Stihl MS400 CFB style

[Canadian farm boy]

Here’s the victim, fresh outta the box

 

[Canadian farm boy]

Some random pics of the tare down

 

[Canadian farm boy]

Stock squish came in at .025”
All set up with TDC

 

[Canadian farm boy]

Stock port timing was
I 75
T 120
E 100

With stock squish at .025” I choose to cut .040” from the squish band and .045” from the cylinder base.

 

[Canadian farm boy]

The sides of the cylinder extension are flat so after cutting the base you have a small ridge left on each side that has to removed. I don’t have any fancy way doing that so I just carefully go at it with a file until the ridge is completely removed. I confirm this with a straight edge. You’ll also need to remove some material from corners of the exhaust side to allow the cylinder to sit down properly on the cases

 

[Canadian farm boy]

Couple pics of where the cylinder base needs to be clearanced. Final squish clearance came in at .0195”

 

[Canadian farm boy]

I like to keep the muffler mod simple on these newer stihl saws. I cut away most of the stock outlet deflector. I open the stock outlet up to about the same size as the spark screen. I then bend up my own defector much larger than the stock one and weld it on overtop the remaining stock deflector. I replaced the factory spark screen with a much much more open screen.

 

[Canadian farm boy]

Here’s that fancy magnesium piston as well as the part numbers for a cylinder base gasket and a muffler gasket

 

[farminkarman]

I like to keep the muffler mod simple on these newer stihl saws. I cut away most of the stock outlet deflector. I open the stock outlet up to about the same size as the spark screen. I then bend up my own defector much larger than the stock one and weld it on overtop the remaining stock deflector. I replaced the factory spark screen with a much much more open screen.
View attachment 454298View attachment 454299View attachment 454300View attachment 454301View attachment 454302

I like that mod…looks nicer than a second outlet.

 

[Canadian farm boy]

For the timing advance I simply put my degree wheel at TDC. I mark the tip of 1 FW fin with a sharpie and mark the case. I then turn the degree wheel 6 degrees and put another mark on the case to line up with the marked FW fin. Remove the FW. Then put the saw back to TDC. I file the FW key until with the piston at TDC the marked FW fin lines up with my 6 degree mark. I then install the FW and tighten the nut down using some recoil string as a piston stop.

 

[Canadian farm boy]

Heres the ported cylinder all ready to be installed.

 

[Canadian farm boy]

I also machined a flat spot on the top of the cylinder. Makes it much easier to dial in and hold in place on the lathe. (Plus it gives me a spot to leave my mark)

 

[Canadian farm boy]

Heat shield and the new muffler gasket both port matched

 

[Canadian farm boy]

I didn’t take many pictures of the reassembly. The clutch drum bearing is bone dry so I gave it a little lube when reassembling

 

[Canadian farm boy]

I know someone is gonna be curious and everybody loves numbers so here’s where I set it
I 80
T 120/124
E 102 and widened to %65 of the bore

I find most all strato saws like a little shorter BD when compared to other saws.

I’m also a fan of keeping the muffler outlet as close to the cylinder as possible. It definitely helps with noise and I find it also helps to keep a wider power band.

 

[Canadian farm boy]

Here’s this saw in wood. I had run about a half tank of fuel through it when I did the video.

 

[Powerstroke Cowboy]

Beautiful work Sir! Thanks for sharing.

 

[cuinrearview]

Want!!

Well done Mike

 

[Squareground3691]

Here’s this saw in wood. I had run about a half tank of fuel through it when I did the video.

She cuts eh ,

 

[Mastermind]

Nice.