One ring vs Two rings on 2 stroke chainsaws

[Tzed250]

Bike engines can get away with one ring easily...the liquid cooling pulls the heat away from the cylinder wall much more efficiently than airflow. Look at the radiator area on an 85cc MXer vs. the cooling fin area on a 385XP. This is the limiting factor on power production. The last of the air-cooled 125 MX bikes (1979-1980) had to have rings put in them every moto (25-40 min.). The piston crown must be kept cool, and the path for this is through the rings.

 

[TRI955]

Great post and info. Thanks.

+1 Have some rep Team Fast!!

 

[Jacob J.]

+1 Have some rep Team Fast!!

+1, it was the best post of this thread.

 

[Crofter]

A longer rod for the same stroke would keep the piston at the top and the bottom of the cylinder for a longer period of time than a shorter rod. I can see the advantage (power not durability) to this for the top of the stroke, but I'm curious...would longer dwell time at the bottom of the stroke have a similar effect to opening up the ports by allowing a longer time for flow to occur?

At the same rpm not really longer duration but it does affect the relative amount of time the port is at full opening which straddles BDC The most critical time periods for a two stroke is time area near opening and closing which is nearer the half way between T and BDC. On a poppet valve four stroke the delay of getting the valves to an effective opening is the same critical factor but occurs near top and bottom of the stroke so the affect of variation in piston speeds because of rod length ratio will be different 2 or 4 stroke. I dont think the shorter rod ratio would have as much effect on a two stroke as far as gas flow is concerned.

Maximum effective rod angle occurs higher on a short rod and probably favours high rpm performance on a two stroke where power stroke is shortened compared to a four stroke. Lots of little things figure into a design and then we go and modify it for a totally different set of operating conditions. Fun!

 

[HolmenTree]

Hi,

I've followed this thread with some interest. I have a background racing karts, specifically 100cc air cooled sprint karts, and roadracing 125cc shifter karts and most recently 250cc Superkarts.

With the 125cc shifter karts, we are using MX derived engines, typically CR-125 and YZ-125 liquid cooled two strokes. The engines are square....54mm bore x 54mm stroke and use nikasil plated cylinders. The hot 125cc setup was a Honda RS-125 cast racing piston using a single Keystone style ring. This is a GP motorcycle racing piston. We spent many hours on the dyno working on the engines, optimizing them for pipes, digital programmable ignitions (with multiple advance curves) and of course the intakes....all used reed valves.
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Good post Erik,
I have been out of the timbersport racing scene for some time now. I built my 1982 Yamaha YZ 125 J liquid cooled hotsaw almost 26 years ago. It too is a 54x54 but it has a cast iron cylinder sleeve liner and 1 ring. When did they switch to nikasil and for what reasons?
Willard.

 

[Team FAST]

Were you running plain half keystones or dykes L

A lot of the 100cc reed and piston port kart engines (IAME, Parilla, TKM, Dap, etc.) use a dykes L style ring.

The RS-125 Grand Prix piston uses a keystone ring. Note that this is an HRC part and it works in both Honda and Yamaha engines. Do Stihl pistons work in Husqvarna engines?

-erik

 

[Team FAST]

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Good post Erik,
I have been out of the timbersport racing scene for some time now. I built my 1982 Yamaha YZ 125 J liquid cooled hotsaw almost 26 years ago. It too is a 54x54 but it has a cast iron cylinder sleeve liner and 1 ring. When did they switch to nikasil and for what reasons?
Willard.

They switched sometime in the early 90's Probably about the time the engines went to water cooling. Obvious reason was for durability and ease of maintenance when doing a top-end refesh. Nikasil plating ensures low wear and reduces friction. However, a properly honed iron bore with the right rings will develop more compression. The nice thing about nikasil is when you stick (seize) a piston, MOST of the time you can acid etch the residue off and save the cylinder.

-erik

 

[HolmenTree]

They switched sometime in the early 90's Probably about the time the engines went to water cooling. Obvious reason was for durability and ease of maintenance when doing a top-end refesh. Nikasil plating ensures low wear and reduces friction. However, a properly honed iron bore with the right rings will develop more compression. The nice thing about nikasil is when you stick (seize) a piston, MOST of the time you can acid etch the residue off and save the cylinder.

-erik

Thanks for the reply. The YZ 125 came out with liquid cooling in 1981, mines a 1982. I like the iron liner for the purpose of reboring, haven't read the service manual for years but I believe the reboring was mainly done to straighten out any bore out of rounding. I got a spare 1 oversize cylinder and the one presently on the saw is new factory standard size with about 5 hrs max on it. I was surprised they did away with the iron liner.

 

[Team FAST]

Thanks for the reply. The YZ 125 came out with liquid cooling in 1981, mines a 1982. I like the iron liner for the purpose of reboring, haven't read the service manual for years but I believe the reboring was mainly done to straighten out any bore out of rounding. I got a spare 1 oversize cylinder and the one presently on the saw is new factory standard size with about 5 hrs max on it. I was surprised they did away with the iron liner.

I started my development work on 1999 vintage YZ-125's so I should have qualified my answer to you as a guess on my part

Are most chainsaw cylinders nikasil plated? Or only the higher qualitity saws?

 

[scotclayshooter]

I started my development work on 1999 vintage YZ-125's so I should have qualified my answer to you as a guess on my part

Are most chainsaw cylinders nikasil plated? Or only the higher qualitity saws?

I think most if not all modern Stihls have Chrome platings.

 

[HolmenTree]

I started my development work on 1999 vintage YZ-125's so I should have qualified my answer to you as a guess on my part

Are most chainsaw cylinders nikasil plated? Or only the higher qualitity saws?

Chrome plating has pretty well gone the wayside except in some of the smaller consumer saws. Mahle [brand] cylinders on the pro Huskys, Stihls, Jonsereds for example are nikasil. But someone correct me if I missed something.

 

[Lakeside53]

Scot's correct. Most of Stihl offerings for many years have been chrome, even those with Marle cylinders (the company that invented Nikersil).

Not all "chome" is equal... it's a bit like saying all steel is the same...

 

[SWE#Kipp]

I think and someone will have to verify that Husky also make 1 ring conversions. My 372xp is 1 ring. It was rebuilt at a Husky dealer this way.

Did not read the whole thread but most of the Euro 372's are one ring piston, one reason for two rings is that it's no as sensitive for low fuel/oil quality and gets longer life but a little less power.
up's and downs with both types

 

[edisto]

Probably not a big consideration for saws, but 2 rings would allow thinner rings for the same amount of heat transfer, and thinner rings means less chance of flutter without having to machine for Dykes rings. Is that right??

 

[stihl only]

I didn't really read the thread, but my wife got one ring. If I ever get rid of her, no woman is ever getting a ring from me, so I guess one ring is better?

 

[2000ssm6]

I didn't really read the thread, but my wife got one ring. If I ever get rid of her, no woman is ever getting a ring from me, so I guess one ring is better?

, sounds like you should have left that one ring in the saw.

 

[Jacob J.]

Do Stihl pistons work in Husqvarna engines?

-erik

I've put a few Husky pistons in Stihl saws...a few Solo pistons in Husky saws...a few Dolmar pistons in Stihl saws...a few Stihl pistons in everything else.

 

[Lakeside53]

I've put a few Husky pistons in Stihl saws...a few Solo pistons in Husky saws...a few Dolmar pistons in Stihl saws...a few Stihl pistons in everything else.

Hmmm.. resulting in transgender mutts...:greenchainsaw:

 

[Jacob J.]

Hmmm.. resulting in transgender mutts...:greenchainsaw:

I plead the fifth your honor....

 

[Lakeside53]

Which reminds me of a joke.. Frenchman in court answering a charge of necrophilia... "but your Honor, I didn't know she was dead; I though she was English"...:greenchainsaw:.


I have no idea why that reminded me of this...