066 porting

[AdamL]

thinking about doing some internal work on my 066, since it's low on compression any way and will be open. how wide can I go on the intake and the exhaust what about squish and transfers... I've ported snowmobile engines in the past but am unsure what specs would perform nicely, I don't really want to just start grinding and trash my cylinder, can anybody share there specs? also if I make myslef a dual outlet exhaust cover what rpm would be a good starting point. Working at a Stihl dealership kinda leaves my hand tied to working on stock saws, they tend to frown on modifactions but this is for my personal saw. thanks

 

[Meadow Beaver]

First off you should replace the rings, maybe the piston if it has low compression. What are you looking to get out of the saw?

 

[AdamL]

I know I'll be replacing the piston and ringss.... Would like to buiuld it into a good stump saw that might be used for a mill in the future. i was debating the BB kit. I'm thinking about just porting the stock cylinder and then a muffler mod.

 

[Meadow Beaver]

Don't go with a bb kit, mill the OEM jug 20/20 (0.020" off the base and combustion chamber). Lighten the piston, widen the exhaust, intake and transefers. You don't have to the upper transfers. Also advancing the timing, but I'm sure you knew that. Good Luck, Mark

 

[nanuk]

huh?

Don't go with a bb kit, mill the OEM jug 20/20 (0.020" off the base and combustion chamber). Lighten the piston, widen the exhaust, intake and transefers. You don't have to the upper transfers. Also advancing the timing, but I'm sure you knew that. Good Luck, Mark

What do you mean by 20/20? I get the .020 off the base, but that WILL take .020 off the combustion chanber.

are you suggesting to take 20 thou off the base then adjust squish to .020?

I'm fairly new :newbie: here and the riddles and acronyms get confusing sometimes.

 

[Meadow Beaver]

20/20, 50/50 etc, you mill 0.020" off the base then mill 0.020" off the combustion chamber. The squish will (should) stay the same, but the smaller combustion chamber adds power and torque. Also when milling like this you will have to raise the ports as well.

 

[nanuk]

sorry!

Still not getting it. How do you mill OFF to reduce combustion chamber?
do you mean mill down the perimeter of the piston to create a domed piston?

thanks for the answers.

 

[Meadow Beaver]

Still not getting it. How do you mill OFF to reduce combustion chamber?
do you mean mill down the perimeter of the piston to create a domed piston?

thanks for the answers.

No, a the combustion chamber is milled down reducing its size, which packs the charge into a smaller combustion chamber, which gives the saw more compression. Compression is the key for a good torquey saw, lightening the piston also increases torque due to the lighter recipricating mass makes the saw work easier. But the whole mod job has to be done right to get the saw to run the way you want it.

 

[nanuk]

hmmm....

No, a the combustion chamber is milled down reducing its size, which packs the charge into a smaller combustion chamber, which gives the saw more compression. .

I get the smaller chamber thing, but how do you milll DOWN a chamber?

are you talking a two piece head?

otherwise it's like making a hole smaller by taking dirt out?

sorry if I sound dense, but I'm just having a mental block on what you said.:bang:

 

[timberwolf]

????

If you are going to use the saw for stumping and milling. then don't get worried about advancing timing and reducing the combustion chamber volume. You don't need to do these things and IMHO you won't want to if you want the saw to last under those conditions.

Put new parts into the saw to get it running like it should first, then...
Check your squish height if if its an older 066 then it is likely close to 0.040, pulling the base gasket will get you down close to 0.020 and give you all the compression you will want, if it is a newer 660 then squish is likely tight from factory.

If you have a tight squish already and want more compression then the easiest decent way is a slight pop up piston and milled base, best done on a metal lathe. You don't want to go over much over 170-175 psi on a saw that is going to get heavy use.

Don't raise the exhaust at all, or at most just the amount the cylinder is lowered. Don't raise the exhaust at all on on a new 660 even if you lower the cylinder the durration is higher than ideal when stock.

Widen your ports out so you keep about 0.100 coverage on each side of the narrowest part of the piston skirt.

Intake durration can be raised a little, but going more than about 1mm deeper on the lower side is not needed on a stump and milling saw and will start to hurt low end. Lowering the cylinder also increases intake durration.

Bell mouth the entry into the transfers, if you have the tools to work the upper transfers they can be re-aimed a bit rearward, but don't raise them.

Open your muffler up, add a good 3/4 of a square inch of outlet area. Reset the carb.

Smooth all the rough edges and match everything up and you will have a runner.


There is much more that can be done to chase a few percentage points but that does not make much sense for a heavy duity work saw.

 

[nanuk]

I understand that now.

Thanks TW. I understand all of what you said, and with all the reading I"ve been doing, I can follow along pretty good, imagining what you said which I've seen on Cyl's done by others through their pics.

it's just the idea of reducing the combustion chamber .020 separate from squish and base..

that's my block.

I've got a couple old saws in poor shape I bought specifically for this kinda hobby. Over winter I'll be taking some apart and having a good look.
as well as getting some tools together for the jobs!

thanks for your post!

 

[93Dakman!]

Gurus correct me if im wrong, I think taking .020 off the combustion chamber goes a bit like this, chuck the whole jug in a lathe and get it spinning, then using the tool all the way up in the jug, take .020 depth off of the squish area (the outermost, flattest area of the top inside of the cylinder).

When you all are saying 0.020 and such numbers, is that in mm or inches. I was trying to set my exhaust width and I ground it down to .035 mm. Im pretty sure I just ruined my jug becasue that is so awfully small....

 

[mdavlee]

.035mm where?

 

[timberwolf]

was trying to set my exhaust width and I ground it down to .035 mm. Im pretty sure I just ruined my jug becasue that is so awfully small....

If you set the width of your exhaust to within 0.35mm of the skirt you have done your saw a disservice.

 

[93Dakman!]

THe .035 im talking about is the clearance between the extreme edges of the exhaust port and the edges of the piston skirt width. I guess if you take the skirt width minus the port width and devide by two that should be the number im talking about. should that be .035 mm or .035 inches.

 

[timberwolf]

Neither. For a work saw you will want something closer to 2mm or 0.080. maybe a hair less if you want to push the limits. Where did you come up with the 0.035 for the sides?

If the exhaust port is perfectly lined up with the skirts you should be ok as long as it is sealed. Don't put any amount of chanfer on the sides of the port though or it will make matters worse. Intake port you cant cut as close as the exhaust due to loading on the piston skirt.

 

[nanuk]

MM vs SAE

Neither. For a work saw you will want something closer to 2mm or 0.080. maybe a hair less if you want to push the limits. Where did you come up with the 0.035 for the sides?

If the exhaust port is perfectly lined up with the skirts you should be ok as long as it is sealed. Don't put any amount of chanfer on the sides of the port though or it will make matters worse. Intake port you cant cut as close as the exhaust due to loading on the piston skirt.

sounds to me like a misunderstanding of a couple issues.
As most of you guys work with these number often, when you see .020 you understand that to mean SAE inches. Usually MM is appended when refering to Metric as machinists in the US are mostly using SAE Inches.

also, you guys JUST KNOW what the reference is to, as you KNOW about what the number should be!

therefore for us :newbie:'s we need to be extra cautious, and ask more questions whereever there is ANY doubt.

I also agree... read read read.... the more past threads I read, the better ejumikaded I become.

the other issue I have noticed is there occasionally seems to be a misunderstanding as to how the cylinder is cast. I assumed also that it was concentric.. but now I see that it isn't even close!

I have come to the conclusion that you can't port the with of I/O by measurement easily. Best method is to draw the port outline on the piston before removal. then you can SEE how much to remove. Then only grind away enough to reach an acceptable level. Seems 2.0 millimeters of port edge to outside of piston skirt is the MAXIMUM we should be looking for, until we get more experienced and understand the relationships better!

JMHO!