Cylinder chrome plating loss

[hotshot]

What's usually the root cause for "peeling" or the loss of chrome plating in a 2 stroke cylinder?

Heat
I've seen it mostly occur around the exhaust ports, so I assume that combustion heat could be a factor,
the plating was originally incorrectly applied, or maybe the cylinder bore was simply contaminated
during the original plating process?

Chemical
Have also seen mud wasps nests built inside the exhaust ports, then discover that the plating was
damaged after the nest was washed out. So acid or an alkali was probably present in the
little bastards spit.

Mechanical
If a wanna be mechanic uses a wrench or screwdriver for a piston stop in the exhaust port,
it causes a "bump" or dent that can be ironed back out. The chrome is always lost on the surface
of the dent, but it never seems to spread or peel past this spot. I've run them with a small
patch gone at the exhaust port, without a problem.

Do any chrome or nikasil plating experts occasionally cruise the AS site?

 

[brokenbudget]

anybody using a screwdriver through the exhaust as a piston stop is an idiot.

 

[excess650]

anybody using a screwdriver through the exhaust as a piston stop is an idiot.

....mebbe worded a bit strongly, but essentially correct...

 

[Roanoker494]

Could be caused by improper preparation of the cylinder before plating. Corrosion could also cause the lining to loosen and the rings do the rest.

 

[Zippy]

Piston the wrong way round

I had a second hand saw that had wear near the top of the cylinder on one side.

The piston was in the wrong way round so it was probably slapping the cylinder every stroke due to the off-set being the wrong way round.

 

[pioneerguy600]

I had a second hand saw that had wear near the top of the cylinder on one side.

The piston was in the wrong way round so it was probably slapping the cylinder every stroke due to the off-set being the wrong way round.

?

 

[WSJchester]

So acid or an alkali was probably present in the little bastards spit.

Not that it makes much difference if it causes chrome to peel, but it's probably slightly alkaline.

 

[IEL]

?

Im with you there... Not sure how thats possible?!..

 

[hotshot]

Thanks

anybody using a screwdriver through the exhaust as a piston stop is an idiot.

Thanks for the replies, I've got a few older saws that the chromed cylinders are non-obtainium & trying to maximize their lives.

Between the different types of pre-cleanings & acid etchings, then platings (i.e. Hexavalent, Trivalent, Nikasil, and Alusil) there were a lot of changes in coatings, and each one has it's own merits. Interesting to note that the Nikasil usage was stopped by BMW for cylinder plating, because the high sulfur content in some of the fuels corrodes it. Mahle was the first to use it.

Nikasil - Wikipedia, the free encyclopedia

On the previous Zippy post with the "piston in the wrong way around", I'll bet the piston had a lot of skirt wear & allowed the piston to rock too much at TDC, causing some wear at the top. If it's in the "wrong way around", the ring ends will usually catch in the exhaust port and quickly cause some piston & cylinder carnage like this http://www.arboristsite.com/chainsaw/146514.htm

 

[Zippy]

Piston slap

The conrod is connected off-centre to the piston to get it more parallel to the up/down motion of the piston during the power stroke. This reduces the tendency for the piston to "tip over" in the bore.

The piston in this saw was back to front (ie arrow pointing away from the exhaust port) so during the power stroke the conrod was at a steep angle to the direction of motion, and it was increasing the tendency to tip in the bore.

The top exhaust side of the bore was worn - I didn't test, but I think it went through the plating.

Well that's what I thought - tell me if I've got it wrong.

Zippy

 

[brokenbudget]

the arrow is there to show you the orientation for the rings. front to back, pistons are the same.

 

[Zippy]

&^%$$£&()&!

I agree the arrow is to make sure you don't have the open gap of the rings fouling a port.

I also believe some pistons have the conrod slightly off-set as I described (maybe automotive?) BUT....

I just rooted around my old bits bin and found the old piston and measured it .... and no offset!

I'd have put money on it - must be getting senile.

I've just done a quick google for piston slap conrod and this came up ..... http://www.motorcycleproject.com/motorcycle/text/cows-offset.html

 

[mopar3]

Has anyone sent a saw cylinder out and had it replated? It is commonly done for mx bike 2 and 4 stroke cylinders. They can strip, weld and replate. Evan badly worn and damaged cylinders can be repaired better than new.

 

[pioneerguy600]

There is only a few shops that can do a blind cylinder replate and hone out. A very few chainsaw cylinders are worthwhile getting this done.

 

[hotshot]

Offset

I've just done a quick google for piston slap conrod and this came up ..... Piston Pin Offset

Good article on the pin offset force theory, but I've only worked on two saw series that had removable rods,
the old Sears 3.7 Roper & the P361/306/245 Poulans. But they're not offset at all, and can run either
way on the crank journal, as long as you get the big end cap re-aligned per the factory stamping marks.

Modified Mark may chime in on other older Poulan rods though...

 

[DUGs-sawshop]

I send snowmobile cylinders into US CHROME co. and have good luck. Wonder if they would do a chainsaw cylinder.

 

[scallywag]

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