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Old 06-18-2010, 07:48 PM
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Piston coatings for better fit?

I have my engine being put together and have an issue. I have new srp pistons w/molly rings but a finished hone for cast rings. I bought a new block with a finished 4.00" bore. The builder said that he will need to get the bores touch honed in order to get a good ring seat. He said this worried him about the piston to wall clearance and that I should have the pistons sent out to be coated. He had a buddy who does this. Teflon. Said, they could build them up up to as much as .005 dia. I, not knowing much about the coatings, said sounds like a good way to solve this issue. Pistons are out getting done as I type this. Today while I was at the builders things started sounding funny. He was saying things like the coating wears off and things like using oil. Now I'm not feeling to good about what seemed to be an easy fix.

What should I do with this issue? I'm not coming down on the builder for this because I think at the time he really thought it to be the right fix. Today I heard doubt. What say you?

Thanks

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Old 06-18-2010, 07:57 PM
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Most of the skirt coatings I have had done are only about .0015. The wear factor only comes into play on a high use engine or an engine assembled in a sand pile.

The change from cast ring hone and moly hone should not be much more than +.001 at most.
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Old 06-18-2010, 08:12 PM
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Thanks,

I shouldn't have any worries then about piston to wall.

I think that I'm worrying way to much about this build.
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Old 06-19-2010, 12:08 AM
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I have been told that moly rings take a little longer to seat properly. If this is true or not, I don't know. If it were me I would drive it normal for a good period of time before stomping on it.

Any one else have an opinion on this? I'd like to know.
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Old 06-19-2010, 09:58 AM
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Moly inlay rings break in quickly. The moly is in a narrow channel cut into the face of a cast iron type ring. Plasma moly will also break in fairly quickly. Moly type rings generally require a less aggressive surface.

Cast iron rings are between moly and chrome, leaning more towards moly, afa how long to break in and finish required.

Chrome rings are much harder and take longer w/a more aggressive finish to seat.
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Old 06-19-2010, 10:05 AM
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Quote:
Originally Posted by Backwoods
I have my engine being put together and have an issue. I have new srp pistons w/molly rings but a finished hone for cast rings. I bought a new block with a finished 4.00" bore. The builder said that he will need to get the bores touch honed in order to get a good ring seat. He said this worried him about the piston to wall clearance and that I should have the pistons sent out to be coated. He had a buddy who does this. Teflon. Said, they could build them up up to as much as .005 dia. I, not knowing much about the coatings, said sounds like a good way to solve this issue. Pistons are out getting done as I type this. Today while I was at the builders things started sounding funny. He was saying things like the coating wears off and things like using oil. Now I'm not feeling to good about what seemed to be an easy fix.

What should I do with this issue? I'm not coming down on the builder for this because I think at the time he really thought it to be the right fix. Today I heard doubt. What say you?

Thanks
Teflon has a poor resistance to friction. On a piston skirt, teflon will not last as long as it takes to type this sentence. To "build up" an undersized piston w/teflon is an exercise in futility. IMO.

I've seen pistons w/coatings that were said to aid in preventing scuffing at start-up/initial break-in. I've never used them, so have no empirical info.

But teflon will not hold up to direct contact w/a cylinder w/o being damaged or worn away. It has no dimensional stability.
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Old 06-19-2010, 10:11 AM
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I must have been thinking about chrome moly. Maybe that was he was talking about.
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Old 06-19-2010, 01:31 PM
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My Speed Pro pistons

My Speed Pro forged pistons came with a molybdenum low friction skirt coating to help prevent skuffing due the faster expansion rate of forged when used with tight bore clearances. It is cheap insurance against piston/bore scuffing on forged pistons with tight bore clearances that must be used in street driven engines. The tight bore clearances was mandated by the EPA in 1970 - Phase two of the Clear Air Act. The same act required that the auto manufacturers delete the oil spit-hole in the rod cap parting line by 1973. The oil pit-hole that lubes the cam lobes also increases pollution by tossing excessive oil on the clyinder walls. The hypereutectic pistons were one way to solve the tight piston to skirt clearnce issue and reduced cylinder wall oiling. However, they are cast pistons and as everyone knows, hypereutectic pistons are merely high silicon cast aluminum pistons and are still not as strong as forged pistons. The hard-core hotrodders are reluctant to use cast pistons of any sort. A forged piston can have the top blown out, yet will hold the pin and rod in place. A hypereutectic piston will disintergrate in a hundred peices and the loose rod will take out the rest of the engine.

Ceramic thermal barrier coated pistons are sometimes used to correct bore clearances. I never have been confronted with that situation.
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