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Moly VS Cast Piston Rings

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Moly VS Cast Piston Rings

Postby chevelle » Wed Jan 12, 2011 10:41 am

I know most later engines run the Moly rings correct?
So they must be better correct?
This will be for a aluminum top ended cyls and heads ,Ironhead sportster, kind of a contradiction isnt it?
Its a 3 ring setup.
Should the Moly handle the heat better?
Whats benefits and downfalls?
Is it only the top ring that is different?
Is the base material different or is a moly just plasma sprayed onto the cast rings?
I am looking at Hastings 2M7003
This is what description reads:
"All ring faces are pre-lapped for easier seating and, in the moly sets, the compression ring features a moly insert on its barrel face design"
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Re: Moly VS Cast Piston Rings

Postby jeff swisher » Wed Jan 12, 2011 12:22 pm

The factory engines i have took apart all have molly face on the top rings..and i deal mostly in v8 stuff..even my 1969 had molly top...Yes ductile iron with a molly coating on the face....used to be anything coarser than a 425grit finish would chip the molly ...do not know if that still holds true .. i have my blocks finished at 625 grit..If the cylinder has a good surface like no major scratches and no ridge,,and a nice golden finish from seasoning with oil for 100,000 miles ..i will not touch the cylinder ,,i just install the molly rings and run it.
I have never had a lick of trouble from any of them..

When i used cast only rings i would notice i was down on power after 18,000 miles or so ..then i would cast ring it again ..
15-18,000 miles down again....after that i went molly and never had the problem
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Re: Moly VS Cast Piston Rings

Postby gem » Wed Jan 12, 2011 4:47 pm

The moly increases the rings scuff resistance temperature in addition to frictional enhancements.

There are moly faced rings and moly filled rings. Most rebuilder ring sets are moly faced. The racing (more expensive) rings are moly filled. The base materials are also different between rebuilder and performance ring offerings with the latter usually starting as a ductile iron and the former just iron.

Maybe Barry R can chime in with some of his knowledge.
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Re: Moly VS Cast Piston Rings

Postby rfoll » Thu Jan 13, 2011 12:07 pm

The two cast irons surfaces are attracted to each other resulting in material loss. It was explained to me that one of the benifits was the dissimilar metals. Add the lubricating properties of the moly and both ring and bore wear are greatly reduced. I pulled down a 100,000 mile 350 with moly rings recently and the end gap was .016".
So much to do, so little time...
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Re: Moly VS Cast Piston Rings

Postby DrillDawg » Thu Jan 13, 2011 12:22 pm

In the old days cast rings are used on used/worn/tapered bores due to the fact they would seal very well on rough honed bores. Someone could cut the ridge out of the top of the bore and then ball hone the bore, install cast rings and have a cheap rebuilt engine. The rings will wear faster. Bike racers (4 stroke flat trackers) who pulled their top ends often used them because they would seal up and were cheap. Moly rings need at least a 400 grit or better hone job or moly may scuff off the ring face. If your bores are not in the best of shape and are ball honed do yourself a favor and run cast rings.

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Re: Moly VS Cast Piston Rings

Postby chevelle » Thu Jan 13, 2011 9:58 pm

The rings I have in the engine now are cast.
I have run them 1 season,about 5000 miles.
I was in a hurry last year so I installed cast rings I found locally.
I removed the heads and the walls look fine, they seem to look dark, but no scuffing or scores.
This is a long stroke Sportster 4.8125", so Im not sure if I should bother pulling the cylinders and replacing w/ Moly rings now that I have the time to do it?
I would ball hone them if I installed new rings.
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Re: Moly VS Cast Piston Rings

Postby DrillDawg » Fri Jan 14, 2011 12:13 pm

If you ball hone the bore use cast rings. Most ball hones are 220 grit, way to rough for moly rings. The other option would be have them honed, not to much, by a shop with the correct equipment and hones, then use moly rings.


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Re: Moly VS Cast Piston Rings

Postby jeff swisher » Fri Jan 14, 2011 12:24 pm

I had a 77 delta 88 pace car and i rebuilt a 350 olds and put in it as the 403 block was junk..
My first rebuild with cast rings...well 18000 miles i was down on power,,replaced rings with another set of cast.after a hone
same thing happened...then i just left the cylinders alone (as they were already polished and had a nice golden color from inpregnated oil) and i installed a set of molly rings,,,again i did not touch the cylinder with any hone...i had no trouble for over 100,000 miles and that includes when i sold it to my sister in law...and then she sold it to a buddy after 2 years and he drove it another 2 and then he disappeared......

I wish i could find another 2 door 1977 delta88 royal
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Re: Moly VS Cast Piston Rings

Postby stock z/28 » Fri Jan 14, 2011 6:12 pm

In almost any thing I build that has fresh or "good" cyl wall I use a moly ring.

A lot of the later OE rings that I have seen are not only a Moly ring (top) but appear to be ductile as well, esecially the the very thin top rings.

In any performance application, even moderate street, I refuse to use any rings that are not a ductile or better top ring. Its just not worth the risk, atleast to me.
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Re: Moly VS Cast Piston Rings

Postby Barry_R » Fri Jan 14, 2011 6:42 pm

Moly is better. The only downside is the initial cost. Only the top ring is different in those low cost sets. Race rings sets - and virtually every OE ring in the past ten years will be moly on a steel or ductile iron top ring mateial. Plain cast rings with no moly have not been used in quality OE passenger car applications for decades...not sure about Yugos...
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Re: Moly VS Cast Piston Rings

Postby stock z/28 » Fri Jan 14, 2011 6:56 pm

Barry_R wrote:Moly is better. The only downside is the initial cost. Only the top ring is different in those low cost sets. Race rings sets - and virtually every OE ring in the past ten years will be moly on a steel or ductile iron top ring mateial. Plain cast rings with no moly have not been used in quality OE passenger car applications for decades...not sure about Yugos...



Im pretty sure the performance model of the Yugo (y-06 option) was fairly exotic and would have used state of the art materials and machining.
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Re: Moly VS Cast Piston Rings

Postby ProPower engines » Fri Jan 14, 2011 10:48 pm

Not sure what the grit is but Speed pro and others say to use a JHU820 sunnen stone to correctly finish the bores for their ductile iron moly race stuff. I use the same for any moly ring regardless of application. Never ever had ring seal issues doing it this way. Hastings rings same thing. for an air cooled engine I would only use a ductile iron top ring and depending on oil control needs that would dictate what the second and bottom ring is to be. Saving a few bucks here does no seem to be a wise idea when longevity is wanted. Japanese engines have been doing this for forever in almost all applications in air and liquid cooled engines. The engines are worn out by the time the rings give up their sealing ability's.Also having a stress plate on those cylinders makes them seal/last waaaaaay longer as well
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Re: Moly VS Cast Piston Rings

Postby pdq67 » Sun Jan 16, 2011 8:19 pm

Fact, "perfectly round, top to bottom cylinders, rough-ball-honed", and Chrome rings will do the best once they are broken in right!

But who has perfectly round, top to bottom cylinders??

Therefore install moly then hone right and go.

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