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Are scored pistons useable?

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Old 10-28-2012, 10:51 AM
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Scota4570
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Default Are scored pistons useable?

I have been purging old junk. I have a couple of chainsaw engines that have scored pistons. The jugs had aluminum stuck over the nicasil that cleaned up like new. The rings are good enough. The pistons have moderate scoring on the aluminum. The intake side of the piston is perfect. One is a 77cc, it intrigues me. Will the scoring on the exhaust side mess up crankcase pumping enough to matter? The compression seems pretty good on the upstroke. With new rings I bet compression would be as original. Before I spend a bunch of time making parts I want to be reasonably certain I have a decent engine. Are these engines ruined?
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Old 10-28-2012, 11:51 AM
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Default RE: Are scored pistons useable?

Post a photo so we can see how bad the piston is scored. Thanks Capt,n
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Old 10-28-2012, 12:35 PM
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Default RE: Are scored pistons useable?

I've already filled off the high spots on a scored piston to smooth the surface affected. As long as the ring groove is not ruined or the upper and lower surface of the ring lands are not distorted, I've reused pistons by installing a new ring. You would be surprised to see a repaired piston working after reinstalling it. I have a few Homies that still run perfectly after a repair. You be the judge as to the extent of the damage and repairability.

John
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Old 10-28-2012, 01:21 PM
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Default RE: Are scored pistons useable?

See attached,

The rings, they are not chewed up. The silver glint is mostly reflection from the sun.

Rather than smoothing it up I would would speculate that leaving it alone would be better than reducing it's diameter.
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Old 10-28-2012, 02:02 PM
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Default RE: Are scored pistons useable?

A scored piston can be no problem at all. Just clean it up using a not too fine file, and clean up the ring groove edges using the rings as tool.
The problem lies with the cylinder bore. If there is any material bonding residue of piston and cylinder in the bore left, then you will be in deep trouble!!!
Carefully clean it out using any means available to you. Normal workshop practice is to use medium grit sandpaper and finger pressure (and lots of oil in the mix thereafter), though chemicals seem to work better.
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Old 10-28-2012, 04:49 PM
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Default RE: Are scored pistons useable?

The cyliner bore cleaned up perfect. The aluminum galling was on top of the liner. #320 paper was working but, I got an idea. I have hardened hook scrapers I made for barrel channels in gunstocks. The profiles are round. They are on a handle and cut on the pull. I touched one up with a hard Arkansas stone to make a keen polished edge. It scraped the aluminum galling off just like scraping teeth by a dental hygienist. I could feel the spots with galling and worked on those until it was smooth. The liner was harder than the steel in the cutter. A little polish with the #320 made it look as new.
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Old 10-28-2012, 05:04 PM
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Default RE: Are scored pistons useable?

Make sure the ring lands are OK and install just one (only one is needed)Frank Boman ring...bet it will run good. Capt,n
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Old 10-28-2012, 05:13 PM
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Default RE: Are scored pistons useable?

OK then. I'll get some new bearings and seals then go to work. The short block weighs only 3# 9 oz. It was a Stihl 046.
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Old 10-28-2012, 07:29 PM
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Default RE: Are scored pistons useable?

Muratic acid will dissolve aluminum from nikasil. Proper safety and a q-tip will do wonders. NOTE if the nikasil is damaged the acid will eat the cylinder and then your hosed.
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Old 10-29-2012, 06:51 PM
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Default RE: Are scored pistons useable?

Good tip on the acid. I should keep a little bottle of con. HCL around for such tasks. Strong base, Lye solution , will do the same. Unfortunately the meth heads have caused there to be restrictions on lye.
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Old 11-01-2012, 03:26 PM
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Default RE: Are scored pistons useable?

A new piston costs $29, done. I also got new rings, a wrist pin, wrist pin bearings, oil seals, and crank bearings. HL Supply, (HLSProParts.com) was very knowledgeable, fast to ship, and very cheap.
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Old 11-01-2012, 05:07 PM
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Default RE: Are scored pistons useable?


Quote:
ORIGINAL: pe reivers

A scored piston can be no problem at all. Just clean it up using a not too fine file, and clean up the ring groove edges using the rings as tool.
The problem lies with the cylinder bore. If there is any material bonding residue of piston and cylinder in the bore left, then you will be in deep trouble!!!
Carefully clean it out using any means available to you. Normal workshop practice is to use medium grit sandpaper and finger pressure (and lots of oil in the mix thereafter), though chemicals seem to work better.
Have you ever knurled a piston to reduce friction...it may or may not work???

https://www.google.com/search?q=knur...iw=835&bih=384
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Old 11-01-2012, 07:31 PM
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Default RE: Are scored pistons useable?

Interesting idea. My intuition would make me think that it would hurt. If you take a couple of glass microscope slides and place a drop of oil between the friction is very low since the pieces are not touching each other. If you turn one up on edge it would scrape along. Knurling it would create little points and that would insure metal to metal contact. I suppose you could put grooves in it like a cast bullet. The trouble I see is that the grooves would fill with oil and gunk and you would still have oil separating the two over the whole surface. Perhaps going to a less viscous oil would reduce the effort needed to move the piston up and down??
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Old 11-02-2012, 03:57 AM
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Default RE: Are scored pistons useable?

In fact, the chromed cylinders of the 1960ties were lightly knurled (pitted) before honing, chroming and honing again to obtain oil pockets. This was before the advent of NIKASIL and the modern flash chroming or vapor deposit techniques.
I would see no harm in doing the same to a piston surface, providing the main piston surface is smoothed and the knurl is not all that deep. Pistons need surprisingly little surface and lubrication to do their job well. It is very much more important to keep them cool, round and parallel with the cylinder bore once up to running temperature.
On our little racers we would check the surface forged pistons for tight spots before harm was done, and file them away. After some running, the pistons showed the file grooves, but had run in smoothly again. Eventually, the grooves filled up with carbon deposits and the piston surface was a very good match with the cylinder surface for best heat transfer. (we ran castor oil 1:15 to 1:20!)
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