merc excessive ring end-gap?

THANK YOU to all who replied. I just got my hands on a .040 set of Hasting rings and they fit tight to bores. I will now take the time and file fit the rings.
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Old 07-06-2020, 02:53 PM   #9
leon bee
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Default Re: 255 merc excessive ring end-gap?

Quote:
Originally Posted by frnkeore View Post
The reason for end gap is expansion of the ring, per inch, per degree.

The top ring runs hotter and needs a little more gap than the second and oil ring.

I don't know the alloy of cast iron rings but, the expansion ratio of gray iron is, .0000058. The circumference of a 3.217 bore is, 10.1035 - .018 = 10.0885

10.0885 x .0000058 x 300 deg = .01755. Not quite touching but, if the ring gets to 350 deg (say a severely over heated engine), it will touch and need a .020+ gap or it will butt together and possibly break.

The 46/48 spec is .014 and I think .016 would be great in a street car that gets reved a lot, on the street, .018 for race and .020 would be fine but, .032 is to much.

The .040 rings should be fine after file fitting.
Frank: Contemporary thought, (I read a lot) seems to suggest the second ring gap should be larger?
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Old 07-06-2020, 03:25 PM   #10
frnkeore
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Default Re: 255 merc excessive ring end-gap?

Yes, I have read that too and that is come about in the last few years. What I read wasn't fully explained. If you can cite, the exact explanation, I would like to read it.

It may be so but, the top ring does get hotter than the second ring so, I still do it this way. I used to make the second ring about .002 less than the top but, after reading that, I make them the same, just in case. In this case, if you have the factory spec of .014/.016, you'll be just fine.

I've bought a set of gapless, second rings, to try out. They have over lapping ends, that would seem to keep any gases from the top ring going past the second and plenty of room to expand.
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Old 07-06-2020, 04:21 PM   #11
bobH
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Default Re: 255 merc excessive ring end-gap?

We're screwing around with old Fords. Me thinks there is too much worrying going on. Are we assembling 'precision' engines, or just old Fords?
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