Cylinder Decking?

[Jed]

Sorry guys.

Just a quick question. I tried to run the little search thing, but I couldn't come up with anything. You guys were so helpful about porting thing, that I just thought I'd give it a shot. What's the point of "decking" the cylinder? I saw the video one of the guys posted of a machine shop which had apparently done a "beautiful job." It just looked line they had shaved some off the bottom to me. What does it do?

 

[MasterBlaster]

Good question.

 

[dastevoe]

It does a few different things. 1st, it will increase compression,2nd, it lowers the intake (increases intake duration....a bit more time to get more fuel mix into crank case),3rd, it lowers the exhaust port which gives a bit more time to complete combustion (increasing torque). Pretty simple explanation but basically I think is what happens.
I see Brian on so maybe he can give the proper explanation !!

Steve

 

[Jed]

Thanks Dastevoe: I guess I shouldn't have asked a question that I wouldn't understand the answer to. Appreciate the response. I have a hard time being a Christian around that girl though. Ever consider a picture of you cutting something?

 

[dastevoe]

Thanks Dastevoe: I guess I shouldn't have asked a question that I wouldn't understand the answer to. Appreciate the response. I have a hard time being a Christian around that girl though. Ever consider a picture of you cutting something?

LOL......if the good lord didn't want us to be looking at them, then he wouldn't have made them so "interesting"

Steve

 

[CurSedVoyce]

I would not have a quarrel with losing my virtue with that young lady at all
We want Steve to keep her up for a while... Shhhhhh Jed..

 

[Timberwolf]

Like Steve said, gaining compression is the main reason, the secondary potential benifit is a slight increase in intake durration which shifts power up in the RPM range. Also it can lower exhaust a little which shortens the exhaust durration gaining lower RPM torque at the expence of high RPM output. These changes can be used to an advantage in porting an engine, but just decking alone is not taking full advantage as the change to the intake is working againt the change to the exhaust, one wants to make the saw run at a higher RPM, the other wants to make it run a lower RPM.

Another advantage of decking the cylinder is to tighten the squish area between the piston and head. Any fuel charge trapped in that area never realy burns soon enough to transfer power to the crank. Making the volume of gas in the squish smaller results in more complete fuel burn. Also a tighter squish increases turbulance within the fuel charge promoting quick efficient combustion. There is a practical limit though and the piston should not come closer than 0.020in to the head at the top of the piston travel. Might get away a couple thou less if machiene work is good, but the tighter the greater the risk of engine problems.

Here is a video for turning down a base to sort of cover the process that was not shown in the 372 video.

http://www.youtube.com/user/Timberwolfsaws#p/u/88/t3Q6OsmMXEU

 

[SkwerI]

Excellent explanation and demonstration, Brian.

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[Al Smith]

I've seen the vid before .About the only thing I can add is the fact different people use different tpes or designs of turning mandrels .Some might use something as simple as a hardwood dowel .Some might use aluminum ,steel or get elborate like myself with replacable sizes of insets .No matter what is used they all get the job done .

The main thing is just make certain the base is running true before you cut it so as not to throw the cylinder out of alignment with the piston .

 

[Jed]

Timberwolf: Thanks for the comprehensive explanation. What I was able to understand made perfect sense. I'm almost motivated to find a machinist. Would it even be worth it on a 440?

 

[Al Smith]

Oh yes ,an 044/440 takes to modifications very well .Dan Henry did one up for Rick that would handle a 36" bar believe it or not .

I ran that thing down through about a 40" log and it never missed a lick .

 

[woodworkingboy]

I've had good success with making up wood and plastic mandrels, but with wood the dimensions can change overnight, so I turn the base down asap.

 

[Al Smith]

The wooden turning mandrel is most likely the method most often used . As long as it turns true though it really doesn't matter what is used .

 

[Timberwolf]

Yep, good way if you just have one cyl to do, But the metal is nice, just set the cylinder on it, move in the live center and it is dead straight every time.

 

[Al Smith]

I've had a picture simialar to this on several times .It's just a slightly more elaborate method of building a turning mandrel .

The shaft is 1" 4140 ground steel chucked in a Hardenge _Jorgenson collett chuck so it runs true as an arrow .The replacable turning head is hard nylon 6/6 .It could just as well be aluminum for that matter.They thread on by using 3/4" threads in the mandrel heads .

The neat part is because the head sets up in the cylinder about 1/4 to 3/8" the chamfer can be cut on the cylinder in one fell swoop whereas with a solid mandrel must often be done as second operation using a die grinder . Just another method of skinning the cat so to speak .