non genuine piston and pot - any advice?

[agg221]

OEM 46mm cylinder will produce more power than virtually all aftermarket options, regardless of 46mm or 47mm.

 

5-10 years ago I would have agreed with you entirely, however today I'm not so sure.

 

If you look at the factors involved, it's a combination of quality control on the alloy (affects thermal conductivity and strength), coating (affects thermal conductivity and wear) and casting tolerance (particularly affects cooling as the fins are most affected) and machining tolerances (affects compression and, to a minor extent, compressed volume).

 

There are many threads on here bemoaning the decline in quality of saws from the big name manufacturers, and I think it's reasonable to assume this arises from a cost driver, which will be passed back to their Tier 1 suppliers, possibly at the price of cutting corners, which could apply to each of the above.

 

Meanwhile, 5-10yrs ago I think the majority of chinese copies would probably have been dodgy to say the least, both in quality of metal and in precision of finishing. Chinese manufacturing has now come on enormously in all respects - a lot of medical devices are now manufactured there, as are an increasing number of aerospace components and, as an aside, so are Omega watches.

 

As such, I would no longer be certain which represented the higher quality product.

 

There's also the point that most people couldn't actually measure power (unless you're set up to do it, e.g. building race saws). For most users it would be masked by factors such as chain and bar condition anyway, along with accurate tach tuning for peak revs relative to altitude and air pressure etc. I suspect that within the tolerances involved here, it would be indistinguishable.

 

Sorry, derailed your thread a bit!

 

Alec

[Megatron]

I agree chicom/aftermarket pistons and cylinders are getting better... and price vs quality aftermarket would likely come out on top.

 

As such, I would no longer be certain which represented the higher quality product.

 

First one thing to point out (i'm guilty of this too), when the word aftermarket is used in the same sentence as saws people assume lower quality. This is often not true for example much better bearings can be had than OEM.

Also the quality of the piston and cylinder of OEM is a lot dependent on what manufacturer the work is outsourced to, if indeed it is. For example Stihl "in house" made cylinders appear to be far inferior to Mahle and Kolbenshmidt (KS) but this may change in time.

 

One of the biggest problems with chicom pistons and cylinders is consistency. Some of the things I have seen/read about (last couple years);

-significant casting flaws to intake/exhaust/transfers/combustion chamber (which would affect power)

-"finish" in general

-pistons with too deep grooves for wrist pin clips, leading to the ears getting busted off (quite common)

-relatively huge variations in squish

-cylinders with a band of coating missing in the squish band, rings don't usually cross it in a stock saw but won't allow pop up crown on piston.

-inconsistent bevelling to ports, causing unnecessary wear to rings and in worse case allowing them to catch...

 

Now on to power;

 

-the casting flaws can often be rectified with a carbide burr but if left will be detrimental to power.

-chicom/aftermarket pistons tend to be heavier than OEM, I can only put this down to inferior alloys being used thus requiring more material in high stress areas.

-I have never seen a wrist pin on an aftermarket piston with a teapered wrist pin like some OEM do, again unnecessary weight. Weight will affect accelaration and likely peak RPM.

-durations, if you can "degree" an aftermarket p+c some are way off OEM. However, some are in fact more aggressively ported and would in theory produce more power but wouldn't meet emissions standards.

-Squish as already mentioned

-Closed vs open port cylinders

 

It is also worth noting I doubt many aftermarket cylinders do in fact have "Nikasil" as it is trademarked, as I suspect "Alusil" is. Some may have similar to these but I have heard of some using chrome instead, the thickness of all of which can vary.

 

My preference;

 

Mahle-->Kolbenshmidt-->Gilardoni-->Tecomec-->Stihl-->Meteor-->Episan-->Golf-->Unnamed

The order of the top three changing depending on the date/model and some not being applicable to all Stihl/Husky saws.

 

Just some food for thought.

Edited June 22, 2011 by Megatron

[agg221]

Thanks Megatron, very interesting comment.

 

Alec

[Chris Sheppard]

Got hold of some hydrochloric acid today and have removed what I think is all the ali from the inside of the barrel now, but I think it's goosed.

 

Tried takign a few photos but they won't come out very clear at all.

 

Kept going with the acid until it stopped reacting and washed it all off and it looks pretty bad still, just not ali coloured now.

 

It's hard to see/feel for definite whether the rough area is on top of the plating or through it.

 

Whats the worst that can happen if I were to leave it with acid on it over night?

[Megatron]

It's hard to see/feel for definite whether the rough area is on top of the plating or through it.

 

Whats the worst that can happen if I were to leave it with acid on it over night?

 

Pass your finger nail around the inside of the jug and see if you fell any resistance or your finger nail catches.

Never a good idea to leave acid in a cylinder unattended. The acid can get into the ports and that ain't good! Also sometimes the cylinder will be pitted as a result of the seizure (coataing missing in small places) and depending where this occurs the cylinder may still be useable. However, if you leave acid in it for longer than necessary it will get into the "pit" and eat away the Al alloy and undermine the plating around the "pit". Most likely again making the cylinder junk.

[agg221]

It may be of interest that I'm working on the issue of recovering genuine pots. I work in the right field to investigate the feasibility of this - we should be setting up some trial kit in a few weeks and if it works I'll report back - may be worth hanging on to any really expensive pots for a bit.

 

One thing that would help if anyone knows - a composition for Nikasil. To start with I'll use nickel, which will be chemically compatible and fill pits and scores if the process is going to work at all, but although it will create a seal it won't be as wear resistant, so I'd like to get closer if I can to sort out matched properties.

 

Alec

[spudulike]

Got hold of some hydrochloric acid today and have removed what I think is all the ali from the inside of the barrel now, but I think it's goosed.

 

Tried takign a few photos but they won't come out very clear at all.

 

Kept going with the acid until it stopped reacting and washed it all off and it looks pretty bad still, just not ali coloured now.

 

It's hard to see/feel for definite whether the rough area is on top of the plating or through it.

 

Whats the worst that can happen if I were to leave it with acid on it over night?

 

You usually need to rub the reclaimed area with 400 grit paper - this should clean it up - if there are vertical scores above the exhaust port then it is probably shot, other than that, it is probably a runner unless small areas of coating are missing!

[Chris Sheppard]

cheers guys, been a bit slack with this one and haven't had chance to have another look since the first go at it - hopefully get another go at it in the next few days

[Chris Sheppard]

Had another go at it last night and used the wet and dry too - can feel the scoring above the exhaust port and also below it, feels quite bad, and with a complete piston and cylinder kit coming in at less than £70 it's still going to be a cheap saw.

 

Just ordered a piston and pot kit from ARB Ireland, they've confirmed it's closed port and that I can just shove a normal Decomp valve in too. it's 1mm bigger bore but other than setting the carb up, is a straight swap and apparently will give a bit more poke (but that's not important)

 

Cheers for your help so far guys - hopefully it'll all be sorted for next week.

[spudulike]

Probably a good choice, Arb Ireland have real fast delivery and are one of the few sellers that do the closed port 346 cylinder which is probably the one you have got.

 

Hope it all goes well - take care the circlips are in place properly and the ring locators are nearest the inlet port - arrow points toward the exhaust port!